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METABOLIC METHANOL: MOLECULAR PATHWAYS
AND PHYSIOLOGICAL ROLES
Yuri L. Dorokhov, Anastasia V. Shindyapina, Ekaterina V. Sheshukova,
and Tatiana V. Komarova
A. N. Belozersky Institute of Physico-Chemical Biology, Moscow State University, Moscow, Russia; and N. I.
Vavilov Institute of General Genetics, Russian Academy of Science, Moscow, Russia
LDorokhov YL, Shindyapina AV, Sheshukova EV, Komarova TV. Metabolic Meth-
anol: Molecular Pathways and Physiological Roles. Physiol Rev 95: 603–644,
2015; doi:10.1152/physrev.00034.2014.—Methanol has been historically con-
sidered an exogenous product that leads only to pathological changes in the human
body when consumed. However, in normal, healthy individuals, methanol and its
short-lived oxidized product, formaldehyde, are naturally occurring compounds whose functions
and origins have received limited attention. There are several sources of human physiological
methanol. Fruits, vegetables, and alcoholic beverages are likely the main sources of exogenous
methanol in the healthy human body. Metabolic methanol may occur as a result of fermentation
by gut bacteria and metabolic processes involving S-adenosyl methionine. Regardless of its
source, low levels of methanol in the body are maintained by physiological and metabolic
clearance mechanisms. Although human blood contains small amounts of methanol and
formaldehyde, the content of these molecules increases sharply after receiving even methanol-
free ethanol, indicating an endogenous source of the metabolic methanol present at low levels
in the blood regulated by a cluster of genes. Recent studies of the pathogenesis of neurological
disorders indicate metabolic formaldehyde as a putative causative agent. The detection of
increased formaldehyde content in the blood of both neurological patients and the elderly
indicates the important role of genetic and biochemical mechanisms of maintaining low levels
of methanol and formaldehyde.
I. INTRODUCTION 603
II. TERRESTRIAL METHANOL 605
III. EXOGENOUS SOURCES OF... 607
IV. HUMAN GUT MICROBIOTA AS A... 611
V. METHANOL METABOLISM IN... 615
VI. METHANOL/FORMALDEHYDE... 624
VII. GENES INVOLVED IN ENDOGENOUS... 628
VIII. CONCLUDING COMMENTS 630
I. INTRODUCTION
Robert Boyle first described wood spirits, or methanol, as
the “sowrish spirit” of boxwood pyrolysis in 1661 (44),
and the function of methanol in plant and animal life has
since been unclear. In higher plants, cell wall (CW) pectin
methylesterase (PME) produces methanol by pectin de-
methylation (248). Terrestrial atmospheric methanol
emission comes from volcanoes, H
2
and CO
2
generation
within seafloor hydrothermal systems, and biomass com-
bustion, but PME-mediated emission from plants is most
likely the largest source of methanol in the atmosphere
(512). Methanol accumulates in the intercellular air
space or the liquid pool when the stomata close at night,
and a large quantity of methanol is released when the
stomata open in the morning (193). Gaseous methanol
was traditionally considered to be a biochemical “waste
product.” However, the effects of PME-generated plant
methanol (“emitters”) on plant defensive reactions (“re-
ceivers”) and plant-animal communication have recently
been shown (111, 112).
In humans, methanol is considered to be a poison because
alcohol dehydrogenase 1b (ADH1b) mainly metabolizes
methanol into toxic formaldehyde (63). Methanol itself is
not toxic to animal cells; however, formaldehyde is respon-
sible for carcinogenesis and age-related damage to neurons
in the brain (472).
Until recently, it was believed that the trace amounts of
methanol and formaldehyde in the blood of healthy people
came only from the consumption of fake or low-quality
alcoholic beverages. However, recent data have indicated
that methanol and short-lived formaldehyde are actually
naturally occurring compounds in normal, healthy human
individuals (413). There are several sources of physiological
methanol in humans (FIGURE 1). Fruits, vegetables, and al-
coholic beverages are likely to be the main sources of exog-
enous methanol in the healthy humans. More than 50 years
ago (126), two other sources were suggested: anaerobic
Physiol Rev 95: 603–644, 2015
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fermentation by gut bacteria (38, 208, 209, 418) and the
transformation of S-adenosyl methionine (SAM) to metha-
nol by certain metabolic processes (16). Although human
blood contains small amounts of methanol and formalde-
hyde, their contents are sharply increased after receiving
even methanol-free ethanol (413). This indicates the exis-
tence of an endogenous source of low levels of metabolic
methanol in the blood, the regulation of which is controlled
by a cluster of genes (246, 413).
Interest in the literature is mainly focused on the mechanism
of ethanol metabolism (63, 347). Ethanol consumption, in
small amounts, can have a beneficial effect (147), but etha-
nol abuse leads to pathological changes in the human body
(355). In the past, methanol was mainly considered an ex-
ogenous product; the consumption of methanol was be-
lieved only to lead to pathological changes in the human
body (453). Researchers did not pay attention to either the
role or the origin of endogenous methanol and formalde-
hyde in humans. Recent studies of the pathogenesis of neu-
rological disorders indicate that metabolic formaldehyde is
a putative causative agent of human pathology (464, 466).
The detection of increased formaldehyde content in the
blood of not only neurological patients but also elderly
people (463) indicates the important role of genetic and
biochemical mechanisms in maintaining low levels of meth-
anol and formaldehyde.
This review will cover the wide spectrum of metabolic
methanol phenomena, both physiological and patholog-
ical, including the origin of human metabolic methanol,
the processes of methanol conversion and clearance, and
the roles of methanol and formaldehyde in human pa-
thology. Ultimately, an imbalance between the inflow
and outflow of formaldehyde may result in the onset of
pathologies.
Regulation of genes
involved in methanol
metabolic clearance
Methanol
Catalase-
H2O2 system
ADHI
Ethanol
Alcoholic
beverages
Aspartame as
a sweetener
Gut
microbiota
Fruits &
vegetables
Pectin/PME
complex
Methionine
+ATP
SAM
methyl
acceptor
1
2
3
4
5
6
7
8
9
10
11
14
15
Formaldehyde
Formic acid
CO2 + H2O
CYP2E1
12 13
FIGURE 1. Overview of physiological methanol biogenesis. This figure summarizes the data on methanol.
The methyl group donor SAM is synthesized via the catalytic activity of methionine adenosyltransferase,
which transfers the adenosyl group of ATP to methionine (step 1). S-adenosyl homocysteine is formed after
SAM transfers a methyl group to a methyl acceptor (step 2) such as DNA; thus methanol is involved in gene
regulation (step 11). Methyl esters such as carboxyl methyl esters are unstable and are readily hydrolyzed
in neutral and basic pH conditions or by methylesterase to produce methanol (step 3). Other sources of
methanol include the human diet, which supplies the methanol-generating pectin/PME complex via fruits
and vegetables (steps 4 and 5), aspartame as a synthetic nonnutritive sweetener (step 6) and alcoholic
beverages (step 7). The human gut microbiota is a putative methanol source (step 8) and takes part in the
generation of human endogenous ethanol (step 9). We suggest that endogenous and dietary methanol
may be involved in the regulation of genes involved in the metabolic clearance of methanol (step 10). The
first stage of the oxidative metabolism of methanol is executed by the catalase-H
2
O
2
system (step 12),
cytochrome P450 (CYP2E1)-mediated oxidation (step 13) and, mainly, the alcohol dehydrogenase I
(ADH1) class of enzymes (step 14). Although ADH1 converts methanol into toxic formaldehyde, physio-
logical ethanol in the bloodstream substantively prevents all formaldehyde production from endogenous
and dietary methanol in humans (step 15).
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II. TERRESTRIAL METHANOL
A. Methanol in the Earth’s Atmosphere
Methanol is an ubiquitous, biogeochemically active com-
pound and a significant component of the volatile organic
carbon in the atmosphere (177, 199, 239, 356, 390, 519).
The atmosphere contains ⬃4 Tg (teragrams, 10
12
g) of
methanol (177). Terrestrial atmospheric methanol emis-
sions come from different sources, including volcanoes and
H
2
and CO
2
generation within seafloor hydrothermal sys-
tems (512). Another source of atmospheric methanol is bio-
mass combustion (131, 315), in which the wood pyrolysis
of plant fibers (i.e., cellulose and lignin) induces methanol
emission (149, 300, 515). Wood pyrolysis, i.e., the decom-
position of wood at elevated temperatures in the absence of
oxygen, was the method that allowed Robert Boyle to ob-
tain the “sowrish spirit” (44) from boxwood; much later,
this became known mainly as methanol and acetone. How-
ever, volatile organic compound emissions from plants are
most likely the largest sources of methanol in the atmo-
sphere (193, 407). After CO
2
and isoprene, gaseous meth-
anol is one of the most abundant carbon-containing volatile
organic compounds in the atmosphere (156, 407).
B. Plant-Made Methanol
Plant-emitted gaseous methanol is an abundant, volatile
organic compound that was considered for a long time to be
a waste product of plant metabolism. Now, the diverse
biological effects of methanol have been discovered and
demonstrated. The main source of plant methanol release is
the above-ground parts of the plant (353). Plant methanol
accumulates in the leaf intercellular spaces in either a dis-
solved or a gaseous state when the stomata are closed at
night. When the leaf stomata open in the morning, a signif-
icant amount of methanol is released (193, 343). Plant
methanol release occurs with varying intensity depending
on the stage of plant development (343), time of day (193),
and other conditions. The quantity of the leaf-emitted
methanol varies from a minimum of 0.38
␮
g·g fresh weight
(FW)
⫺1
·h
⫺1
at night to a maximum of 7
␮
g·g FW
⫺1
·h
⫺1
in
the morning, while the average value is ⬃3
␮
g·g FW
⫺1
·h
⫺1
.
It should be noted that under certain types of stress, meth-
anol emission is dramatically increased to up to 100
␮
g·g
FW
⫺1
·h
⫺1
. In addition, damaged plants emit significant
amounts of methanol (48, 230). For example, in alfalfa
fields, high levels of gaseous methanol were detected after
mowing, and methanol emission levels continued to rise
over the next 3 days (505).
Plant tissues have been shown to metabolize methanol
(115). The majority of endogenous methanol reaches the
leaf surface and evaporates, and a minor amount is nonen-
zymatically oxidized to formaldehyde, which could later be
involved in the synthesis of serine, methionine, and phos-
phatidylcholine. In addition, methanol could be enzymati-
cally oxidized to CO
2
and then directed to the Calvin cycle
(151). Methanol metabolism in plants can be accompanied
by significant increases in biomass; in some C3 plants, this is
often accompanied by an increased photosynthetic effi-
ciency and developmental rate (151, 346). Moreover, plant-
generated methanol could be involved in leaf growth during
plant development (247). Small amounts of methanol emit-
ted through the stomata are oxidized to carbon dioxide by
methylotrophic bacteria either directly on the leaves or later
in the soil (244). In general, methanol is a rather stable
substance under normal conditions with a half-life of ⬃10
days (199).
Plants produce methanol in the CW pectin de-methyl-ester-
ification reaction (FIGURE 2). Pectin is a matrix-forming
component of the plant CW. Cellulose and xyloglucan fi-
brils are immersed in the pectin matrix (90). Pectin is mainly
formed from homogalacturonan blocks of
␣
-1,4-linked ga-
lacturonic acids, but rhamnogalacturonan I, rhamnogalac-
turonan II, and xylogalacturonan are the other major com-
ponents of the pectin matrix (165, 369). Pectin homogalac-
turonan is a compound that is synthesized by plant cells and
secreted into the CW in a highly methyl-esterified form
(FIGURE 2).
During the growth and development of cells or CW-me-
chanical damage pectin in the CW can be de-methyl-esteri-
fied, resulting in the production of methanol and the accu-
mulation of negatively charged galacturonic acid residues
(FIGURE 2). Due to the interaction of Ca
2⫹
with the charged
carboxyl groups, the rigidity of the CW can be increased if
the de-methylated residues are arranged in blocks. On the
other hand, in the case of the random de-methyl-esterifica-
tion of pectin, CW softening can occur, i.e., homogalac-
turonan becomes available for hydrolysis by pectin-degrad-
ing enzymes of the CW, resulting in the initiation of pectin
cleavage (511). Both PME-associated modulations of the
CW and the degree of pectin methylation play essential
roles in mediating the plasticity of the CW, cellular adhe-
sion, ionic composition, and pH (371). All of these aspects
are very important in the growth and development of
plants, pollination, and fruit ripening, as well as in the re-
sponses of plant cells to stress factors.
While they are conserved proteins, plant PMEs comprise a
multigene family (371). Some PME genes are ubiquitously
expressed, while others are specifically expressed during
fruit ripening, microsporogenesis, and germination of the
pollen grain, or stem elongation (304). Most higher plant
PMEs are synthesized as a precursor protein that contains
an NH
2
terminus pre-pro-sequence, which is necessary for
the correct processing to form a mature enzyme and deliver
it to the CW. The pre-sequence, which may be presented
with a signal peptide and/or a transmembrane domain, is
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cleaved at the stage of precursor delivery through the endo-
plasmic reticulum to the CW (290, 303), and the pro-se-
quence is removed at the next step of maturation (114,
513). Only group 2 PMEs possess the pro-sequence, which
is necessary for the correct folding of the enzyme and is
believed to function as an intramolecular chaperone (318,
371). In addition to the importance of PMEs during pro-
cesses of plant growth and development, PMEs play a sig-
nificant role in the protection of plants against external
stresses. PMEs are involved in modifications of the CW,
which is a natural barrier that protects and separates the cell
from the environment. Aside from the direct effects of
PMEs on the CW, the enzymes also act indirectly through
different compounds, including the methanol released via
pectin de-esterification (488a, 112, 371). Pectin with a
higher degree of methylation provides better resistance to
fungal and bacterial pectolytic enzymes. Thus plants that
are transgenic for the PME inhibitor (PMEI) gene exhibit
increased resistance to pathogenic fungi and bacteria com-
pared with wild-type plants (282, 283, 488). On the other
hand, PME is activated in response to the invasion of patho-
genic fungi and bacteria, and increased PME activity occurs
as a part of pattern-triggered immunity (31). CW integrity
damage by pathogenic pectolytic enzymes results in the ac-
cumulation of oligogalacturonide fragments, which play a
role in damage-associated molecular patterns (140). Oli-
gogalacturonide de-methylation by PMEs leads to the en-
hancement of their activity as elicitors and, thus, the im-
provement of plant defense responses to the invasion of
pathogenic fungi and bacteria (31, 383). With regard to
viral pathogens, PMEs have been shown to participate in
the virus cell-to-cell movement (74), which is necessary for
successful viral infection. PME is known to interact with the
movement protein of tobacco mosaic virus (TMV) (75,
113), and this interaction is required for the intercellular
transport of TMV. The important role of PME in the
spreading of viruses through the plant was confirmed by the
fact that Arabidopsis transgenic for the PMEI gene exhibits
decreased intercellular transport of TMV and lower suscep-
tibility to the virus (284). On the other hand, PMEs were
also shown to enhance virus-induced gene silencing (113)
and to indirectly interfere with nucleocytoplasmic transport
(245).
One of the indirect ways by which PME affects host-patho-
gen relationships is via gaseous methanol that is formed
during the pectin demethylation reaction in response to in-
jury. The effects of PME-generated methanol from plant
emitters on the defensive response of neighboring plant re-
ceivers were recently studied (111). It was shown that an
increase in methanol emission from PME-transgenic or
nontransgenic wounded plants restrains the growth of the
pathogenic bacteria Ralstonia solanacearum in neighboring
receiver plants. However, at the same time, plants exposed
to gaseous methanol became more sensitive to the viral
infection, likely due to the general activation of intercellular
transport. Such antibacterial resistance and virus suscepti-
bility are accompanied by increased activities of the genes
responsible for stress control and intercellular communica-
tion in receiver plants. These results led to the conclusion
that methanol is a signaling molecule that is involved in the
intra- and interplant communication (111, 247, 248). The
first portion of methanol emitted by damaged leaves is likely
produced by PME that is preexisting in the CW; this allows
for rapid methanol release into the atmosphere (250, 488a).
However, simultaneous de novo PME synthesis is induced,
which allows a high level of methanol emission to be main-
tained (111). Based on the available data, we can conclude
that methanol, which is released into the air by damaged
plants or plants attacked by herbivorous insects, serves as
O
O
OOCH
3
C
OH
COO–
HO
HO HO
O
O
PME
MeOH
PME
O
OO
O
CH3
C
OH
COO–
HO
HO HO
O
O
O
O
O
OO–
C
OH
COO–
HO
HO HO
O
O
O
OO–
O
C
OH
COO–
HO
HO HO
O
O
O
FIGURE 2. Demethylesterification of pectin ho-
mogalacturonan by PME with methanol formation.
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an alarm to help neighboring plants or adjacent leaves pre-
pare for defense.
C. Roles of Methanol and Ethanol in Plant-
Animal Communication
The role of methanol in plant-herbivore relationships re-
mains rather controversial. As was mentioned above, plants
release methanol during their growth and development, as
well as in response to wounding. Gaseous methanol plays a
role in signaling in the plant community and is also likely to
play a role in plant-animal communication. Methanol is
toxic to some insects (106, 288) and mediates defense reac-
tions against some herbivores (22). On the other hand, gas-
eous methanol attracts certain insects such as bark beetles
(Hylurgops palliatus, Tomicus piniperda, and Trypoden-
dron domesticum), while long-chain alcohols do not act as
attractants (56). Methanol emission occurs in response to
Manduca sexta larvae feeding on Nicotiana plants and ap-
pears to play a beneficial role for that insect: the treatment
of plants with the same amount of methanol as is released
during larvae feeding led to decreased plant defense reac-
tions and increased larvae attacking performance (488a).
Thus methanol release may induce metabolic changes that
influence the susceptibility of plants to herbivores. Further-
more, laboratory mice prefer the odor of methanol to that
of other plant-emitted volatiles or ethanol. Methanol emit-
ted by wounded plants was shown to be an attractant: mice
in a two-choice Y-maze chose methanol over other plant
volatiles. Moreover, inhalation of vapors released by
wounded plants led to increased blood methanol levels in
mice and changes in the expression profiles of the so-called
methanol-responsive genes in the brain (112, 246). This
discovery led to the conclusion that methanol emitted from
damaged plants should be regarded not only as a signal of
plant-to-plant communication but also as a plant-to-animal
cross-kingdom signaling molecule that modulates both be-
havior and mammalian gene expression (112).
As for ethanol, another alcohol, its increased emission
mainly accompanies fruit ripening due to the fermentation
of sugars by yeasts. Ethanol, among other volatiles, is an
important component of the scent of ripe fruit. Moreover,
an optimal ethanol concentration is a significant indicator
of the quality of fresh citrus fruit (331, 351). However, it
seems that increased ethanol levels in fruit adversely af-
fected the taste (80). Dominy (108) analyzed the ethanol
content of some Asian fruits and showed that small
amounts of ethanol (from 0.005 to 0.48%) were detected in
fruits of all developmental stages; the ethanol levels posi-
tively correlated with the soluble sugar levels. In this case,
ethanol is regarded as an olfactory cue for frugivores to
locate food of good quality with high carbohydrate content.
Frugivores such as Egyptian fruit bats (Rousettus aegyptia-
cus) were shown to locate fruits by assessing the degree of
ripeness and quality based on the intensity of the scent of
alcohol (methanol and ethanol). However, the Egyptian
fruit bats avoided overripe fruits, or fruits with ethanol
contents exceeding 1% (401). Moreover, yellow-vented
bulbuls (Pycnonotus xanthopygus) were shown to decrease
their food intake by 36% if the ethanol concentrations in
fruits exceeded 3% (309). The frugivorous tropical butter-
fly Bicyclus anynana locates fruits by using ethanol odor
cues as long range signals that guide the butterflies in the
direction of food sources containing soluble sugars (101).
An interesting relationship has been observed between the
bertam palm (Eugeissona tristis) and its pollinator, the pen-
tailed tree shrew (Ptilocercus lowii). The palm exudes nec-
tar from its flowers when the petals are still closed. This
nectar contains a high ethanol concentration and is pro-
duced by the plant for periods of up to 46 days. This facil-
itates the development of yeasts living in the nectar. Thus
these flowers release a strong ethanol odor that attracts tree
shrews, which then act as pollinators (509).
Thus plant-associated methanol and ethanol both play roles
in plant-animal communication as either attractants or re-
pellents, depending on the particular case.
III. EXOGENOUS SOURCES OF
PHYSIOLOGICAL METHANOL IN
HUMANS
A. Methanol Content in Healthy Human
Blood and Breathing Air
Methanol and ethanol are integral components of life for
humans and mammals (299). Healthy human blood con-
tains small amounts of metabolic methanol (112, 246, 413)
and ethanol (269, 362). Gaseous methanol and ethanol are
also detected in the air exhaled by healthy people (126, 212,
213, 474). The methanol concentration in the blood is 400 –
1,000 times less than the toxic concentration (214, 251) and
has been estimated by different researchers to range from
0.20 ⫾0.035 to 5.37 ⫾0.08 mg/l (24, 25, 78, 84, 112, 385,
413). Methanol is also detected in the urine, saliva (263),
and breast milk (370). The methanol concentration in ve-
nous blood is, on average, 3 times higher than that in saliva
(413) and 1.3 times lower than that in urine (263).
B. Plant Food as a Source of Exogenous
Methanol
Fruits and vegetables are the main sources of methanol in
the human body. Methanol is known to accumulate in fruit
during ripening as a result of PME activity, which among
other CW degrading enzymes plays a significant role in fruit
softening and ripening (367, 384). Methanol is detected
among other volatiles in the headspace of cut fruits (133).
Freshly squeezed citrus juices contain an average of 20–40
mg/l methanol, whereas methanol is detected in a lower
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concentration of 6.2 mg/l in processed juices or juice con-
centrates (293).
The consumption of vegetables such as leafy salads results
in increased blood methanol contents, by ⬃30% (112). The
ingestion of pectin, fruits with different degrees of ripeness,
or fruit juices leads to the rise of the methanol content of
exhaled air (126, 451, 474) and blood (278). This occurs
mainly due to the demethylation of pectin contained in
vegetables by PME, which is also present in the plant CW.
When citrus pectin in the presence of PME activity was
administered to mice, their blood methanol levels increased
by more than 15-fold compared with those administered
pectin without PME activity just 10 min after ingestion
(112). Pectin demethylation is also believed to occur in the
gut by the gut microbiota (418), as some strains of gut-
inhabiting bacteria possess pectolytic enzymes. However,
the administration of pectin containing no active PME in
the gastrointestinal tract of mice did not lead to significant
differences in the concentrations of methanol in their blood
compared with the control; thus it can be concluded that the
contribution of the intestinal microflora in methanol gener-
ation from pectin is rather low compared with the methanol
production from ingested plant food by PME (112).
Pectin is a water-soluble fiber that is resistant to hydrolysis
by gastrointestinal tract enzymes but can be fermented by
the microflora in the large intestine. This fermentation re-
sults in the formation of short-chain fatty acids, which are
then absorbed and metabolized (232). The positive effect of
apple pectin on the human gastrointestinal microflora was
demonstrated: the daily intake of two apples increases the
amount of Bifidobacterium and Lactobacillus, most strains
of which are capable of utilizing pectin (414). Moreover,
the daily consumption of whole apples, pomace, or unclari-
fied juice, i.e., products containing water-soluble fibers such
as pectin, was shown to lead to lower levels of low-density
lipoproteins in the blood (389, 456). This positive effect of
dietary pectin can be explained by the ability of pectin to
inhibit the reabsorption of cholesterol and bile acids in the
lower gastrointestinal tract due to the good gel-forming
properties of pectin, as well as by the ability of short-chain
fatty acids to decrease cholesterol biosynthesis by the liver
(207).
The beneficial effects of plant food are generally recognized,
and according to the World Health Organization’s (WHO)
recommendations, the minimal daily intake of plant food
should not be less than 400 g (250 g vegetables and 150 g
fruits). Fresh fruits and vegetables are particularly rich in
pectin complexed with enzymatically active PME. Despite
that PME is a rather thermostable enzyme, its activity sig-
nificantly decreases after boiling (112). On the other hand,
more intense pectin deesterification and methanol forma-
tion occurs after the moderate heating of plant food to
45–65°C (blanching) (5). Pectin, mainly from apples and
citrus fruits, is used as a dietary supplement to improve
digestion and also as a therapeutic and prophylactic agent
in several diseases, including atherosclerosis (52, 468) and
cardiovascular diseases (458), due to its role in slightly de-
creasing blood cholesterol levels (51, 160). Pectin is also
believed to lower the risk of cancer by decreasing tumor cell
proliferation (30, 477), inducing apoptosis (198), and sup-
pressing the metastatic ability of tumors (146). Notably,
citrus pectin consumption led to significantly increased
blood methanol levels in volunteers; this resulted in changes
in the expression profiles of genes in human white blood
cells, as demonstrated by microarray analysis (413). Pectin
can modulate detoxifying enzymes, stimulate the immune
system, modulate cholesterol synthesis, and act as an anti-
bacterial, antioxidant, or neuroprotective agent (261).
With consideration of the recommendations of nutritionists
to include a large proportion of plant food and vegetable
fiber in the diet, as well as the popularity of vegetarianism,
it can be concluded that fresh fruits and vegetables are two
of the main sources of exogenous methanol in human
blood.
C. Plant Food as a Source of Ethanol
Plant food, mainly ripe fruit, can also be a source of ethanol.
It is known that plants are able to synthesize ethanol mainly
via anaerobic fermentation by alcohol dehydrogenases
(ADHs) (67, 141, 237, 329). Transcription from ADH pro-
moters is induced in response to a lack of oxygen and to
cold stress in maize and Arabidopsis (107). When respira-
tion is impeded or inhibited, pyruvate is formed as a result
of glycolysis. Then, pyruvate is converted to lactate by lac-
tate dehydrogenase. Pyruvate decarboxylase is a key en-
zyme in the alternative pyruvate metabolic pathway, which
results in acetaldehyde formation followed by its reduction
to ethanol by ADH. Ethanol is much less toxic to plant cells
than lactate or acetaldehyde. This metabolic switch allows
plant cells to continuously regenerate ATP and NAD
⫹
, but
is less effective than normal conditions (437). Usually, the
Michaelis constant (K
m
) of ADH for acetaldehyde is less
than or equal to 1 mM, while the K
m
for ethanol is more
than 10-fold higher (33). Thus one of the main functions of
ADH in plant cells is the disposal of toxic acetaldehyde.
Another function of ADH is the synthesis of C6 and C9
volatiles that define fruit flavor (406), which attracts fruit-
eating, seed-spreading animals (frugivores). Thus the roles
of ADH in plants include its participation in ethanolic fer-
mentation in hypoxic or anoxic conditions and, in normal
aerobic conditions, in fruit and seed ripening to allow for
the production and emission of the characteristic scent cues
used by pollinators or frugivores. Nowadays, to prolong the
storage lengths of fruits and vegetables, they are kept in a
low-oxygen atmosphere (for example, with an increased
CO
2
content) and are covered with special waxes that also
induce hypoxia. These treatments lead to increased ethanol
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contents in the stored fruits and vegetables (11, 12, 80).
Notably, the methanol concentrations in such fruits and
vegetables also rise (11, 12).
Still, the main source of ethanol from fruits is the fermen-
tation of fruit sugars by yeast. The increased ethanol con-
tent in fruits during ripening is likely to be an evolutionary
adaptation because raised ethanol concentrations restrain
active bacterial propagation (118). Thus ethanol is ubiqui-
tous in ripe fruits, and its concentration ranges from 0.04 to
0.72% (7–125 mM) (117, 270). Thus frugivorous animals
consume significant amounts of ethanol, as they consume
ripe fruits.
D. Alcoholic Beverages as Sources of
Ethanol and Methanol
The amount of ethanol obtained by humans from fruits and
vegetables is at least two orders of magnitude less than that
from alcoholic beverages (beer, wine, distilled spirits),
which appear to be the main sources of exogenous ethanol
for people. The first evidence of artificial fermented-bever-
age production comes from China, nearly nine millennia
ago (312). Later, fermented beverages started to appear in
different cultures; today, they are a part of human life.
At present, the production of different alcoholic beverages
is under strict governmental control, and there are regula-
tions that define the maximum acceptable concentrations
(MAC) of congeners (methanol, formaldehyde, acetalde-
hyde, etc.) in produced alcoholic beverages. Methanol has
been detected in beer (6–27 mg/l of product) (153), wine,
and distilled spirits. On the basis of data available since
1949 on the methanol content in different wines, reviewed
by Gnekow and Ough (148), the highest concentrations of
methanol, up to 635 mg/l, were found in Spanish and Italian
red wines. The average methanol content of the analyzed
wines was ⬃100 mg/l of the final product. The maximum
limits for the methanol content in wines allowed by Inter-
national Organization of Vine and Wine are 400 mg/l for
red wines and 250 mg/l for white and rosé wines (354).
Higher methanol concentrations are allowed in distilled
fruit spirits due to the nature of the raw material, which is
derived from pectin-rich fruits. As was mentioned above,
PME activity increases during fruit ripening; thus the meth-
anol content is elevated. Methanol released from pectin
after demethylation accumulates in ripe fruits and ends up
in the final product, distilled fruit spirits (34) Under labo-
ratory conditions, it was shown that fruit spirits made from
cherries and plums could contain from 18.3 to 25 g meth-
anol/l of the 100% vol ethanol (405). Undoubtedly, such
high methanol contents exceed the MACs. In the United
States, the allowed methanol concentration for distilled
fruit spirits is 6–7 g/l of 100% vol ethanol (34, 477a). Since
2008, there have been EU limits on methanol (per liter of
100% vol ethanol) in different distilled spirits, as follows:
0.1 g for vodka; up to 1.35 g for some fruit spirits; 1.5 g for
fruit marc spirits; and 2 g, the highest allowed methanol
content, for wine spirits and brandy (130a).
Thus the methanol concentration in alcoholic beverages is
strictly controlled by legislation, and manufacturers are ea-
ger to reduce this concentration in their beverages. Despite
this, shockingly, when an individual drinks wine or distilled
spirits even with very low or undetectable methanol con-
tents, the endogenous methanol level increases significantly
and is comparable to the increased ethanol concentration
that results from ingesting ethanol (413). Earlier, it was
shown that the ingestion of methanol-free whisky or grain
alcohol by a volunteer led to increased methanol content in
his breath (451). Healthy volunteers who drank a glass of
red wine (150 ml) containing 13.7% vol of ethanol and 33
mg/l of methanol showed nearly twofold increases in their
blood methanol content. Moreover, the orders of magni-
tude for methanol and ethanol concentrations were the
same or at least comparable 60–120 min after ingestion.
Furthermore, when 50–90 ml per person (1 ml of 40% vol
ethanol per 1 kg of weight) of methanol-free 40% vol eth-
anol was ingested by the same volunteers in another exper-
iment, the methanol contents in their blood were also of the
same order of magnitude as ethanol and were even compa-
rable 90 min after ingestion, ⬃350–400
␮
M (413). Thus
the ingestion of alcoholic beverages always leads to in-
creased blood methanol levels, regardless of how low the
methanol content is in the beverage. This likely happens due
to the partial depletion of liver ADH by ethanol, while a
methanol level continues to rise via endogenous sources.
From these data, the level of endogenous methanol pro-
duction can be estimated to be at least 1.7 mg·kg
⫺1
·h
⫺1
(413). The above-mentioned methanol concentrations
observed after red wine or 40% vol ethanol consumption
or even 5-fold higher methanol contents could be toler-
ated without negative consequences; only a nearly 20-
fold higher concentration is regarded as dangerous (364).
Despite the fact that alcoholic beverages contain ethanol,
which “distracts” ADH from metabolizing methanol,
blood formaldehyde levels also rise significantly after
ethanol ingestion, but with slight retardation; they start
to increase 60 min after ingestion and reach concentra-
tions comparable to methanol and ethanol 60–90 min
after red wine or 90–120 min after ethanol consumption
(413). Thus the ingestion of alcoholic beverages, even
those of high quality, always leads to increased ethanol,
methanol, and formaldehyde contents in the blood.
Moreover, plant food, which is undoubtedly believed to
be healthy, also results in increased methanol and form-
aldehyde levels in the blood. Taking into account that
plant food was the main food source during human evo-
lution, such oscillations of methanol and formaldehyde
concentrations might have some beneficial effects in hu-
mans.
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E. Aspartame
Aspartame, a synthetic nonnutritive sweetener, is a dipep-
tide composed of aspartic acid and phenylalanine methyl
ester. Compared with sucrose, aspartame is 200-fold
sweeter, which allows it to be consumed in much lower
doses to make foods or beverages sweet; for this reason,
aspartame is regarded as a low-calorie sweetener (298). As
aspartame is a methyl ester, its degradation could result in
methanol formation. The stability of aspartame depends on
the storage conditions: it becomes unstable after heating in
aqueous solutions and at neutral or alkaline pH. None of
the degradation products of aspartame is sweet. Thus as-
partame is usually used in dry form or in carbonated bev-
erages and is not recommended for heat-treated foods. Fur-
thermore, the appropriate storage conditions and the expi-
ration dates of foods and beverages with aspartame are very
important. Despite the fact that aspartame was approved as
a safe sweetener by the United States Food and Drug Ad-
ministration (FDA) in 1996 (136, 477b) and in Europe in
1994 (130), the debates around this compound have not
subsided, and research into the effects of aspartame on hu-
man health are in progress; the question of its safety re-
mains acute (423). The main safety concern regarding as-
partame is the formation of methanol as a result of the
inevitable degradation of phenylalanine methyl ester. As
methanol is essentially converted to formaldehyde in mam-
malian organisms, excessive aspartame consumption may
be hazardous because of its contribution to the formation of
formaldehyde (467).
Currently, the acceptable daily intake (ADI) of aspartame in
Europe is 40 mg/kg body weight (bw) and in the United
States is 50 mg/kg bw. The highest aspartame consumption
is observed in teenagers and diabetics; the maximum intake
was estimated at ⬃13.6 mg·kg bw
⫺1
·day
⫺1
for teenage girls
consuming high amounts of soft drinks (9), which, never-
theless, is at least two times lower than the ADI. For chil-
dren and adults with diabetes, the daily consumption of
aspartame in the worst-case scenario also did not exceed the
ADI (298). When volunteers consumed 600 mg aspartame
eight times a day at 1-h intervals, there were no detectable
differences in their blood methanol levels (434). It is worth
mentioning that the average body weight of the volunteers
was 70.8 ⫾13.5 kg; thus the doses of aspartame used in the
experiment exceeded the present ADI. According to Ste-
gink’s (433) data, the ingestion of 34 mg/kg bw of aspar-
tame (which is very close to the ADI) did not lead to a
detectable increase of blood methanol levels, likely due to a
lack of sensitivity of the detection method. Only doses from
100 mg/kg bw (twice as high as the ADI) resulted in detect-
able rises of blood methanol levels up to a mean value of
12.7 mg/l (433). Notably, a comparable methanol concen-
tration of ⬃11 mg/l in the blood is obtained from endoge-
nous sources after methanol-free 40% vol ethanol ingestion
(413). In another study, the ingestion of 7.5–8.5 mg/kg
aspartame (the FDA estimation for the average daily con-
sumption) resulted in a mean rise over endogenous values of
1.06 mg/l serum (which is ⬃0.5 mg/l blood), but this in-
crease is of the same order of magnitude as the variations in
endogenous methanol levels and between-individuals dif-
ferences (95). Thus it can be concluded that moderate as-
partame consumption within the recommended doses leads
to a rise in the blood methanol content, similar to the con-
sequence of plant food, juice, or alcoholic beverage inges-
tion.
F. Food as an Exogenous Source of
Acetaldehyde and Formaldehyde
Aside from being a source of ethanol and methanol, food
may also be an exogenous source of acetaldehyde and form-
aldehyde. For example, the acetaldehyde content in apples
ranged from 0.4 to 2.3 mg/kg, in bananas ranged from 1.88
to 18.27 mg/kg, in fresh orange juice was 5.89 mg/kg, and
in fermented dairy products, such as fruit-flavored yogurt,
was up to 17 mg/kg. Daily acetaldehyde consumption with
food (excluding ethanol) is estimated to be ⬃40
␮
g/kg bw,
which is considerably lower than the exposure from ethanol
consumption or tobacco smoking (476). The average daily
exposure to acetaldehyde from alcoholic beverages was es-
timated at 0.112 mg/kg bw (257). Significant amounts of
acetaldehyde (up to 37 mg/l) were detected in different sam-
ples of beer, whiskey, and other distilled spirits (up to 10
mg/l) (440).
Some food could be also a source of exogenous formal-
dehyde. The formaldehyde contents in numerous prod-
ucts were analyzed with chromatographic methods. In
different fruits and vegetables, the formaldehyde concen-
trations ranged from 3 to 26 mg/kg (469). High levels of
formaldehyde were detected in fish of the Gadidae fam-
ily, ranging from 6.4 to 293 mg/kg (32); in milk (0.027
mg/kg in fresh milk and 0.164 mg/kg in processed milk)
(222) and milk products [in Italy, formaldehyde is used
as a bacteriostatic agent in cheese production (391)]; and
in coffee [3.4–4.5 mg/kg in commercially brewed and
10–16.3 mg/kg in instant coffees (171)]. Moreover, nat-
ural antioxidants that are O- and N-demethylated by
cytochrome P450 in humans (125) could be regarded as a
source of formaldehyde. The same applies to some drugs
such as those containing codeine (98, 490). According to
various assessments, the daily intake of direct formalde-
hyde is estimated to be in the range of 1.5–14 mg/day for
the average adult (98).
One significant non-food source of exogenous methanol
and formaldehyde for humans that cannot be ignored is
cigarette smoke, which has an estimated methanol content
of 180
␮
g/cigarette and formaldehyde content of 45–73
mg/cigarette (157, 302).
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G. Food Modulating Activity of Alcohol-
Metabolizing Enzymes
Aside from the food- and beverage-related intake of alco-
hols (methanol and ethanol) and aldehydes, changes in the
contents of these compounds can also result from ADH and
aldehyde dehydrogenase (ALDH) activity modulation.
Agents that affect these enzymes could also be food compo-
nents. Plants that contain inhibitors or activators of ADH
and ALDH are used in traditional Chinese medicine to re-
lieve hangovers or to treat alcoholism. Recently, several of
these phytotherapy-based products also appeared in West-
ern societies (313). The most popular herb for treating al-
coholism is Kudzu (Pueraria lobata). It contains daidzein
(4=,7-dihydroxyisoflavone), puerarin (8-C-glucoside of
daidzein), and daidzin (7-glucoside of daidzein), which re-
versibly inhibits mitochondrial ALDH2 (235). The struc-
ture of the daidzin/ALDH2 complex was resolved at 2.4 Å
and showed that the daidzin isoflavone moiety is placed
very closely to the substrate-binding site and glycosyl to a
hydrophobic patch immediately outside the isoflavone-
binding pocket (291). Daidzein is a dietary phytoestrogen
that is also found in soy products. Soy products that are
popular in Asia contain another isoflavone, genistein,
which together with daidzin lowers ethanol and acetalde-
hyde concentrations in the blood due to the enhancement of
ethanol metabolism; this was shown in rats that ingested
soy milk and fermented soy milk together with ethanol
(225). In humans, daidzein is metabolized to daidzin by
bowel bacteria (313). High levels of serum daidzein, 240–
280
␮
M, are characteristic for the Japanese population and
are ⬃20 times higher than those in the United Kingdom
population (330); this difference can be explained by di-
etary preferences.
Thus the risks of acetaldehyde and formaldehyde intoxica-
tion as a result of ethanol consumption or smoking are
higher for the Asian population.
Another plant, Hovenia dulcis, also known as the Japanese
raisin tree, is used in traditional Asian medicine for detox-
ification after ethanol consumption. Extracts from the fruits
of this plant significantly decreased the blood ethanol con-
centration by stimulating the activity of liver ADH, ALDH,
and glutathione S-transferase in mice and rats. Hovenod-
ulinol is believed to be the compound responsible for this
effect (194), but the exact mechanism of its action remains
unknown.
Other foods have also been shown to affect ethanol metab-
olism. In experiments in which mango flesh or mango peel
was administered to mice, followed by ethanol ingestion,
blood ethanol levels were lower by more than 50% com-
pared with the control group. This difference in ethanol
concentration likely occurred due to the activation of ADH
and cytosolic ALDH in the liver (238). The authors as-
sumed that the effect observed could be mediated by fruc-
tose and aspartate from the mango, which produce NAD
⫹
and thus stimulate ADH and ALDH.
Some phytophenols were also shown to modulate ethanol
metabolism. For example, vanillin, syringaldehyde, caffeic
acid, and ellagic acid appeared to strongly inhibit liver
ADH1 in mice. When these phytophenols were adminis-
tered to mice together with ethanol, they prevented the
elimination of blood ethanol through the ADH metabolic
pathway (170). This effect led to reduced acetaldehyde ac-
cumulation after ethanol ingestion. Interestingly, all of
these compounds were found in mature whisky, and their
concentrations rose with the time of maturation. These
compounds are also present in different foods: vanillin is
found in vanilla beans and as a flavoring agent in confec-
tions; caffeic acid is found in coffee beans, soy beans, argan
oil, and barley; and ellagic acid is present in blackberries,
cranberries, pecans, pomegranates, raspberries, strawber-
ries, walnuts, grapes, and peaches.
Thus methanol, ethanol, formaldehyde, and acetaldehyde
are natural components found in the human body. People
come into daily contact with all of these compounds via
food and inhaled air.
IV. HUMAN GUT MICROBIOTA AS A
SOURCE OF METHANOL
A. Overview of Methanol Metabolism in
Bacteria
In mammals, the intestinal flora is the most relevant candi-
date for the production of methanol followed by its oxida-
tion to formaldehyde, formic acid, and carbon dioxide. To
prove this, it was recently tested whether the intestinal mi-
crobes in rats generate methanol (246). Removal of the
bowel in rats resulted in a lessened methanol increase in the
blood following the administration of 4-methylpyrazole
into the liver compared with the control group (246). Cur-
rently, a large number of bacteria and several types of yeast
are known to be capable of growing on medium wherein
methanol is the sole carbon source (TABLE 1). Bacteria uti-
lize three classic mechanisms of include methanol metabo-
lism: the Calvin cycle, the ribulose monophosphate cycle,
and the serine cycle (387). First, the most energetically
costly pathway of methanol metabolism begins with the
oxidation of methanol to carbon dioxide, which is then
incorporated into the Calvin cycle as a carbon source. The
process of methanol utilization through the Calvin cycle
was detected for the first time in Micrococcus denitrificans
(91). Later, this process was observed in the bacteria Xan-
thobacter autotrophicus (21), Methylacidiphilum inferno-
rum (358), as well as in bacteria belonging to the NC10
phylum (129). The classical method of methanol oxidation
to carbon dioxide takes place in three stages: the oxidation
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of methanol to formaldehyde, formaldehyde to formic acid,
and formic acid to carbon dioxide (491). In some facultative
methylotrophs, instead of three reactions for converting
methanol to carbon dioxide, formaldehyde is oxidized to
carbon dioxide via the dissimilatory hexulose phosphate
cycle (82). In this section, we focus on bacterial enzymes
that either use methanol as substrate or produce methanol
as a product.
Bacteria oxidize alcohols via a few different groups of en-
zymes. The first group includes mammalian-like ADHs
with different electron acceptors (i.e., NAD, NADP, heme).
ADHs from the aerobic bacterial intestinal flora actively
participate in ethanol metabolism. The intestinal flora in
piglets produces great amounts of acetaldehyde from etha-
nol in vivo (211). Bacteria contain a branch of enzymes that
metabolize ethanol, but ADHs oxidize most of it. Although
ADHs play an important role in methanol metabolism in
mammals, bacteria utilize methanol in alternative ways.
Bacterial NADP-dependent ADH (493), NAD-dependent
ADH (124, 348), quinohemoprotein ADH (70), and mem-
brane-associated quinohemoprotein ADH (412) have little
or no activity toward methanol and mostly oxidize C2–C6
alcohols. An exception is NAD-dependent ADH (EC
1.1.1.1) from Bacillus stearothermophilus, DSM 2334,
which is able to oxidize methanol with an efficacy compa-
rable to that of horse liver ADH (411).
Enzymes that use hydrogen peroxide to metabolize alcohols
(methanol and ethanol) comprise a second group of en-
zymes that catalyze the first step of alcohol oxidation in
bacteria. The first catalase enzyme (EC 1.11.1.6) is likely
the most ancient alcohol-oxidizing enzyme and can be
found in many organisms, from archaea to human. In bac-
teria, catalase is encoded by the katE gene, which is an
ortholog of the human CAT gene. The second catalase en-
zyme (EC 1.11.1.21) is encoded by the katG gene, which is
specifically found in bacteria and fungi. According to the
KEGG database, both genes are present in all phyla of bac-
teria and in more than 1,000 species, highlighting their
crucial role in bacterial alcohol metabolism. Along with
ADHs, bacterial catalases from the intestinal flora partici-
pate in ethanol oxidation and metabolize ⬃26–32% of the
total ethanol in vitro (459). Although the data on methanol
oxidation by catalase in microorganisms are very limited, it
is known that methanol can act as a hydrogen donor in
catalase reaction in some bacteria and yeast (166, 395,
482b). Based on the number of intestinal bacteria that carry
the katG and katE genes (TABLE 1) and the experimental
evidence of methanol utilization by the catalase enzymes,
Table 1. Summary of the most abundant bacteria that carry genes encoding methanol-producing (PME, BioH, PMG) and methanol-
oxidizing enzymes (katG, katE) in stool samples
Bacteria Name
Number of Samples
Containing Bacteria
(Out of 5)
Average Number of
Clones per Sample PME BioH PGM katG katE
Eubacterium rectale 5 3,412 ⫹
Escherichia coli 5 3,183 ⫹⫹⫹⫹
Bacteroides vulgatus 5 2,293 ⫹⫹
Alistipes finegoldii 5 589 ⫹
Bacteroides thetaiotaomicron 5 538 ⫹⫹
Parabacteroides distasonis 5 527 ⫹
Bifidobacterium breve 5 402 ⫹
Butyrivibrio fibrisolvens 5 156 ⫹
Eubacterium eligens 5 143 ⫹⫹
Bacteroides fragilis 5 119 ⫹
Butyrate-producing bacterium SS3/4 472 ⫹
Ruminococcus albus 359⫹⫹
Clostridium phytofermentans 528⫹
Geobacillus thermoglucosidasius 523 ⫹⫹
Alistipes shahii 518⫹⫹
Xanthomonas campestris 418⫹⫹⫹⫹
Klebsiella pneumoniae 315⫹⫹ ⫹⫹
Enterobacter cloacae 217⫹⫹⫹ ⫹
Bacillus cereus 34⫹⫹
Geobacillus thermodenitrificans 33 ⫹⫹⫹
For the analyses, we took the 16S RNA sequencing data from citizens of New Zealand, United States, Hungary,
Norway, and Hong Kong. The data were obtained from the MG-RAST database, loaded and performed by the
American Gut Project (http://metagenomics.anl.gov/linkin.cgi?project⫽6594).
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we hypothesize that catalases may participate in methanol
oxidation by the intestinal microflora.
Bacteria mainly oxidize methanol to formaldehyde by the
following methanol dehydrogenases (MDH): cytochrome
c-dependent methanol dehydrogenase, which contains a
pyrroloquinoline quinine cofactor (PQQ-MDH); NAD-de-
pendent methanol dehydrogenase (NAD-MDH) and
NADPH-dependent methanol dehydrogenase (NADPH-
MDH). On the basis of the analysis of bacteria from human
stool samples, three types of bacteria that oxidize methanol
by MDHs were revealed: Methylobacterium extorquens
and Methylobacterium nodulans, using PQQ-MDH, and
Bacillus methanolicus, using NAD-MDH (10). However,
the MDH-containing bacteria are not typical residents of
the human microflora, and few clones of these bacteria were
detected in analyzed stool samples.
Formaldehyde that is produced in the methanol and meth-
ane oxidation reaction is either assimilated in the ribulose
monophosphate (RuMP) or the serine cycle or further oxi-
dized into carbon dioxide, which is then incorporated into
the Calvin cycle (FIGURE 3). In the RuMP cycle, formalde-
hyde and D-ribulose 5-phosphate are condensed by hexu-
lose-6-phosphate synthase to form hexulose 6-phosphate.
Bacillus subtilis (521) and Brevibacillus brevis (528) are
two examples of intestinal flora bacteria that assimilate
formaldehyde via the RuMP cycle. In the serine cycle, serine
hydroxymethyltransferase uses formaldehyde and tetrahy-
drofolate as cofactors to form serine from glycine. The ser-
ine pathway of formaldehyde assimilation was discovered
in Methylobacterium extorquens AM1 (76), a gram-nega-
tive Alphaproteobacteria that can be found in human intes-
tines.
There are two classes of reactions that result in methanol
production: methyl ester hydrolysis and redox reactions
(TABLE 2). Despite the versatility of methanol-producing
reactions, only a few are typical for bacterial metabolism
under normal conditions. On the other hand, some of the
methanol-producing reactions can be found only in a few
species of bacteria. For example, methane monooxygenases
(soluble, sMMO; membrane-bound form, pMMO) are en-
zymes specific to methanotrophs, a small group of bacteria
Methanol
Formaldehyde Formaldehyde
Methane
Formic acid
NAD(P)H + H+, H2O
O2, NAD(P)H + H+
FDMMD/ADH
RuMP
CYCLE
SERINE
CYCLE
GSH-FaDH
FAE
MtdB FolD MtdA
Formate dehydrogenases
sMMO, pMMO
Substrate: 4-MO
Enzyme: 4-MO esterase
Substrate: Pectin
Enzyme: PE
Substrate: PAM
Enzyme: BioH
Carbon dioxide
Methyl esters
CH2 = H4FCH2 = H4MPT
FIGURE 3. Methanol metabolism and production by bacteria. 4-MO, 4-methyl oxaloacetate; PAM, pimelyl-
[acp] methyl ester; FDM, formaldehyde dismutase; MDs, methanol dehydrogenases; ADHs, alcohol dehydro-
genases; sMMO/pMMO, soluble/membrane-bound methane monooxygenase; GSH-FaDH, glutathione-linked
formaldehyde oxidation; FAE, formaldehyde activating enzyme; FolD, 5,10-methylene-H
4
folate dehydroge-
nase/5,10-methenylH
4
folate cyclohydrolase.
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able to grow on methane as the sole carbon and energy
source (163).
Methanol concentrations in the blood of volunteers of dif-
ferent ages, genders, diets, as well as smoking and alcohol
habits are very similar. Additionally, methanol accumulates
with similar kinetics in humans and mice after the admin-
istration of ADH inhibitors. Both parameters indicate that
endogenous sources of methanol work constitutively under
normal conditions. Here, we will review the reactions that
result in methanol production and 1) occur under normal
conditions, 2) are common among intestinal bacteria, and
3) work constitutively.
B. Bacterial Pectin Methylesterase
Pectin methylesterase (PME; EC 3.1.1.11) is generally asso-
ciated with plant physiology and CW structure. PME from
bacteria is of particular interest due to its role in the degra-
dation of the CW in plants (260) and its various industrial
applications (187). Additionally, bacterial PME is an essen-
tial part of pectin fermentation in humans because our en-
zymes cannot break down pectin (514). PME is one of the
most abundant enzymes participating in methanol produc-
tion by intestinal bacteria. Approximately 30 species of in-
testinal bacteria carry the PME gene, and some of them are
commonly present in the microflora (TABLE 1). However,
little is known about characteristics of the intestinal bacte-
ria that produce methanol via PME, and few works have
demonstrated the ability of the mammalian intestinal flora
to produce methanol as the result of pectin breakdown by
PME under aerobic and anaerobic conditions (38, 208,
209, 418). As a result of the digestion of dietary pectin
(from vegetables and fruit) and pectin capsules by the mi-
croflora and/or by pectin-associated PME, methanol levels
in human blood increased up to 1.5 times than that before
exposure (112). However, PME only catalyzes methanol
production in the presence of exogenous pectin; thus PME
activity alone cannot explain the basic, similar methanol
concentration found in the diverse group of volunteers and
the constitutive production of methanol in mammals.
C. Protein-Glutamate Methylesterase
Protein-glutamate methylesterase (PGM) is another com-
mon enzyme in the microflora (TABLE 1) that hydrolyzes
methyl esters of glutamate that bind to proteins. Methyl-
ation and demethylation are well-known mechanisms of
controlling expression, translation, and protein activities.
Depending on the type of bind between methyl radicals and
biopolymers, active demethylation can produce either
formaldehyde or methanol. The product of carbonic acid
(amino acids, glutamate, oxaloacetate, etc.) or methyl ester
demethylation is methanol, while DNA demethylation gen-
erates formaldehyde. PGM is of particular interest due to its
ability to produce detectable levels of methanol in vivo
(461), and, in contrast to PME, only endogenous molecules
are substrates of PGM for methanol production. The CheB
gene, which encodes PMG, is present in more than 50 spe-
cies of intestinal bacteria from stool samples, including
abundant species (TABLE 1). However, CheB expression
correlates with decreased chemotactic stimuli (43), and
PMG does not maintain permanent methanol concentra-
tions over time. PMG seems to participate in methanol for-
mation by intestinal bacteria, but it is unlikely that PMG
plays a crucial role in this process.
D. Pimeloyl-[Acyl-Carrier Protein]-Methyl-
Ester Hydrolase
Pimeloyl-[acyl-carrier protein]-methyl-ester hydrolase (BioH)
is an enzyme with hydrolysis activity that might also
participate in methanol production by microflora. BioH
participates in biotin (vitamin H) production in bacteria
and other organisms. The demethylation reaction termi-
nates the part of the biotin biosynthesis pathway that is
catalyzed by fatty acid synthesis enzymes (281) and results
in methanol and biotin precursor production. Vitamin H
serves in all living organisms as a covalently bound enzyme
cofactor that is essential for the introduction of carboxylic
acid groups to substrates via carboxylases. Mammals have
no enzymes capable of biotin synthesis and thus use micro-
flora- and food-derived biotin. Hence, bacteria containing
genes that encode components of the biotin biosynthesis
pathway (including BioH,TABLE 1) are present in the intes-
Table 2. List of methanol-producing enzymes, their substrate,
and the type of reaction they catalyze in bacteria
Ferment Name [EC
Number] Substrate
Reaction
Class
Pectinesterase
[3.1.1.11]
Methyl esters of
pectin
Hydrolysis
sMMO/pMMO
[1.14.13.25]
Methane, oxygen Redox
reaction
Formaldehyde
dismutase
[1.2.99.4]
Formaldehyde Redox
reaction
Protocatechuate 4,5-
dioxygenase
[1.13.11.8]
3-O-Methylgallate Redox
reaction
4-Methyloxaloacetate
esterase [3.1.1.44]
4-Methyloxaloacetate Hydrolysis
Pimeloyl-[acyl-carrier
protein]-methyl-ester
hydrolase (BioH)
[3.1.1.85]
Pimeloyl-[acyl-carrier
protein]-methyl
ester
Hydrolysis
Protein-L-glutamate-O4-
methyl-ester
acylhydrolase
[3.1.1.61]
Protein glutamate
methyl ester
Hydrolysis
sMMO, soluble methane monooxygenase; pMMO, particulate meth-
ane monooxygenase.
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tinal flora. BioH activity was originally demonstrated in
Escherichia coli, which is one of the major microflora bac-
teria. Thus BioH participates in an essential biosynthetic
pathway and is widespread in the microflora; hence, BioH is
the best candidate for methanol production by the intestinal
flora.
To summarize, there are a few strong candidates among
bacterial enzymes that might participate in methanol me-
tabolism and serve as the source of methanol metabolism in
mammals. Catalase seems to oxidize most of the methanol
in bacteria, while ADH and MDH are less important. PME
and PGM require exogenous sources to produce methanol,
so they cannot be an efficient endogenous source. Because
BioH participates in an essential biosynthetic pathway and
is widespread in the microflora, it is the best candidate for
methanol production by the intestinal flora. This analysis is
in agreement with the experimental data on the microflora
of rats (246) and gives new clues for further investigation of
methanol metabolism by intestinal bacteria in mammals.
V. METHANOL METABOLISM IN HUMANS
Along with dietary methanol and methanol produced by the
intestinal microflora, processes that involve SAM participa-
tion also contribute to the pool of physiological methanol
(265). SAM is a universal, endogenous methyl donor for
several reactions, including the methylation of proteins,
phospholipids, DNA, RNA, and other molecules involved
in basic epigenetic mechanisms (196). Genome-wide meth-
ylation analysis has identified DNA methylation profiles
specific to aging and longevity. Moreover, analysis of the
DNA methylation landscape revealed that DNA obtained
from a 103-yr-old donor was more unmethylated overall
than DNA from the same cell type isolated from a neonate
(181). Differentially methylated genes are strikingly en-
riched in loci associated with neurological disorders, psy-
chological disorders, and cancers (531). Protein carboxym-
ethylation involves the methylation of amino acid COOH
groups; this reaction is catalyzed by methyltransferases,
which produce carboxyl methyl esters that are readily hy-
drolyzed under neutral and basic pH conditions, or by
methyl esterase, which produces methanol (104). Protein
carboxymethylase is highly localized to the brain and pitu-
itary gland in several mammalian species (105).
In a healthy person, no matter from what sources methanol
is derived in the body, it is eventually displayed and kept at
a low physiological level via physiological and metabolic
clearance mechanisms.
A. Nonmetabolic (Physiological) Clearance of
Methanol
Because methanol metabolism involves the same enzymes
as ethanol metabolism, information about the nonmeta-
bolic clearance of ethanol may be relevant to our consider-
ation of methanol metabolism. Approximately 90% of eth-
anol is removed by oxidation, and ⬍10% of ethanol is
excreted in breath, sweat, and urine (63, 347). Animal stud-
ies have reported that inhaled methanol is eliminated
mainly by metabolism (70–97% of absorbed dose), and
only a small fraction is eliminated as unchanged methanol
in the urine and expired air (⬎3–4%) (42, 110, 188). The
determination of methanol and ethanol contents in the hu-
man breath after the consumption of alcoholic beverages
and various amounts of fruits revealed that methanol con-
centrations increased from a natural (physiological) level of
⬃0.4 to ⬃2 ppm a few hours after eating ⬃0.5 kg fruit
(451). The same concentration was reached after drinking
of 100 ml brandy containing 24% ethanol and 0.19%
methanol. We estimated the levels of endogenous methanol
generation by considering the probable removal of metha-
nol via pulmonary and renal excretion (413). Assuming
renal methanol clearance in the average volunteer of 1.0
ml/min (231, 483), we believe that a clearance of 60 ml of
blood in 1 h occurs and has little effect on the total blood
methanol content. For the estimation of the pulmonary ex-
cretion of methanol, we used estimates (200) indicating that
each minute, 5.6 ml of blood is hypothetically completely
cleared of methanol, i.e., the effect of the pulmonary clear-
ance of plasma methanol is also negligible. Thus, following
exogenous uptake or endogenous production, unchanged
methanol either is excreted (direct excretion) in the urine or
exhaled breath or enters a metabolic pathway.
B. First Phase of Methanol Catabolism
The main way methanol is eliminated from the body is via
its oxidation to formaldehyde and then to formic acid,
which can then be either excreted in the urine or further
oxidized to carbon dioxide. The first phase of the oxidative
metabolism of methanol and ethanol is similar in insects
(162, 503, 504), animals, and humans (63, 168, 453). Eth-
anol and methanol are converted into formaldehyde and
acetaldehyde, respectively, via one of at least three separate
pathways (FIGURE 4). The first involves cytochrome P450
monooxygenases (CYP) such as CYP2E1 in humans, which
catabolizes ⬃9% of exogenous alcohol, especially at high
concentrations (57, 87, 495). The second process, which
oxidizes up to 1% of exogenous alcohol, is greatly depen-
dent on catalase (64, 97). The third pathway, which oxi-
dizes up to 90% of alcohol, is catalyzed by cytosolic alcohol
dehydrogenase I (ADH I) via NAD
⫹
-dependent oxidation
(63, 168, 297). The contribution of each of these mecha-
nisms in the catabolism of alcohol varies by human organs.
In the liver, most alcohol catabolism occurs via ADH1b
(122), while in the brain, the first phase of methanol and
ethanol oxidative metabolism is mainly carried out by two
other mechanisms involving CYP2E1 and catalase-H
2
O
2
compound I (89, 143, 272). Although there are species dif-
ferences in methanol and formic acid pharmacokinetics in
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mice, rabbits, and primates (442, 443), methanol is metab-
olized to formaldehyde primarily by ADH in humans. Ro-
dents are reported to rely on the peroxidative activity of
catalase (45, 64, 229).
1. ADH-mediated methanol catabolism
ADH is a zinc-containing enzyme family that consists of
two subunits of ⬃40 kDa (345). The function of ADH in
the human body consists mainly of the oxidation of ethanol
and methanol produced in the body by microorganisms of
the gastrointestinal tract and taken in via fruit, vegetable,
and alcoholic beverage consumption. The enzyme is located
in the cytosolic fraction of cells, primarily in the liver, gas-
trointestinal tract, kidney, nasal mucosa, testes, and uterus,
and is absent in brain cells (143). Approximately 90% of
ingested ethanol and methanol is metabolized via hepatic
ADH1b (TABLE 3), which catalyzes an oxidative pathway
(65) according to the general scheme presented in FIGURE 4.
The control of ADH activity is complex and includes 1)
dissociation of the product NADH, which is the rate-limit-
ing step; 2) product inhibition by NADH and acetaldehyde
or formaldehyde; and 3) substrate inhibition by high con-
centrations of ethanol and methanol. Mammalian ADHs
(TABLE 3) catalyze the reversible oxidation of alcohols to
aldehydes, acting on a wide variety of substrates ranging
from methanol to long-chain alcohols and sterols (120).
Mammalian liver ADH1b is a well-characterized enzyme
that uses ethanol as a substrate. Relatively little is known
about the interaction between ADH1b and methanol, but
there is extensive structural homology between methanol
and ethanol. ADHs were isolated from the human liver by
Vallee et al. (489) 50 years ago; this resulted in partially
purified preparations. Subsequent efforts improved upon
the purity, which allowed the study of human liver ADH
enzymology (286). Later, its primary and tertiary struc-
tures, catalytic mechanisms, and enzymatic properties were
well studied (301, 376, 381). In vitro, ADH1b catalyzes the
transfer of a hydride ion from an alcohol substrate to the
NAD
⫹
cofactor, yielding the corresponding aldehyde and
the reduced cofactor NADH. ADH1b is also an excellent
catalyst of the reverse reaction. In enzymatic tests carried
out in vitro, methanol is oxidized to formaldehyde by
NAD
⫹
in the presence of ADH; then, formaldehyde can be
oxidized to formate by NAD
⫹
or reduced to methanol by
NADH. These reactions follow a rapid equilibrium random
mechanism. Among these three reactions, the reduction of
formaldehyde is the most rapid. The rate of formaldehyde
oxidation is faster than that of methanol oxidation (380).
Each of the two liver ADH1b subunits contains one cata-
lytic and one structural zinc ion (83). The catalytic zinc ion
is situated at the bottom of the cleft between the coenzyme
binding domain and the catalytic domain. It is accessible
through two channels, one that accommodates the coen-
zyme and another through which the substrate approaches
the catalytic site (382). When the ternary complex forms,
the catalytic domain rotates ⬃10° to allow the residues
in the catalytic domain to move closer to the coenzyme do-
main, and the cleft between domains closes (83). There are
significant differences in the enzymatic behavior for meth-
anol and ethanol. In vitro, methanol is oxidized by live
ADH1b at a much slower rate. The rate of methanol oxi-
dation catalyzed by purified ADH1b is only ⬃3% of the
rate of ethanol oxidation. Within the alcohol binding site of
liver ADH1b, there are two binding regions: a hydroxyl
binding region and a hydrophobic binding region (99, 100).
Methanol binds to a similar site as ethanol in ADH1b.
Pyrazole, an efficient liver ADH1b inhibitor, binds to the
enzyme-NAD
⫹
complex and blocks the substrate binding
site (378). The inhibition patterns and inhibition constants
of pyrazole are similar for both methanol and ethanol
(380). This confirms that the binding of methanol to the
catalytic zinc is similar to that of ethanol. On the other
A
Ethanol Acetaldehyde Acetate
NAD+NADH
ADH
O2 + NADPH NADP+ + H2O2
H2O2H2O
CYP2E1
Catalase
B
NAD+NADH
ADH
O2 + NADPH NADP+ + H2O2
H2O2H2O
CYP2E1
Catalase
FormaldehydeMethanol Formate
FIGURE 4. The first phase of ethanol (A) and methanol (B) catab-
olism to acetaldehyde and formaldehyde, respectively, by three en-
zymatic systems.
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hand, the second step, hydride transfer, which is the con-
version of the ternary complex, is the rate-limiting step for
methanol oxidation. Because of the lack of an appreciable
hydrophobic chain, methanol exhibits different kinetics
than ethanol (50, 122, 380). The hydrophobic interaction
in the active site plays a fundamental role in substrate bind-
ing. When a substrate binds to the enzyme, the interaction
between the hydrophobic binding region and the hydropho-
bic chain on the substrate stabilizes substrate binding.
Methanol, with a chain of only one carbon atom, cannot
bind to the hydrophobic region as ethanol can. Because the
methyl group is not held in the proper orientation for hy-
dride transfer, methanol exhibits both a weak binding con-
stant and a slow turnover rate. The binding of NAD
⫹
to the
enzyme does not exhibit a significant effect on the binding
of methanol, nor does methanol affect the binding of
NAD
⫹
(380). Pyrazole and its derivatives such as 4-meth-
ylpyrazole are potent inhibitors of ADH1b and block the
interactions of all class I ADH isozymes with ethanol (523)
and methanol (378). These compounds function as ligands
of zinc, which is essential for the catalytic activity of the
native forms of ADH. They form slowly dissociating ter-
nary complexes with ADH and NAD
⫹
and act as compet-
itive inhibitors with respect to ethanol and methanol.
4-Methylpyrazole, the most potent inhibitor, binds to the
enzyme-NAD
⫹
complex as an inner sphere ligand of the
catalytic zinc (416), thus blocking the substrate binding
site and the formation of a dead-end ternary complex with the
enzyme and NAD
⫹
(455). The strong sensitivity of class I
ADHs to pyrazole inhibition explains the powerful inhibi-
tion of ethanol metabolism in humans by these agents. The
K
i
(4-methylpyrazole) value of class I ADHs is ⬃0.1
␮
M
(63) with ethanol as a substrate, and the K
i
value for meth-
anol is similar, equal to 0.09
␮
M (378). The
␲␲
ADH
isoform (TABLE 3), named for its pyrazole insensitivity, is
inhibited much less effectively by 4-methylpyrazole (K
i
⫽2
mM), although the pyrazole derivatives 4-bromo-, 4-nitro-,
or 4-pentylpyrazole are more efficient with ethanol (K
i
val-
ues between 4 and 27
␮
M) (40).
In addition to ethanol and methanol, ADH isoforms also
oxidize several metabolic alcohols with high catalytic effi-
ciency, including retinol,
␻
=-hydroxy fatty acids, hydroxy-
steroids, and hydroxyl derivatives of dopamine and epineph-
rine metabolites (37, 382). Vitamin A and its derivatives
(retinoids) are essential components in vision. Animals are,
in general, unable to synthesize vitamin A de novo. Retinol,
retinal, and retinoic acid are C20 isoprenoids that are met-
abolically derived from the oxidative cleavage of C40 caro-
tenoids. In the presence of NAD
⫹
, the enzyme oxidizes the
alcohol retinol to the aldehyde retinal, which is necessary
for vision. ADH utilizes a mechanism for retinol-retinal
interconversion that is similar to that used for ethanol/
methanol oxidation and acetaldehyde/formaldehyde reduc-
tion (37, 382). In contrast to the methanol-formaldehyde,
ethanol-acetaldehyde, and other short-chain alcohol-alde-
hyde systems, the equilibrium constant for the retinol-reti-
nal interconversion is ⬃200 times higher. The K
m
value (60
␮
M) indicates that the interactions between the long carbon
chain of the retinoid and the hydrophobic pocket of the
enzyme provide a major driving force for the binding pro-
cess. The oxidation of these alcohols can be inhibited by
ethanol and methanol, and therefore, the role of primary
alcohol substrate competition is an important issue in the
effects of alcohol on humans.
2. Human ADH genes
The human genome contains seven genes encoding ADH
(122) (TABLE 3) (FIGURE 5). They form a cluster of genes
located in chromosome 4 (4q21–23), spanning ⬃370 kb
(359, 360) and encoding multiple forms of ADH with dif-
Table 3. ADH genes and proteins
Gene Name Class
Amino Acid Differences
Between Alleles Protein Name K
m
(Ethanol), mM K
m
(Methanol), mM
ADH1A I〈4.0
a
to 4.2
d
150.0
b,c
ADHB*1 Arg48, Arg370
␤
1
0.05
a,d
to 1.2
b
6.0
d
to 7.0
e
ADH1B*2 His48, Arg370
␤
2
0.9
d
ND
ADH1B*3 Arg48, Cys370
␤
3
34.0
d
ND
ADH1C*1 Arg272, Ile350
␥
1
1.0
d
ND
ADH1C*2 Gln272, Val350
␥
2
0.6
d
to 1.0
b
30.0
b
ADH1C*352Thr Thr352 ND ND ND
ADH4 II ⌸34
d
No activity
b
ADH5 III
␹
or FDH ⬎1,000.0
f
No activity
b
ADH6 V ADH6
g
ND ND
ADH7 IV ⌺30.0
f
ND
K
m
indicates the concentration of alcohol (ethanol or methanol) at which the enzyme works at 50% capacity.
ND, not determined; FDH, formaldehyde dehydrogenase.
a
Bosron et al., 1983 (41);
b
Wagner et al., 1983
(492);
c
For
␣␥
1 heterodimer;
d
Crabb et al., 1987 (92);
e
Pietruszko, 1975 (378);
f
Edenberg, 2007 (122);
g
Ostberg et al., 2013 (361).
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ferent substrate specificities (63, 92). The human liver con-
tains seven ADH isoforms, which are divided into five
classes (TABLE 3).
Class I ADHs are encoded by three genes, ADH1,ADH2,
and ADH3, which encode these subunits:
␣
(ADHIA),
␤
1,
␤
2,
␤
3 (ADHIB), and
␥
1 and
␥
2 (ADHC). The ADH1 sub-
units share ⬃94% sequence identity. These different units
and polymorphs can form homodimers or various het-
erodimers (e.g.,
␣␣
,
␤
1
␤
1,
␣␤
2,
␤
1
␥
2) (41, 393, 492). The
class I ADH isoforms play the most important role in alco-
hol oxidation. The study of ADH1-null mice showed that
ADH1 enzymes together are responsible for ⬃70% of the
total ethanol-oxidizing capacity of liver, indicating that the
non-ADH1 pathway accounts for the remaining 30%
(169). The relative contributions of each of the ADH
isozymes to ethanol oxidation change with the hepatic eth-
anol concentration. The K
m
of human class I ADH for eth-
anol is very low (TABLE 3). Therefore, the enzyme becomes
saturated after the intake of just 28–30 ml of ethanol, and
ethanol is removed from the human body at a constant rate,
independent of the concentration (347). Hence, the rate of
disappearance of ethanol from the blood is virtually inde-
pendent of the prevailing blood-ethanol concentration
(zero-order kinetics) (92). All isozymes of class I ADH show
substrate inhibition ([ethanol] ⬎20 mM) because an excess
of ethanol decreases the speed of NADH dissociation from
the enzyme (223).
The ADH
␣
-subunits, encoded by the gene ADH1A, are
monomorphic, whereas the other two subunits were iden-
tified as isoforms that differ in ADH catalytic activity levels.
The ADH1B gene was shown to comprise two functionally
important polymorphisms (TABLE 3):1) one encoding
ADH1B*2, which occurs in human populations of North
Africa, Eurasia, and Oceania, and 2) one encoding
ADH1B*3, which is found only in African populations.
Experiments on isolated preparations of human liver ADH
showed that the arginine amino acid at position 48 partic-
ipates in the binding of the pyrophosphate group NAD
⫹
,
but the replacement of arginine for histidine leads to a lower
optimal pH and a 100-fold increase in the
␤
2-ADH turn-
over (215, 308). The study of the role of this change on the
metabolism of ethanol in the human body did not provide
such an unambiguous picture. The first-pass study showed
that the hepatic ethanol clearance of ADH1B*2 individuals
was higher than that of the ADH1B*1 individuals (267).
Evidence of the high enzymatic activity of
␤
2-ADH was
also obtained in the study of another ADH1B*2/*2
group (224); the blood acetaldehyde concentrations of
ADH1B*2/*2 individuals were higher than those of
ADH1B*1/*2 individuals only in the ALDH2*1/*2 group.
However, the blood ethanol concentrations of the
ADH1B*2/*2 group were higher than those of the
ADH1B*1/*2 group regardless of the ALDH2 genotype.
These findings are unexpected and are difficult to explain
from the viewpoint of ADH enzyme activity alone.
The Arg370Cys amino acid replacement in the ADH1B*3
allele, which almost never occurs in populations of Euro-
pean and Asiatic origins, also leads to an increased reaction
speed due to a changed efficiency of the interaction between
the
␤
3
␤
3
dimer and NAD
⫹
(55).
ADH1C also contains single nucleotide polymorphisms, of
which ADH1C1 and ADH1C*2 are the most studied. The
enzyme encoded by ADH1C*1 (
␥
1-ADH) contains Arg at
position 272 and isoleucine (Ile) at position 350, whereas
that encoded by ADH1C*2 (
␥
2-ADH) contains glutamine
(Gln) at position 272 and valine (Val) at position 350 (359,
360). The kinetic differences between
␥
1-ADH and
␥
2-
ADH are small (185) (TABLE 3).
Classes II, III, and IV enzymes are homodimeric forms of
the
␲
,
␹
, and
␴
subunits, respectively. The ADH4 gene
encodes class II ADHs, including the
␲␲
ADH isoform,
named for its pyrazole insensitivity. The
␲␲
ADH isoform is
more labile than the other ADH isoforms and was first
detected in liver biopsies and fresh autopsy samples. Due to
its rather high K
m
for ethanol (TABLE 3), this enzyme may be
more important in the metabolism of high concentrations of
ethanol (92).
␲␲
ADH accounts for nearly 30% of the total
ethanol-oxidizing capacity of the liver (192) and exhibits a
more limited substrate specificity than do the other molec-
ular forms of the human liver. Methanol, glycerol, and eth-
ylene glycol, even at concentrations up to 100 mM, cannot
be oxidized by
␲␲
ADH (40).
The ADH5 gene encodes class III ADHs, including the
␹␹
ADH isoform, which is widely distributed in the tissues
ADH1C ADH1B ADH1A
ADH7 1C 1B 1A ADH6 ADH4 ADH5
Class 1 ADH
1B*1: R48, R370
1B*2: H48, R370
1B*3: R48, C370
1C*1: R272, I350
1C*2: Q272, V350
1C*352Thr: T352
FIGURE 5. Relative map of the SNP sites
and ADH alleles. Top panel represents the
whole ADH gene cluster, and bottom panel
shows the genes of the class 1 ADH cluster
with the different alleles and amino acid sub-
stitutions indicated. [Modified from Osier et
al. (360) and Edenberg (122), both with per-
mission from Elsevier.]
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(63).
␹␹
ADH is virtually not inhibited at all by 4-meth-
ylpyrazole (492), has a very high K
m
for ethanol, and
likely plays almost no role in ethanol and methanol elim-
ination (63).
3. Cytochrome P450s-mediated oxidation of
methanol and ethanol
The CYP enzymes are named CYP for cytochrome P450,
followed by an Arabic number denoting the family, a letter
designating the subfamily and, finally, an Arabic numeral
representing the individual gene in the subfamily (236, 341,
530). The P450s are arranged in families based on sequence
homologies (340, 342) and are very versatile catalysts that
activate dioxygen and insert a single oxygen atom into al-
most any organic compound imaginable (255). Cyto-
chrome P450s are a family of heme-containing monooxy-
genases that are involved in the oxidation of ethanol, meth-
anol, steroids, fatty acids, and numerous xenobiotics
(acetone, benzene, and other alcohols) ingested from the
environment (62, 249, 252, 295, 530). The heme iron of the
CYP enzymes binds two oxygen atoms. However, in con-
trast to the iron found in hemoglobin, this iron atom is
bound rather tightly to the anionic thiolate sulfur of cys-
teine. This binding mode gives heme the properties neces-
sary for splitting the dioxygen molecule into two atoms.
Because one oxygen atom forms a water molecule, the CYP
enzymes were also named mixed function oxidases. The
second oxygen atom is activated for introduction into the
substrate molecule; therefore, CYP enzymes have been in-
cluded in the monooxygenase enzyme class. Hence, the
splitting of the dioxygen molecule to two atoms results in
the formation of a hydroxylated product (6). Most P450s
are considered to operate according to a general scheme
(FIGURE 6) (26, 27, 86, 508). There are many P450 isoforms
encoded by more than 100 gene families (249, 252, 340,
342, 530, 538). P450 functions in conjunction with micro-
somal enzymes such as NADPH-cytochrome P450 reduc-
tase and cytochrome b
5
(342) (FIGURE 6).
CYP2E1 is the P450 enzyme with the highest activity for
oxidizing ethanol and methanol to acetaldehyde and form-
aldehyde, respectively. CYP2E1 accounts for ⬃6% of the
total P450 content in the human liver and catalyzes the
metabolism of 2% of commercially available drugs (150,
538). The human gene encoding CYP2E1 is the only gene of
the CYP2E subfamily located at chromosome 10q26.3; it
contains 9 exons and comprises several polymorphisms
(236). Several different CYP2E1 polymorphisms have been
identified (266). Human CYP2E1 expression is undetect-
able in the fetal liver but is the highest expressed cyto-
chrome P450 in the adult liver, where it is present mainly in
the endoplasmic reticulum (microsomal fraction) (271,
272, 274, 275) but are also found in mitochondria (15, 18,
19, 242, 273). Aside from the liver, CYP2E1 is also ex-
pressed in the brain (137, 138, 164, 178, 216, 246, 349,
482) and lungs (227). CYP2E1 expression has been found
also in the nasal mucosa, kidney cortex, testes, ovaries, and
gastrointestinal tract at lower levels (273, 454) and in car-
diac tissue at somewhat higher levels (317, 532, 533).
CYP2E1 is regulated by several mechanisms, including
transcriptional and posttranscriptional processes (350,
530). CYP2E1 induction appears to occur via two steps: a
posttranslational mechanism at low ethanol concentrations
and an additional transcriptional mechanism at high etha-
nol concentrations (17, 397).
Fe3+ ROH
FeOH3+ R•
FeO3+ RH
Fe-OOH RH Fe2+-O2 RH
O2
O2•–
Fe2+ RH
H+
NADPH-P450 reductaseox
NADPH-P450 reductaseox
NADPH-P450 reductasered
NADPH-P450 reductasered
Fe3+
Fe2+-O2– RH
b5ox
b5red
H2O
–H2O
–ROH RH
H2O2
2e–
1e–
1e–
Fe3+ RH
1
2
3
4
5
6
7
8
FIGURE 6. The general scheme for P450-
catalyzed oxidation reactions (26, 27). RH,
substrate; ROH, product. The reversibility of
some of the latter steps is unknown. The
outlets for the uncoupled reduced oxygen
products O
2
,H
2
O
2
, and H
2
O are shown. Fol-
lowing substrate binding (step 1), ferric
P450 receives 1 electron via NADPH-P450
reductase (step 2). The ferrous form of hem
binds O
2
(step 3) before undergoing a sec-
ond 1-electron reduction to begin O
2
activa-
tion (step 4). Although this second electron
originates from NADPH-P450 reductase,
the accessory protein cytochrome b
5
takes
part in the delivery of the electron to P450.
Insertion of the activated oxygen into the sub-
strate occurs via C-H bond cleavage (step 6)
followed by rapid oxygen rebinding to form
the product (step 7). Step 8 is the release of
the product from the active site of the en-
zyme.
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Ethanol and methanol are both inducers and substrates of
CYP2E1 (61, 63, 226, 357, 398). Acetaminophen, caffeine,
chlorzoxazone, and an array of carcinogens such as ben-
zene, styrene, acrylonitrile, and nitrosamines are also me-
tabolized by CYP2E1. The K
m
of CYP2E1 for ethanol is 10
mM, which is 10-fold higher than the K
m
of ADH for eth-
anol. Therefore, liver ADH has a much higher capacity for
ethanol oxidation than the CYP2E1 system at low ethanol
concentrations, but CYP2E1 may be the basis of the meta-
bolic adaptation to high concentrations of ethanol that de-
velops upon chronic ethanol consumption (272). CYP2E1
is likely to provide ⬃10% of the total ethanol-oxidizing
capacity of the liver during modest ethanol intake (62).
Alcohol oxidation increases at higher ethanol concentra-
tions, and much of this increase is due to the metabolism of
ethanol by CYP2E1. CYP2E1 levels are increased by
chronic ethanol administration by a mechanism that largely
involves protection of the enzyme against proteolysis.
CYP2E1 expression is also induced in diabetics, in the
fasted nutritional state, and in nonalcoholic steatohepatitis
(NASH). NASH is a progressive liver disorder that occurs in
patients with high hepatic CYP2E1 levels but without sig-
nificant ethanol consumption (506).
CYP2E1 is highly conserved within the human population
(271), suggesting significant physiological functions, in-
cluding nutritional support via the catalysis of fatty acid
␻
=-1 and
␻
=-2 hydroxylation reactions (2, 4, 259). Thus
CYP2E1 plays dual physiological roles, one in the conver-
sion of ketones into glucose and one in detoxification (271).
An important feature of ethanol and methanol oxidation
via CYP2E1 is the generation of reactive oxygen species
(ROS), which contribute to the damage of liver cells (57,
272, 516). CYP2E1 is an effective generator of ROS such as
the superoxide anion radical and hydrogen peroxide as a
result of the uncoupling of oxygen consumption and
NADPH oxidation. The significant levels of CYP2E1 within
the mitochondria could further contribute to ROS deleteri-
ous effects (18). Moreover, metabolism by CYP2E1 results
in a significant release of free radicals that, in turn, diminish
reduced glutathione levels (18) and other oxidative stress
defense systems that play major pathogenic roles in alco-
holic (60, 242, 272) and nonalcoholic liver disease (14, 66).
CYP2E1 is a causative player in alcoholic liver disease, as
well as in NASH, likely through the enhancement of hepatic
lipid peroxidation (14, 60, 242).
4. Catalase-H
2
O
2
system
The ability of peroxisomal catalase to oxidize short-chain
alcohols such as methanol and ethanol was discovered long
ago (64, 81, 234, 274). With the use of hydrogen peroxide
(H
2
O
2
) as a cosubstrate, the heme-containing enzyme cat-
alase forms the catalase-H
2
O
2
system (also known as com-
pound I), which is also the major pathway of formaldehyde
oxidation. The role of this system in the oxidation of alco-
hols is small in the liver (63) but significant in the brain
(499, 536, 537), which lacks ADH activity (143). Ethanol
in the brain is oxidized into acetaldehyde by the action of
the catalase-H
2
O
2
system (7, 8, 81, 145, 203, 228) and
cytochrome P4502E1 (CYP2E1) (537). It is estimated that
the catalase-H
2
O
2
system in the brain is responsible for
60–70% of the acetaldehyde generated in this organ, while
CYP2E1 accounts for 10–20% (537). Inhibitors of cata-
lase, aminotriazole, and sodium azide depress the oxidation
of ethanol and methanol to acetaldehyde and formalde-
hyde, respectively, in the brain (64, 537). Acetaldehyde de-
rived from the catalase-dependent oxidation of ethanol in
the brain has been suggested to play a role in the mediation
of many effects of ethanol in the brain, including behav-
ioral, neurochemical, and neurotoxic actions, and plays a
crucial role in the development of alcoholism (89, 334).
Participation of the catalase-H
2
O
2
system in the oxidation
of ethanol and methanol in the brain was confirmed by
experiments with alpha lipoic acid (ALA, 1,2-dithiolane-3-
pentanoic acid). ALA is considered to be an H
2
O
2
scaveng-
ing agent and thus an inhibitor of the catalase-H
2
O
2
com-
plex; treatment with ALA prevents the formation of ac-
etaldehyde in the brain and, therefore, prevents its
neurochemical and neurotoxic actions (264). Indeed, on the
one hand, ALA reduced ethanol self-administration in rats
(368) and mice (264), and on the other hand, ALA treat-
ment prevented methanol exposure-induced oxidative dam-
age of tissues of the rat nervous system (388). However,
these results can also be explained by another property of
ALA, namely, the ability of ALA to activate ALDH2 (116,
171a, 285, 310). The ALA-mediated decreases of formal-
dehyde and acetaldehyde in animal tissues can be induced
by both interfering with their formation and accelerating
their oxidation by ALDH2. It could be suggested that the
beneficial effects of ALA on ethanol metabolism are related
to the capacity of ALA to control both reactions.
C. Second Phase of Methanol Catabolism
Formaldehyde is a naturally occurring biological com-
pound that is present in all tissues, cells, and biological
fluids (174). The concentration of endogenous formalde-
hyde in the blood of rats (176), monkeys (58), and humans
(176, 413, 463) does not exceed 0.1 mM, and that in the rat
and human brain is 0.2–0.4 mM (464, 466). Most formal-
dehyde is produced as the oxidation product of methanol.
However, another endogenous source of formaldehyde is
the oxidative deamination of methylamine derived primar-
ily from creatine (526) by semicarbazide-sensitive amine
oxidases (SSAOs) (201, 436, 527), which generate formal-
dehyde together with ammonia and hydrogen peroxide,
as follows (233, 363, 525, 527): CH
3
NH
2
⫹O
2
⫹
H2O
→
SSAO
HCHO ⫹H
2
O
2
⫹NH
3
.
In mammals, among the SSAOs, vascular adhesion protein
1 is one of the most extensively studied members of this
group of enzymes (119, 202, 524). It is interesting that some
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monoamine oxidase inhibitors such as phenelzine with an-
tidepressant properties (20) are able to protect neurons and
astrocytes against formaldehyde (424).
Formaldehyde is also part of the one-carbon pool, which is
utilized for the biosynthesis purines, thymidine, and several
amino acids, which are incorporated into DNA, RNA, and
proteins during macromolecular synthesis (220). Besides
the oxidation of methanol, formaldehyde is formed by the
conversion of serine and glycine in the presence of tetrahy-
drofolate (35). Formaldehyde is also generated during chro-
matin structure remodeling in the reactions of removing
methyl groups from lysine residues in histones that are cat-
alyzed by lysine-specific demethylase 1 and JmjC domain-
containing histone demethylases (79, 189, 287, 470, 497).
Aside from endogenous sources, formaldehyde can also be
produced and released from different exogenous products
that naturally contain formaldehyde, including food prod-
ucts such as coffee, codfish, meat, poultry, and maple syrup
(95b, 98, 298, 391). Drugs also may be sources of formal-
dehyde. For example, Selegiline (Anipryl, L-deprenyl, El-
depryl, Emsam, Zelapar), which is used to treat Parkinson’s
disease, gives off formaldehyde as a byproduct as a result of
the metabolic transformation circuit (219).
Efficient systems of formaldehyde oxidation exist in mam-
mals. Despite the multiple endogenous and exogenous
sources of formaldehyde, a low physiological level of form-
aldehyde in bodily fluids and tissues is maintained by the
continuous action of cellular formaldehyde-metabolizing
enzymes (FIGURE 7).
The oxidation of formaldehyde to formate occurs by at least
three separate pathways with the participation of P450
monooxygenases, mitochondrial aldehyde dehydrogenase
2 (ALDH2), and the gene encoding ADH5,
␹␹
ADH,
(TABLE 3), also called ADH3 or formaldehyde dehydroge-
nase (FDH) (473) (FIGURE 7). Liver ADH1b plays the main
role in the first phase of methanol oxidation but has a neg-
ligible effect on the reduction of formaldehyde to methanol
because of its higher K
m
value for formaldehyde (⬃30 mM)
(419).
Similarly to other aldehydes, formaldehyde can be oxidized
by P450s (27, 183, 255, 289, 452), as shown in FIGURE 6.
Interestingly, transgenic Petunia hybrida plants harboring
the CYP2E1 gene have improved resistance to formalde-
hyde (533).
In humans, two members of the divergent ALDH superfam-
ily, a cytosolic (ALDH1A1) and a mitochondrial (ALDH2)
enzyme (204, 420, 427, 435, 484, 502), can directly metab-
olize formaldehyde (142, 297, 452, 453, 473, 485). Mito-
chondrial ALDH2, which is especially important for the
oxidation of high concentrations of formaldehyde, has a
high K
m
for formaldehyde (0.2–0.5 mM) compared with
that of FDH (see below) (⬍0.01 mM) (59, 175, 336, 432).
The intracellular concentration of free formaldehyde is
likely too low to result in significant oxidation by ALDH2
(175).
Human FDH or ADH5 gene encoding (FIGURE 6)
␹␹
ADH
belongs to the family of medium-chain zinc-containing
ALDH1A1
ALDH2
Glutathione
Glutathione
S-hydroxymethyl glutathione
S-formylglutathione
FDH
NAD+
NADH
NAD+
NADH
NAD+
NADH
S-formylglutathione
hydrolase
MethanolEndogenous
sources
Exogenous
sources
Formaldehyde
Formate
CYP450s
CO2 + H2O
FIGURE 7. Formaldehyde metabolism. Overview
of the pathways of the conversion of formaldehyde
into water and CO
2
. For details, see text.
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ADHs (TABLE 3). In the literature, different ADH nomen-
clatures have been used in early reports, in which ADH3
was referred to as glutathione-dependent FDH,
␹␹
ADH or
class III ADH (218, 432). We will use glutathione-depen-
dent FDH in our review because this name more precisely
reflects its formaldehyde oxidizing function.
In contrast to other mammalian ADH isoenzymes that are
active toward ethanol and methanol, FDH shows very poor
activity toward ethanol and methanol and prefers long-
chain alcohol substrates such as
␻
=-hydroxy fatty acids,
including 12-hydroxydodecanoic acid and retinol (vitamin
A) (243, 268, 320, 332, 432). Compared with the other
ADH classes, FDH has two additional activities: a glutathi-
one-dependent formaldehyde oxidizing activity and an S-
nitrosoglutathione (GSNO) reductase activity (186).
Human FDH is the most efficient formaldehyde-metaboliz-
ing enzyme among all of the enzymes elucidated so far (432)
and was first purified from human liver and partially char-
acterized by Uotila and Koivusalo (479, 480). FDH is the
most conserved class of all of the ADHs (186). Like other
human ADH isoenzymes, FDH exists as a dimer and con-
tains two covalently bound zinc ions per subunit (520).
FDH is considered to be the main scavenger of exogenous
formaldehyde and is ubiquitously expressed with relatively
little inter-tissue variation in mammals, in contrast to other
ADHs. FDH is primarily a cytosolic enzyme, although was
revealed in nucleus where it likely protects DNA from form-
aldehyde-mediated damage (139, 195).
FDH is a glutathione-dependent
␹
-ADH and oxidizes form-
aldehyde to formate in a two-step process (FIGURE 7). The
polarized carbonyl group formaldehyde is a compound
with high reactivity with thiols, spontaneously forming S-
hydroxymethyl glutathione after the interaction of formal-
dehyde with glutathione (256). Thus, in the first step of
FDH-mediated formaldehyde oxidation, S-hydroxymethyl
glutathione is formed and is subsequently used as an FDH
substrate to generate S-formylglutathione (168, 297, 431,
457). The conjugate S-formylglutathione is then hydrolyzed
byaS-formylglutathione transferase to generate formate
and glutathione (167, 168, 297, 430, 431, 452, 457, 473).
The formate generated by formaldehyde oxidation can
undergo further oxidization to carbon dioxide (FIGURE 7)
in metabolic pathways involving catalase (85, 419), 10-
formyltetrahydrofolate dehydrogenase, also known as
ALDH1L1, or ALDH1L2, its mitochondrial isoform
(419, 473).
Formaldehyde interacts with GSNO and functions as a
GSNO reductase in NO homeostasis (431, 432, 457).
GSNO depletion is associated with various diseases, includ-
ing asthma. Importantly, FDH-mediated S-(hydroxymeth-
yl)glutathione oxidation is accelerated in the presence of
GSNO, which is concurrently reduced by immediate cofac-
tor recycling (430, 432, 457).
D. Features of Methanol Metabolism in the
Brain and in Embryos
The metabolism of methanol and formaldehyde in brain
cells is important for the function of this vital organ (472).
Moreover, the elevation of brain formaldehyde levels is
likely to result in neurodegenerative diseases (386, 448,
450, 463, 464, 517, 518). Methanol and the products of its
metabolism in brain cells may be synthesized in situ or be
introduced from peripheral organs via the bloodstream by
overcoming the blood-brain barrier.
1. Production of methanol and formaldehyde by brain
cells
Cultured brain cells, including astrocytes and neurons, con-
tain mRNAs encoding SSAO and lysine-specific demethyl-
ase 1, as well as for the enzymes involved in formaldehyde
metabolism (472, 473). Synthesis of formaldehyde by brain
cells can occur via protein carboxymethylation (69, 265), as
well from the participation of SSAOs and in the reactions of
removing methyl groups from lysine residues in histones,
which are catalyzed by lysine-specific demethylase 1 and
JmjC domain-containing histone demethylases (79, 189,
287, 470, 497).
SAM might be transformed to methanol and S-adenosyl
homocysteine in the bovine pituitary gland and other
animal brain tissue (16, 422). SAM is a universal endog-
enous methyl donor and is a limiting factor in various
methylation reactions, including protein carboxymethy-
lation (135). Protein carboxymethylase is highly ex-
pressed in the brain and pituitary gland of several mam-
malian species (103, 105). The effects of SAM-induced
protein carboxymethylation on the formation of metha-
nol, formaldehyde, and formic acid in rat brain striatal
tissues were investigated; this study directly showed that
excessive SAM-dependent methylation increased levels
of methanol, formaldehyde, and formic acid in rat brain
striatal homogenates (265).
Formaldehyde can also be generated in the brain by the
reactions catalyzed by lysine-specific demethylase 1 and
JmjC domain-containing histone demethylases (79, 189,
288, 470, 472, 497).
In other metabolic pathways, SSAOs located in the outer
membranes of vascular smooth muscles and the endothe-
lium of the brain catalyze the deamination of methylamine
and generate formaldehyde together with ammonia and hy-
drogen peroxide (73, 233, 363, 527). It has been shown that
the incubation of methylamine in the presence of SSAO-rich
tissues, e.g., human brain meninges, results in cross-linked
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proteins. This cross-linkage can be completely blocked by a
selective SSAO inhibitor (155). Recently (386), the partici-
pation of brain SSAOs in formaldehyde generation was
shown in a well-established mouse model of Alzheimer’s
disease, the senescence accelerated mouse-prone 8 (SAMP8)
strain (466). Formaldehyde levels were elevated in the
brains of 3-mo-old SAMP8 mice (94.43 ⫾1.32
␮
mol/g)
compared with those of age-matched senescence accelerated
resistant mouse 1 (SAMR1) mice (85.21 ⫾1.39
␮
mol/g).
In contrast to serine hydroxymethyl transferase and
CYP450 2E1, SSAOs play a major role in the generation of
brain formaldehyde in SAMP8 mice. Levels of formalde-
hyde-producing SSAOs are higher in the brains of SAMP8
mice compared with SAMR1 mice, whereas the FDH
mRNA content was reduced in SAMP8 mice. Both the re-
duced ADH3 activity and SSAO mRNA/protein levels cor-
relate well with the observed formaldehyde accumulation in
the brains of SAMP8 mice; these data allowed the authors
to suggest a causal relationship between the two phenom-
ena (386).
2. Metabolism of methanol/formaldehyde introduced
into the brain through blood-brain barrier
One pertinent question is whether methanol and formalde-
hyde synthesized in the gastrointestinal tract and liver can
enter the blood and ultimately reach the brain by crossing
the blood-brain barrier (BBB) (472). The BBB ensures the
optimal control of homeostasis of the brain’s internal envi-
ronment. The anatomical structure of the BBB comprises
endothelial cells of arterioles, capillaries, veins, and epithe-
lial cell surfaces, with the presence of tight junctions be-
tween the endothelial cells of brain capillaries and oligoden-
drocytes. The endothelial cells enable the very selective
transport of substances from the blood to the brain, and
vice versa (326, 481). Methanol and ethanol from the blood
apparently reach the brain cells without hindrance (112,
159, 197). At the same time, peripherally produced acetal-
dehyde and formaldehyde penetrate the BBB to enter the
brain with difficulty. Although the physical-chemical prop-
erties of formaldehyde and acetaldehyde suggest that these
molecules would easily penetrate the BBB, the presence of
the “metabolic” barrier of ALDH and FDH hinders their
crossing of the BBB (89, 334, 410, 535).
3. The modes of brain methanol and formaldehyde
catabolism
ADH is responsible for the metabolism of ethanol and
methanol in the liver but has not been definitively demon-
strated to play a significant role in the brain. The dominat-
ing view is that ADH1 does not participate in methanol
metabolism in the brain (143, 217, 472). This conclusion is
strongly supported by the direct determination of the local-
ization of different classes of ADHs in the brain by North-
ern blot and enzyme activity analyses (143). This study did
not reveal any ADH1 activity in the brain, whereas class III
␹␹
ADH or FDH had high activity in the brains of adult
humans, mice, and rats. No ADH1 mRNAs were revealed
in the mouse brain following methanol inhalation (246).
Although ADH1 activity was not detected in the whole
brain homogenate, it was found to be expressed specifically
in granular and Purkinje cells of the cerebellum, indicating
that ADHs are likely to play a role in particular regions of
the brain (143). On the other hand, brain cell malignization
results in increased total ADH activity and, specifically,
increased class I ADH activity. The other tested classes of
ADH and ALDH enzymes did not show statistically signif-
icant differences in activity in cancer and normal cells (262).
The literature does not provide conclusive or reliable infor-
mation about phase I metabolism of methanol in the brain.
However, considering that ethanol is converted into acetal-
dehyde by the system that includes the participation of
CYP450s and the catalase-H
2
O
2
system, it is safe to assume
that these systems are also involved in methanol conversion
in the brain (178). Catalase and CYP450s represent the
major enzymes in the brain that catalyze ethanol oxidation.
CYP450s are abundantly expressed within the microsomes
of certain brain cells and are localized to particular brain
regions. There is evidence that catalase serves as the primary
ethanol-metabolizing enzyme in the brain (486) and ac-
counts for 60% of the ethanol oxidation in brain (537). The
studies of ethanol metabolism conducted by perfusing liv-
ing rat brains confirmed the participation of catalase in
acetaldehyde formation. Moreover, the addition of the cat-
alase inhibitor aminotriazole to the perfusing fluid in-
creased the ethanol level in the perfusate, i.e., ethanol elim-
ination was significantly decreased. The presence of
CYP450s in the brain has been established (178), but the
total level of CYP450s in the brain is much lower than that
in the liver, and these enzymes are concentrated in specific
regions and cell types, indicating their potentially consider-
able impact on local metabolism (138). The expression of
CYP2E1 and CYP2B6 within the brain can be substantially
increased in response to ethanol (137, 400). Brain CYP450s
are regulated by transcriptional, posttranscriptional, and
posttranslational mechanisms (138). Experiments on ani-
mals harboring genetic deficiencies in CYP2E1 indicated
that CYP2E1 is responsible for ⬃20% of the ethanol me-
tabolism in the brain (537), but the mechanism responsible
for metabolizing the remaining 20% of ethanol remains
unclear (178, 375, 537).
Although no direct studies of the metabolic conversion of
formaldehyde in the mammalian brain have been con-
ducted, experiments with exogenous formaldehyde primar-
ily indicated its neurotoxicity (472). The extent of damage
depends on the dose of formaldehyde and the duration of
the exposure (13, 425, 426, 450). The conversion of form-
aldehyde into formic acid apparently occurs with the par-
ticipation of ALDHs (472). Fifteen Aldh isozymes and their
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mRNAs were revealed in the mouse brain (3). After meth-
anol inhalation by mice, Aldh 1 and 2mRNA contents were
dramatically increased in the brain (246). The contribution
of ALDHs in the oxidation of formaldehyde in the human
brain is unknown. However, the mitochondrial ALDH2
activity toward formaldehyde in the liver was essentially
less in human subjects with mutant alleles (ALDH2*2)
compared with those with wild-type alleles (502).
The enzymatic degradation of formaldehyde in the brain is
also regulated by FDH/ADH3. In the human brain, FDH is
also likely involved in the oxidation of formaldehyde be-
cause its activity was detected in histological preparations
(328). The most intense FDH immunostaining was ob-
served in hippocampal pyramidal neurons, cerebellar Pur-
kinje cells, cerebral cortical neurons in layers IV and V, and
perivascular and subependymal astrocytes.
Studies conducted in adult rats and mice showed that Adh3
is the only ADH class present in the rodent brain (507). The
important role of this enzyme for mammalian life was
proven by studies of mice with a mutated Adh3 gene. Adh3-
knockout mice have a high sensitivity to formaldehyde and
a significantly reduced LD
50
value (96). Moreover,
Adh3⫺/⫺mice exhibited smaller litter sizes, smaller body
weights at 14 wk of age, and lower postnatal survival rates
compared with wild-type mice. Less than 20% of these mice
survive past postnatal day 6 (320). These features indicate
the inability of Adh3⫺/⫺mice to provide metabolic clear-
ance of endogenous formaldehyde. Reduced expression of
the Adh3 gene was also revealed in SAMP8 mice; along
with increased SSAO activity, high formaldehyde contents
in the brain and cognitive function disruption were ob-
served in these mice (386).
The metabolism of methanol in embryos is important for
mammalian procreation and has a great deal in common
with those in the central nervous system. In general, prena-
tal ethanol exposure has been found to be a risk factor for
fetal mortality, stillbirth, and infant and child mortality.
The elimination of ethanol from the fetus relies on the met-
abolic capacity of the mother (54). The main feature of
methanol metabolism in embryos is the lack of ADH1 ac-
tivity (507). Testing for Adh class mRNAs by in situ hybrid-
ization revealed the complete absence of Adh1 and Adh4
mRNAs from all brain structures at all prenatal stages in-
vestigated during the embryonic development of mice and
rats. Adh3 is the only ADH class investigated that was
found to be present in the rodent brain during embryogen-
esis. These data were in agreement with another in situ
hybridization study (143) that demonstrated only the pres-
ence of Adh3 in the brains of adult mice, rats, and humans.
The ubiquitous expression of Adh3 mRNA found in both
humans and mice from the earliest stages examined suggests
a housekeeping role for this gene. The important role of
Adh3 in the detoxification of formaldehyde also strength-
ens this suggested housekeeping function (432).
Thus the metabolic clearance of methanol and formalde-
hyde in the central nervous system and the embryo occurs
via two strategies. The first strategy aims to prevent the
oxidation of methanol, i.e., preventing in situ formaldehyde
formation. In accordance with this strategy, there is no
ADH1 activity in the brain and in embryos, thereby pre-
venting the creation of endogenous formaldehyde, i.e.,
formaldehyde synthesized in situ; furthermore, any metha-
nol introduced through the BBB is made nontoxic by phys-
iological clearance. The second strategy involves the oxida-
tion of any formaldehyde in the brain and in embryos that is
synthesized in situ or inserted into bloodstream by enzymes,
including FDH and ALDH2.
VI. METHANOL/FORMALDEHYDE AND
HUMAN PATHOLOGY
This section discuss the results of studies pointing to the
possible involvement of metabolic methanol and formalde-
hyde in human pathology. We believe that the mechanisms
of human poisoning by exogenous methanol, as well as the
practical therapies in cases of accidental methanol intake,
cannot be ignored.
A. Formaldehyde Neurotoxicity
Formaldehyde easily penetrates cell membranes, possesses
neurotoxic potential, and causes deficits in memory and
learning (386, 425, 466, 472). High levels of exogenous
formaldehyde without a concurrent elevation in the capac-
ity to clear formaldehyde raise the formaldehyde levels in
the body and lead to formaldehyde stress (171b, 472). Al-
though low concentrations of metabolic formaldehyde can-
not cause acute toxicity in brain cells, experiments on cul-
tured cells have provided some idea of how the metabolism
in the brain is changed by the putative impact of formalde-
hyde. It has been shown that the metabolism of cultured
astrocytes is altered after formaldehyde exposure (471). Al-
though the cells efficiently oxidized formaldehyde to for-
mate, likely by using ALDH2 and FDH, enhanced glyco-
lytic flux was observed due to the inhibition of mitochon-
drial respiration. In cultured cerebellar granule neurons
exposed to formaldehyde, a high rate of formaldehyde ox-
idation accompanied by significant increases in the cellular
and extracellular formate concentrations increased glucose
consumption, and lactate release strongly accelerated the
export of the antioxidant glutathione (473). Hydrogen sul-
fide (H
2
S) is considered the third gaseous neuromediator,
along with nitric oxide and carbon monoxide (327, 500,
501). In the mammalian brain, H
2
S is formed from the
amino acid cysteine by the action of cystathionine
␤
-syn-
thase, the key enzyme in the transsulfuration pathway that
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processes homocysteine, which induces neurotoxicity (449)
and defects in the learning and memory of rats (277). In
Alzheimer’s disease brains, H
2
S was found to be decreased,
but homocysteine was increased (128). Formaldehyde has
also been shown to decrease endogenous H
2
S generation
and to induce neurotoxicity and intracellular ROS accumu-
lation in PC12 cells derived from rat adrenal medulla pheo-
chromocytoma (448). The interaction of formaldehyde
with H
2
S was the trigger of learning and memory dysfunc-
tions in rats (448).
B. Formaldehyde and Neurodegenerative
Diseases
Although the concentration of formaldehyde is low in the
blood, it is high in the brain, suggesting that formaldehyde
plays roles in brain function and dysfunction (441). There is
a direct correlation between elevated formaldehyde levels in
the brain and memory impairment (463, 464, 466); it is
currently unclear whether formaldehyde or a disruption in
gene expression is the cause of the pathogenesis of the men-
tal disease. Notably, brain formaldehyde concentrations
were significantly elevated to pathological levels (⬃0.5
mM) in aged rats. It has been suggested that excess formal-
dehyde is associated with a global decline DNA methylation
(DNA demethylation) in the hippocampus during aging
(464). Intrahippocampal injection with excess formalde-
hyde induces memory deficits in adult rats. Furthermore,
intrahippocampal injection with excess formaldehyde (0.5
mM) not only obviously impaired the ability of spatial
learning in the adult rats but also damaged the memory
abilities of the rats (464, 465). In humans, a correlation was
observed between the degree of senile dementia and the
levels of urine formaldehyde in different patients (466).
Moreover, brain formaldehyde was increased in Alzhei-
mer’s disease patients (463, 466) and animal models (365,
386). The cortex and hippocampal formaldehyde levels of
Alzheimer’s disease patients were significantly higher than
those of age-matched controls or young people, reaching
⬃0.5 mM (463). Interestingly, regularly drinking water can
decrease endogenous formaldehyde. Furthermore, this ac-
tivity can mitigate age-related cognitive impairment (460).
Highly reactive formaldehyde can be both a cause and a
consequence of pathological processes in humans. To un-
derstand the mechanism of its pathological effects on brain
cells, the chemical properties of formaldehyde must be
taken into account. The polarized carbonyl group of form-
aldehyde reacts with the amine group of lysine or arginine,
forming Schiff bases and producing irreversibly covalently
cross-linked complexes between proteins, as well as be-
tween proteins and single-stranded DNA (73, 155, 171b,
307, 432, 534). Formaldehyde, which is found at elevated
levels in Alzheimer’s disease (386, 439, 448, 450, 463, 464,
517, 518), may play important roles in
␤
-amyloid (A
␤
)
aggregation and cerebral amyloid angiopathy related to
Alzheimer’s disease pathology (319, 421, 428, 478, 525).
The potential implications of endogenous aldehydes in A
␤
misfolding, oligomerization, and fibrillogenesis was con-
firmed by in vitro experiments in which formaldehyde was
capable not only of enhancing the rate of formation of A
␤
oligomers and A
␤
-APOE protein complexes but also of in-
creasing the size of the aggregates (72, 73, 394). In line with
this hypothesis, the overexpression of SSAOs, which were
colocalized with A
␤
deposits, was detected in the cerebro-
vascular tissue of patients with Alzheimer’s disease (210,
478).
Another biomarker of Alzheimer’s disease is neuronal tau,
an important protein in the promotion and stabilization of
the microtubule system that is involved in cellular transport
and neuronal morphogenesis (429). Whereas tau normally
contains 2–3 mol of phosphates per mole, tau phosphory-
lation levels in Alzheimer’s disease brains are three- to four-
fold higher. Formaldehyde at low concentrations induces
tau polymerization into globular amyloid-like aggregates in
vitro and in vivo (191, 344). As shown in the study of mouse
neuroblastoma (N2a) cells treated with formaldehyde, tau
hyperphosphorylation and DNA damage occurred (296).
Tau became hyperphosphorylated not only in the cyto-
plasm but also in the nucleus of mouse brains (292).
Chronic methanol feeding led to pathological changes that
were related to Alzheimer’s disease development, including
tau hyperphosphorylation in the brains of mice (517) and
rhesus macaques, non-human primates (518).
Indirect evidence of the involvement of formaldehyde in the
pathogenesis of Alzheimer’s disease was obtained by inves-
tigating ALDH2 activity, a key enzyme that oxidizes alde-
hydes, including formaldehyde. The hypothesis that a de-
crease in ALDH2 activity contributes to Alzheimer’s disease
was supported by studies of transgenic mice with low
ALDH2 activity; these mice exhibited age-dependent neu-
rodegeneration and memory loss (352).
In humans, ALDH2 dysfunction is likely to be one of the
risk factors for Alzheimer’s disease. The ALDH2*2 allele is
a risk factor for late-onset Alzheimer’s disease. Approxi-
mately 0.6–1% of the Japanese population are estimated to
belong to the group with a combination of ALDH2*2 ge-
notypes, and nearly all of these people are expected to de-
velop Alzheimer’s disease (221, 352). A Chinese survey con-
firmed these data (498).
C. Formaldehyde and Hangover
Methanol and formaldehyde are known to participate in
the development of a hangover, known for unpleasant sen-
sations and uncomfortable “morning after” symptoms in
people following excessive ethanol intake (373, 377).
Hangovers after binge drinking are likely more common in
the young than in older aged persons (462). Among hang-
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over causes, which include imbalances in the immune sys-
tem (373), effects of dehydration such as headaches (415)
and sleep disturbances, acetaldehyde accumulation, dys-
regulated cytokine pathways, and hormonal alterations
(487, 510), the metabolism of methanol and production of
formaldehyde are likely the most important (28, 446). The
first support for the contribution of methanol to hangovers
comes from data showing that brandies and whiskeys,
which are more frequently associated with the development
of a hangover, contain the highest methanol concentra-
tions. The first excellent study of methanol metabolism and
hangover found that methanol accumulated in the blood of
alcoholic subjects during a 10- to 15-day period of chronic
ethanol intake (299). The disappearance of blood methanol
lagged behind the linear disappearance of ethanol by ⬃6–8
h, and complete clearance of blood methanol took several
days. Importantly, the accumulation and clearance patterns
of methanol and ethanol were similar in subjects who con-
sumed either whisky (bourbon) with a high methanol con-
tent or grain alcohol with low methanol content. The au-
thors suggested that methanol accumulates in the blood as a
result of the well-known competitive inhibition of ADH by
ethanol (263) and the presence of endogenous methanol
(413), which may contribute to the hangover severity (299).
An experimental study with healthy subjects who con-
sumed red wine containing 100 mg/l of methanol also
showed that elevated blood methanol levels persisted for
several hours after the ethanol was metabolized and that
this corresponded to the time course of hangover symptoms
(212, 213). The half-life of methanol in healthy men during
a hangover was estimated to be 142 min. This indicates that
elevated methanol concentrations in the blood persist for
⬃12 h (213). The author suggested that methanol lingers
after ethanol levels drop because ethanol competitively in-
hibits methanol metabolism. Ethanol readministration
(“hair-of-the-dog” drinking) (377) fends off the hangover
effects that may be based on the ability of ethanol to block
methanol metabolism, thereby slowing the production of
formaldehyde and formic acid (213, 413).
D. Effect of Alcoholic Beverages on the
Cardiovascular System: U-Shape
The consumption of ethanol has an effect on the cardiovas-
cular system in humans and can cause coronary heart dis-
ease (CHD) (333, 355). A plethora of epidemiological evi-
dence has demonstrated a J- or U-shaped association be-
tween alcoholic beverages consumption and all-cause
mortality, as well as cardiovascular morbidity and mortal-
ity. On the other hand, moderate alcoholic beverages con-
sumption was inversely associated with CDH mortality
(144, 147, 241, 305, 335, 337, 366, 402). According to the
Dietary Guidelines for Americans (314), moderate alco-
holic beverages consumption is considered an intake of no
more than 1 drink per day for women and no more than 2
drinks per day for men, where 1 drink is equal to ⬃12 g
ethanol. Moderate ethanol consumption also has a benefi-
cial effect in reducing the risk of vascular disorders of the
brain. Heavy alcoholic beverage consumption increases the
relative risk of any type of stroke, whereas light or moderate
ethanol consumption may be protective against ischemic
stroke (241, 366). The mechanism of this phenomenon is
not completely understood. Numerous hypotheses have
been proposed to explain the benefit of light-to-moderate
ethanol intake on the heart and brain, including an increase
of high-density lipoprotein cholesterol (47) and the promo-
tion of antioxidant effects with the participation of a pro-
tein kinase B/nuclear factor (erythroid-derived 2)-like 2-de-
pendent mechanism (77, 241, 494). A recent study also
suggested the involvement of the ADH1b gene in the U-
shaped curve. A survey of over 260,000 individuals showed
that carriers of the rs1229984 A-allele (ADH1B*2,TABLE
3) with a 100-fold increase in
␤
2-ADH turnover (215, 308)
consumed less ethanol and had a reduced frequency of cor-
onary heart disease compared with noncarriers of this allele
(184). In the search for mechanisms to explain the U-shaped
relationship between ethanol consumption levels and coro-
nary heart disease, researchers have focused on methanol
and its oxidation product, formaldehyde. Recently, formal-
dehyde was hypothesized to participate in the process
(322). The proposed mechanism relies on the fact that the
common ADH1b enzyme carries out one-phase catabolism
of methanol and ethanol. Moderate ethanol consumption
competitively inhibits the conversion of methanol to form-
aldehyde, thus reducing the endogenous formaldehyde con-
tent in the organs. Another possible mechanism for the
beneficial effects of moderate ethanol intake was indicated
by the results of a study in rats showing that ALDH2 oxi-
dizes aldehydes, including formaldehyde, and may serve as
a potential endogenous neuroprotective target and a prom-
ising therapeutic strategy for the management of stroke
(438). Ethanol administration activated ALDH2 and en-
hanced the detoxification of aldehydes (161). The study of
ALA also indicated that ALDH2 participates in the formal-
dehyde and acetaldehyde detoxification process (116, 171a,
285, 310).
E. Methanol Fatalities and Antidote Therapy
Many cases of poisoning that result in fatalities are caused
by methanol ingestion. When methanol was discovered,
nothing was known about the toxicity of its metabolites in
humans; thus the use of methanol as a substitute for ethanol
became widespread in industry (453). Quickly thereafter,
the dangers of methanol intoxication through skin absorp-
tion and inhalation became apparent (311), as numerous
cases of industrial poisoning were reported (263). More-
over, as the odor and appearance of methanol are almost
the same as those of ethanol, these two alcohols were often
confused, and methanol was used instead of ethanol as a
beverage, thereby increasing the cases of poisonings and
fatalities.
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Today, methanol is more widely used in industry, but the
number of industrial poisonings are reduced due to the
existence of the necessary precautions and safety instruc-
tions. Nevertheless, methanol poisoning and fatal cases re-
main a problem. These cases are mainly due to the intended
or unintended ingestion of “surrogate alcohols,” namely,
non-beverage alcohol (e.g., industrial spirits, fire -liquids,
windshield washer liquid, antifreeze), moonshine, home-
made alcohol, samogon, etc. (258). Cases of homicide and
suicide with methanol-containing liquids have also been
reported (214). Fewer cases are related to methanol intox-
ication through percutaneous methanol absorption or va-
por inhalation (316a).
The minimal dose of methanol that could lead to a lethal
outcome without adequate treatment is currently consid-
ered to be ⬃1 g/kg (396), and 10–15 ml may cause blind-
ness (316). According to the literature, the toxic effects of
methanol could be observed after the ingestion of 15 ml of
40% methanol (29) to 600 ml of pure methanol (253).
These significantly different results could be explained by
insufficient and incorrect data regarding the amount of al-
cohol consumed; usually, the ingested volume is self-re-
ported by the patients or is indirectly obtained. Moreover,
the important role of different factors associated with the
ingestion of methanol, including joint ethanol-methanol
consumption, for example, should not be underestimated.
Furthermore, the variability of toxic or lethal doses is de-
pendent on the general state of health, diet, and the amount
of tetrahydrofolate, which is known to take part in metha-
nol metabolism, of the organism (396, 453).
Upon ingestion or inhalation, methanol initially has a nar-
cotic effect as ethyl alcohol, followed by an asymptomatic
period of ⬃10–12 h. After this period, methanol metabo-
lites may cause nausea, vomiting, dizziness, headaches, re-
spiratory difficulty, abdominal pain, visual disturbances,
and metabolic acidosis (396). The main consequences of
methanol intoxication are metabolic acidosis, hyperosmo-
lality, formic acid, lactic acid, formaldehyde and ketones
accumulation in the organism, and retinal damage with
blindness (253). The high level of formic acid (formate) is
believed to be the main cause of the onset of the clinical
signs of methanol poisoning (23). Formate accumulates in
bodily fluids and tissues, leading to metabolic acidosis and
blood bicarbonate depletion (409). Formate inhibits mito-
chondrial cytochrome oxidase, resulting in tissue hypoxia;
this effect is enhanced when the pH is low. Notably, for-
mate injection per se leads to optic disk damage, indepen-
dently of acidosis; the ocular effects associated with meth-
anol poisoning appear to be due to hypoxia in areas of the
cerebral and distal optic nerves (276). One study directly
demonstrated this by intravenously injecting monkeys with
formate at concentrations similar to those observed in
methanol poisoning. The blood formate levels in these ani-
mals were constantly elevated for 24–48 h, whereas the
blood pH maintained at physiological levels. All animals
developed ocular toxicity within 24 h, and its symptoms
were the same as those of methanol poisoning. Thus ocular
toxicity is caused by formate but not by methanol or acido-
sis (306). Further studies showed that acidosis correction
did not prevent ocular toxicity.
One of the first massive methanol poisoning outbreaks to be
documented was in the 1940s. During the Second World
War, more than 100 fatal cases were registered in the Ger-
man army. Lachenmeier (258) summarized further out-
breaks of methanol poisoning from surrogate alcohols that
have been reported in the scientific literature since the
1950s.
Modern toxicology handbooks recommend treating the
consequences of methanol intoxication when blood metha-
nol concentrations are more than 0.2 g/l in a nonacidotic
patient. However, it is likely that this threshold was chosen
arbitrarily (251), as this approach does not take into ac-
count the time passed since the moment the methanol-con-
taining liquid was ingested. For example, if the blood meth-
anol content is 0.2 g/l 12 h after ingestion, then the peak
level 1 h after ingestion could be estimated as more than 1
g/l. The same concentration of 0.2 g/l measured 24 or 48 h
after ingestion corresponds to peak levels of 2.15 or 4.2 g/l,
respectively (251). According to this approach, the amount
of ingested methanol might be significantly underestimated,
which could lead to improper treatment. Thus the time and
circumstances of the ingestion should be taken into account
whenever possible. The levels of blood formate should also
be assessed because the relative concentrations of methanol
and formate in the blood are functions of the amount of
methanol ingested. Different methods, including enzymatic
measurements and gas chromatography, for formic acid
analysis in both ante-mortem and postmortem blood sam-
ples were used to provide additional information to assist
with the interpretation of methanol fatalities (190, 495).
The best picture of toxic and lethal blood methanol concen-
trations was obtained from the analysis of postmortem
methanol and formate levels. Blood methanol and formate
contents estimated ante-mortem and postmortem were
shown to be very close. The case report study performed by
Jones (214) analyzed postmortem methanol and formate
concentrations in 73 cases; blood methanol levels varied
significantly, ranging from 0.7 to 7.9 g/l, whereas formate
concentrations showed a more narrow range between 0.53
and 1.40 g/l. In 97% of cases, the formate concentrations
ranged from 0.60 to 1.10 g/l. Similar ranges were detected
by Wallage (495). Another study of 15 cases of methanol
poisoning (no fatalities included) reported approximate
blood methanol levels ranging from 0.14 to 4.5 g/l and
formate levels ranging from 0.01 to 1.48 g/l. The authors
also noted that there was no correlation between methanol
and formate levels. Nonetheless, the levels of both com-
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pounds are strongly recommended to be monitored in hos-
pitals for patients with methanol poisoning (190).
Methods of treatment for methanol poisoning are highly
dependent on how quickly treatment is initiated and the
stage of poisoning. Gastric lavage, vomiting induction, and
the use of activated charcoal can only be conducted within
1 h after methanol ingestion (253); after 1 h, the methanol
is fully absorbed by the gastrointestinal tract. For those
patients, therapy with inhibitors of subsequent methanol
metabolism can be applied. After methanol is rapidly and
completely absorbed by the gastrointestinal tract, it pene-
trates all bodily fluids. With the participation of liver ADH,
methanol then is metabolized to formaldehyde and formic
acid. The following methods can be applied as therapies for
methanol poisoning (399): 1) intravenous bicarbonate in-
jection to reduce metabolic acidosis; 2) the use of antidotes
such as ethanol and fomepizole (4-methylpyrazole) to ter-
minate the conversion of methanol to formic acid; and 3)
hemodialysis to remove methanol and formic acid from the
blood, used for uncomplicated poisoning with methanol
(316).
Therapy with the use of ethanol or fomepizole can be used
in cases in which the time of methanol intake is known and
the methanol levels in the blood rise above 10–20 mg/dl
or the osmolal gap is ⬎10 mosmol/kgH
2
O. In cases in which
the time of methanol consumption is not known, any two of
the following criteria should be present: a bicarbonate con-
centration in the blood serum of ⬍20 meq/l, an arterial
hydrogen rate (pH) less than 7.3, and an osmolal gap of
⬎10 mosmol/kgH
2
O (22, 87, 227, 230). Treatment with
ethanol is the most accessible, due to cost effectiveness and
ease of use (peroral or intravenous), but to use ethanol as an
antidote, high serum levels of 100 mg/dl must be main-
tained. Thus it is not recommended for patients with liver
disease and stomach ulcers. Additionally, ethanol should be
used with caution in patients who use drugs that suppress
the central nervous system because ethanol may exacerbate
their condition (1, 23, 253).
Fomepizole, on the other hand, has a higher affinity for
ADH than ethanol (by close to 1,000 times) and can com-
pletely inhibit ADH1b at significantly lower serum concen-
trations (46, 316). Fomepizole concentrations of more than
0.8 mg/l (10 mM) in the serum provide permanent inhibi-
tion of ADH1b (23). For patients not undergoing hemodi-
alysis, the recommended doses of fomepizole are known
(392): an initial dose of 15 mg/kg, followed by 10 mg/kg
every 12 h, and then an increased dose of 15 mg/kg at 48 h,
followed by another 15 mg/kg dose after 12 h.
For patients undergoing hemodialysis, two options have
been proposed for fomepizole administration. In the first
case, the dosage remains the same as for patients not under-
going hemodialysis but with a reduced time interval be-
tween successive doses: the second dose is given 6 h after the
first dose, and subsequent doses are given every 4 h. In the
second case, continuous intravenous infusion of 1.0–1.5
mg·kg
⫺1
·h
⫺1
fomepizole is performed after the primary
dose. However, the use of fomepizole to treat methanol
poisoning is not possible for all patients at every clinic.
Most people with methanol poisoning are alcoholics who
cannot afford such an expensive drug; thus ethanol is more
commonly used in such cases (253, 392).
VII. GENES INVOLVED IN ENDOGENOUS
METHANOL CATABOLISM AND
HUMAN PATHOLOGY
A. Cluster of Genes Involved in Endogenous
Methanol Catabolism
Carbon dioxide and nitrogen oxide are signaling molecules
with small molecular weights that are widespread in the
environment. Methanol might be another example of a
small regulator of gene expression. Recently, a number of
genes were shown to undergo expression changes due to
increased methanol levels in the plasma of humans and
mice. Most of the genes affected by methanol fluctuation
are involved in ethanol metabolism (ADH1, ALDH2,
GSTO1, MGST3, GSTP1,CYP2E1), Alzheimer’s disease
pathogenesis (PSENEN, APOE, SNCA, MME), and hemo-
globin production (HBB2, HBA1) (246, 413). Although
ethanol was known to alter the gene expression of alcohol-
metabolizing enzymes in mammals, our data reveal the sig-
naling function of methanol or formaldehyde in mammals
for the first time. There are many examples, including form-
aldehyde, in which a product or substrate of a reaction can
affect the gene expression of an enzyme. Alcohol oxidase
from the methylotrophic yeast Pichia pastoris was shown to
undergo expression changes in the presence of methanol
(482a). Further investigation revealed a formaldehyde-sen-
sitive transcriptional factor that can alter the expression of
the methanol-oxidizing enzymes in yeasts (65, 374). At the
same time, increased methanol in the blood of mice led to
overexpression of the brain genes involved in formaldehyde
clearance (Aldh2,Gsto1,Mgst3,Gstp1) and downregulation
of Adh1, which participates in formaldehyde formation (246,
413). These changes in gene expression of brain tissue in
mice may control toxic formaldehyde concentrations.
However, Cyp2E1, which oxidizes ethanol and methanol
to aldehydes, was upregulated (246). Thus methanol or
formaldehyde can alter the expression of the genes involved
in methanol oxidation.
B. Alcoholism
Polymorphisms in genes that affect ethanol utilization are
of particular interest in heavy drinkers. Mutations in ADH
and ALDH genes that affect the enzymatic activity and
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protein structure could be associated with the risk of alco-
holism. Twin surveys revealed that the genetic influence on
the development of alcoholism was as important as that of
environmental factors (172, 173). Polymorphisms in en-
zymes such as ADH1b and ALDH2 that metabolize ethanol
are mostly associated with alcoholism. Genotypes with
ADH1 and ALDH2 alleles that lead to greater acetaldehyde
accumulation are protective against excessive ethanol
consumption. For example, the ADH1B*2 (Arg48His,
rs1229984) and ADH1B*3 (Arg370Cys, rs2066702) vari-
ants are able to oxidize ethanol to acetaldehyde much faster
(122) and are associated with a protective effect against
alcoholism in East Asian (122), African American (121),
and American Indian populations (372, 496).
Another way to increase acetaldehyde formation is to block
the activity of ALDH2, which is the primary enzyme that
oxidizes acetaldehyde to acetic acid. East Asian populations
with inactive ALDH2 accumulate acetaldehyde to a greater
extent while drinking, which confers a protective effect
against ethanol dependence (182). Conversely, carriers of
the SNPs in the ADH4 promoter were found to have a
higher risk of alcohol dependence (158, 294). Edenberg et
al. (123) demonstrated that one of the SNPs (⫺75A) in-
creased the expression of corresponding gene compared
with the ⫺75C variant (123). Individuals with cysteine at
the ⫺75 position are three times more likely to develop
ethanol dependence (158).
C. Neurodegenerative Diseases
Thus far, the scheme of Alzheimer’s disease pathogenesis is
incomplete; however, there is strong evidence that oxidative
stress triggers and participates in Alzheimer’s disease devel-
opment. Mitochondrial dysfunction is strongly associated
with oxidative stress and neurodegenerative disorders, in-
cluding Alzheimer’s disease (279). Thus changes in mito-
chondrial proteins with antioxidative stress implications
may be involved in neurodegenerative diseases. Examples
include the cytochrome coxidizing complex, manganese
superoxide dismutase, and catalase and ALDH2, two en-
zymes that are important in alcohol metabolism. Whereas
catalase mainly oxidizes ethanol and methanol at high con-
centrations, ALDH2 is a central protein in the oxidation of
relational aldehydes. One of the major polymorphisms of
ALDH2 is the substitution of glutamate at the 487th posi-
tion by lysine. This variant, ALDH2*2, is widespread
among the Japanese population, in which up to 30% of
people are heterozygous (447). In individuals who carry at
least one ALDH2*2 allele, a lower K
m
results in higher
acetaldehyde concentrations, whereas ALDH2*2 homozy-
gotes show no ALDH2 activity (447). Because ALDH2 lo-
calizes to the mitochondrial matrix and is implicated in the
oxidation of aldehydes generated in oxidative stress,
ALDH2 was proposed to play a role in Alzheimer’s disease
pathogenesis. The low activity of ALDH2*2 causes the ac-
cumulation of highly toxic 4-hydroxy-2-nonenal (4-HNE),
which was shown to cause neuronal death (254) and was
observed in Alzheimer’s and Parkinson’s disease patients
(325, 404, 522). Analysis of the ALDH2*2 polymorphism
in the 472 Alzheimer’s disease patients and 472 nonde-
mented controls revealed a small but statistically significant
difference in the frequency of the allele (48.1 and 37.4%,
respectively) (221). Interesting, APOE-
⑀
4 homozygotes car-
rying at least one ALDH2*2 allele have a 31-fold higher
chance of developing Alzheimer’s disease than those with
neither allele (352). Because both proteins control the con-
centration of 4-HNE, the dysfunction of ALDH2*2 dra-
matically increases the levels of 4-HNE. While the apolipo-
protein E-␧4 (APOE-␧4) polymorphism is found in more
Alzheimer’s disease patients compared with healthy indi-
viduals than the ALDH2*2 allele, the coexistence of both
alleles dramatically increases the probability of developing
Alzheimer’s disease. Thus ALDH2*2 itself is not a strong
prognostic factor for Alzheimer’s disease, but dramatically
increases the risk of the Alzheimer’s disease development in
APOE-␧4homozygotes (352).
D. Cardiovascular Disease
Recent data revealed a crucial protective role of ALDH2 in
ischemic injury. ALDH2 is activated by protein kinase C-␧
via phosphorylation (71) and then detoxifies poisonous al-
dehydes such as 4-HNE and acetaldehyde (36, 88). Because
4-HNE can trigger necrotic cell death in the reperfused
myocardium (338) and blocks important metabolic pro-
teins such as glyceraldehyde 3-phosphate dehydrogenase
(475), Na
⫹
-K
⫹
-ATPase (417), and the 20S proteasome
(134), its deactivation by ALDH2 has a cytoprotective ef-
fect in ischemic patients. Additionally, ALDH2 participates
in the bioactivation of nitroglycerin, leading to increased
blood flow (321). Based on these data, ALDH2 is a prom-
ising target for the treatment of ischemic injury by activa-
tors such as ALDH2 activator 1 (Alda-1) and ALA (71,
171a). Alda-1 not only increases ALDH2 levels and restores
ALDH2*2 activities (53), but also prevents 4-HNE-medi-
ated blocking the function of ALDH2 in ischemic conditions
(71). Undoubtedly, the mechanisms regulating the transcrip-
tion, translation, and enzymatic activity of ALDH2 require
further investigation.
E. Tumoral Diseases
ADH1 and ALDH1 can oxidize a variety of the alcohol
metabolites, including aliphatic alcohols and retinol (vita-
min A). ADHs and ALDHs are implicated in developmen-
tal, apoptotic, metabolic, and signaling processes. The dys-
function of ADHs and ALDHs on different levels can lead
to or be associated with different disorders. Due to the
crucial role of ADH1 and ALDH1 in the retinoic acid path-
way, this pathway is of particular interest. The retinoic acid
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pathway controls the processes of differentiation, prolifer-
ation, and apoptosis and was found to participate in the
development of various tumors (132, 280, 339).
1. Uterine fibroid
Among women, uterine fibroid (UF) is a widespread benign
tumor of the smooth muscle cells that rarely transforms into
malignant leiomyosarcoma. Due to uncontrolled fibroid
growth, UF can cause various pains, frequent urination, and
infertility in 1.0–2.4% of patients (109). Molecular factors
involved in UF development include retinoic acid, insulin
growth factor 2, transforming growth factor
␤
pathways,
and extracellular matrix formation. In UF samples, ADH,
ALDH1, CRBP1, RXR
␣
, and RXR
␥
mRNA and protein
levels are significantly lower than in healthy controls (93,
529). ADH1 and ALDH1 oxidize retinol to retinal and
retinoic acid; thus decreased RA levels are observed in UF
samples. The reduced concentration of retinoic acid could
lead to typical tumor characteristics: decreased differentia-
tion and apoptosis together with increased proliferation
(529). Thus ADH and ALDH may play an important role in
UF development and could be targets in UF treatment.
2. Head and neck cancer
Acetaldehyde is a carcinogenic metabolite, and its concen-
tration is controlled by the enzymatic activities of ADHs,
ALDHs, catalase, and CYP2E1 (49). Because the main
source of acetaldehyde is ethanol consumption, this carcin-
ogen mostly affects the upper aerodigestive tract (408).
Variants of the ADH and ALDH genes that lead to changes
in the activities of their protein products are associated with
the development of head and neck, esophageal, and liver
cancers. Although variants that produce acetaldehyde in
greater concentration would be expected to increase the risk
of cancer, this is not always the case. Homozygotes for
the ADH1B*2 (Arg48His; rs1229984) and ADH1C*1
(Arg272Gln; rs1693482) allele variants were shown to pro-
duce 40- and 2.5-fold more acetaldehyde from ethanol, re-
spectively (39). However, meta-analyses revealed an inverse
correlation between the head and neck cancer risk and car-
rying the variants of ADH1C and ADH1B that metabolize
acetaldehyde faster (68). The authors provide three possible
explanations for this unexpected finding: 1) a decreased
opportunity for the oral microflora to produce acetalde-
hyde locally by the prolonged systemic circulation of etha-
nol, 2) the prevention of ethanol acting as a solvent for
other carcinogens, and 3) decreased amounts of ethanol
consumption due to the discomfort caused by the conse-
quent peak in systemic acetaldehyde. We propose that
formaldehyde might also affect the results: large concentra-
tions of formaldehyde can be found in human body after
alcoholic beverage consumption, and formaldehyde is
known to have a carcinogenic effect. Although little evi-
dence is available regarding formaldehyde causing NPC
(445), studies have demonstrated that chronic exposure to
formaldehyde causes squamous cell carcinoma in rats (323,
324, 444).
3. Colorectal cancer
ADH1 and ALDH may contribute to colorectal cancer
(CC). Biochemical tests reveal increased ADH1 activity in
CC samples compared with healthy controls (205). More-
over, ALDH activity in cancer cells is slightly lower than in
healthy tissues; the ratio between ADH1 and ALDH activ-
ities is 20.5:1.0 in cancer cells and 10:1 in healthy cells.
Acetaldehyde seems to affect the relationship between the
ethanol-oxidizing system and CC. In CC cells, carcinogenic
acetaldehyde accumulates roughly twofold faster than in
healthy cells, indicating that acetaldehyde may participate
in the progression of CC. Additionally, ADH1 activity in
the sera of CC patients is increased and rises with the stage
of cancer (206).
VIII. CONCLUDING COMMENTS
Over the past decade, significant progress has been made in
elucidating the functions and physiological roles of human
metabolic methanol. Moreover, recent advances in the field
of metabolic methanol research have provided insights into
the mechanisms by which low levels of formaldehyde are
maintained in human plasma. However, numerous aspects
of metabolic methanol synthesis and the regulation of meth-
anol-metabolizing genes are not understood.
Methanol is an integral and inevitable component of human
life. In the morning woods (193) or on a mowed meadow
(95a, 230, 505), inhaled air features a noticeable content of
plant methanol vapors. The blood of fasting individuals
contains small amounts of methanol and its oxidative prod-
uct, formaldehyde; the levels of these molecules increase
sharply even after the ingestion of vegetable pectin (112,
413). There are several metabolic sources of methanol (FIG-
URE 1). In addition to fruit and vegetables, methanol can be
consumed in soft drinks containing aspartame and alco-
holic beverages, which contain methanol as a product of the
fermentation of vegetative raw materials with PME. In ad-
dition to the processes of methylation-demethylation, an
important source of metabolic methanol is the intestinal
microflora. The detection of methanol even after inhibition
of liver ADH1b by 4-methylpyrazole (246, 403) or admin-
istration of ethanol-free methanol (413) clearly indicates
the existence of internal sources of methanol.
The mechanisms of phase I of the catabolism of methanol
share several features of ethanol catabolism. In humans, the
oxidation of methanol and ethanol requires several stages
of conversion. In the first stage, methanol is oxidized to
formaldehyde in three ways. The first is via oxidation by
CYP450s (57, 87, 495). The contribution of CYP450s to
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the metabolism of methanol and ethanol in the liver is low
(⬍9%) and increases at higher doses of methanol and eth-
anol. The second form of methanol oxidation occurs via the
catalase-H
2
O
2
complex (64, 97). Its contribution to the
metabolism of methanol and ethanol in the liver is small
(⬍1–2%) but increases at higher doses of methanol and
ethanol. The third means of methanol oxidation involves
ADH1b, which catalyzes up to 90% of methanol and eth-
anol oxidation in the liver (63, 168, 297). ADH1b activity
has not been detected in brain extracts (143). The enzymatic
reaction occurs in two stages. The first is the formation of
the methanol/ADH1b/NAD
⫹
ternary complex with the
participation of Zn. At this stage, methanol and ethanol
enter the complex equally effectively; thus ethanol func-
tions as an antidote for methanol poisoning (382). In the
second stage, hydride transfer occurs with the formation of
CHO ⫹NADH ⫹H
⫹
. Because ADH1b is involved in both
methanol and ethanol catabolism, ethanol functions as a
powerful competitive inhibitor at low concentrations, and
the enzyme has a strong preference for converting ethanol
to acetaldehyde over converting methanol to formaldehyde.
Methanol itself is not toxic to human cells; however, its
oxidative product, formaldehyde, is a toxin that is believed
to play a role in carcinogenesis and age-related neuronal
damage in the brain (472). Although formaldehyde is pri-
marily produced by the oxidation of methanol, there are
several sources of metabolic formaldehyde, including
SSAO-mediated oxidative deamination and the removal of
methyl groups from lysine residues in histones catalyzed by
histone demethylases.
The second phase of methanol catabolism, i.e., the conver-
sion of formaldehyde to formic acid, occurs via three mech-
anisms, including the participation of CYP2E1 (27, 183,
255, 289, 452), cytosolic AlDH1 and mitochondrial
AlDH2 (142, 297, 452, 453, 473, 485), and FDH (432).
There are two mechanisms by which metabolic formalde-
hyde can be maintained at low levels. The first is to decrease
ADH activity to prevent the oxidation of methanol to form-
aldehyde. The second method is the rapid and effective
oxidation of formaldehyde to the end products carbon di-
oxide and water. These two methods are implemented in
the brain and embryo, both of which are highly sensitive to
formaldehyde. ADH activity is decreased or absent in the
brain and the embryo (143), and increased FDH (432) and
ALDH2 (472) activity oxidize formaldehyde introduced via
the bloodstream or formed in situ.
In conclusion, human methanol and formaldehyde catabo-
lism is complex, multilayered, and highly effective. Meta-
bolic control can be achieved at the gene or protein level,
i.e., at the level of enzyme activity. Methanol is not poison-
ous itself but rather acts as a signaling molecule that regu-
lates the activity of a gene cluster involved in the mainte-
nance of a low level of formaldehyde (112, 246, 413). The
disruption of formaldehyde metabolism control may be a
causative factor in neurodegenerative diseases. Low levels
of formaldehyde have been associated with some human
pathologies. Increased formaldehyde content in the blood
and brain has been detected in neurological patients as well
as in the blood of the elderly, suggesting a disruption of the
genetic and biochemical mechanisms responsible for main-
taining low formaldehyde levels (463, 466). To determine
the beneficial or detrimental effects of methanol, we must
also consider the favorable role of fruits and raw vegetables
in human health. A vegetarian diet is the main source of
exogenous methanol in a healthy individual (112). The role
of methanol-generated pectin in atherosclerosis and cancer
prophylaxis is well-known (389). It has been suggested that
pectin can modulate detoxifying enzymes, stimulate the im-
mune system, modulate cholesterol synthesis, and act as an
antibacterial, antioxidant, or neuroprotective agent. There
is also increasing evidence to suggest that regular fruit and
vegetable consumption may play an important role in pre-
venting or delaying the onset of dementia, age-associated
cognitive decline, and Alzheimer’s disease (127).
Thus advances in modeling and an analysis of human meth-
anol metabolism will extend our knowledge of the role of
methanol in health and disease, permitting the customiza-
tion of existing and future therapeutic and prophylactic
modalities.
ACKNOWLEDGMENTS
Address for reprint requests and other correspondence:
Y. L. Dorokhov, A. N. Belozersky Institute of Physico-
Chemical Biology, Lomonosov Moscow State University,
Leninsky Gory 1, Laboratory Building A-40, Moscow
119991, Russia (e-mail: dorokhov@genebee.msu.su).
GRANTS
This work was supported by Russian Science Foundation
Grants 11-04-01152, 12-04-33016, and 14-04-00109; the
Russian Foundation for Basic Research; and a stipend from
the President of the Russian Federation for young scientists.
The funders had no role in data collection and analysis,
decision to publish, or preparation of the manuscript.
DISCLOSURES
No conflicts of interest, financial or otherwise, are declared
by the authors.
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doi:10.1152/physrev.00034.2014 95:603-644, 2015.Physiol Rev
Tatiana V. Komarova
Yuri L. Dorokhov, Anastasia V. Shindyapina, Ekaterina V. Sheshukova and
Physiological Roles
Metabolic Methanol: Molecular Pathways and
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