Behavioral and neurochemical effects of proline

Abstract

Proline is an amino acid with an essential role for primary metabolism and physiologic functions. Hyperprolinemia results from the deficiency of specific enzymes for proline catabolism, leading to tissue accumulation of this amino acid. Hyperprolinemic patients can present neurological symptoms and brain abnormalities, whose aetiopathogenesis is poorly understood. This review addresses some of the findings obtained, mainly from animal studies, indicating that high proline levels may be associated to neuropathophysiology of some disorders. In this context, it has been suggested that energy metabolism deficit, Na(+),K(+)-ATPase, kinase creatine, oxidative stress, excitotoxicity, lipid content, as well as purinergic and cholinergic systems are involved in the effect of proline on brain damage and spatial memory deficit. The discussion focuses on the relatively low antioxidant defenses of the brain and the vulnerability of neural tissue to reactive species. This offers new perspectives for potential therapeutic strategies for this condition, which may include the early use of appropriate antioxidants as a novel adjuvant therapy, besides the usual treatment based on special diets poor in proline.

Full text

REVIEW ARTICLE
Behavioral and neurochemical effects of proline
Angela T. S. Wyse & Carlos Alexandre Netto
Received: 1 March 2011 /Accepted: 12 May 2011 /Published online: 4 June 2011
#
Springer Science+Business Media, LLC 2011
Abstract Proline is an amino acid with an essential role for
primary metabolism and physiologic functions. Hyper-
prolinemia results from the deficiency of specific enzymes
for proline catabolism, leading to tissue accumulation of
this amino acid. Hyperprolinemic patien ts can pre sent
neurological symptoms and brain abnormalities, whose
aetiopathogenesis is poorly understood. This review
addresses some of the findings obtained, mainly from
animal studies, indicating that high proline levels may be
associated to neuropathophysiology of some disorders. In
this context, it has been suggested that energy metabolism
deficit, Na
+
,K
+
-ATPase, kinase creatine, oxidative stress,
excitotoxicity, lipid content, as well as purinergic and
cholinergic systems are involved in the effect of proline
on brain damag e and spatial memory deficit. The discus-
sion focuses on the relatively low antioxidant defenses of
the brain and the vulnerability of neural tissue to reactive
species. This offers new perspectives for potential thera-
peutic strategies for this condition, which may include the
early use of appropriate antioxidants as a novel adjuvant
therapy, besides the usual treatment based on special diets
poor in proline.
Keywords Proline
.
Hyperprolinemia
.
Brain damage
.
Antioxidants
.
Alpha-tocopherol
.
Ascorbic acid
Introduction
L-Proline (Pro) is a non-essential amino acid for human
infants and adults (Hiramatsu et al. 1994; Young and El-
Khoury 1995). It can be endogenously synthesized either
from glutamate or ornithine, but these synthetic pathways
are not utilized to provide substrate for protein synthesis
because Pro is also present in food regularly consumed
from the diet. Low levels of Pro (51–271 μM) are normally
found in the plasma (Phang et al. 2001). However, genetic
defects can be found in the enzymes of Pro metabolism that
can lead to the increase in Pro levels, namely hyper-
prolinemia. Mutations in proline o xidase and delta-1-
pyrroline-5-carboxylate dehydrogenase are associated with
excess levels of Pro (>500 μM), mental retardation and
epilepsy. Although these mutations are rare, mild or high
elevations of Pro levels have been associated with cancer
and predispositions to psychiatric disease (Phang et al.
2001). However, to understand the potential relevance of
the role of Pro in the central nervous system, it is important
to briefly review pathways involved in metabolism of this
amino acid.
Proline metabo lism—an overview
In contrast to other amino acids, Pro has no primary
amino group but an imino group, since only possesses
one hydrogen atom inserted in its pyrroline ring, giving
rise to a molecule with an exceptional conformational
rigidity. Based on this fact, Pro is excluded from the
pyridoxal-5-phosphate coenzyme catalyzed decarboxyl-
ation and transaminations reactions that are important
for amino acid metabolism. As such, Pro is metabolized
by enzymes with properties and regulatory mechanisms
A. T. S. Wyse (*)
:
C. A. Netto
Laboratório de Neuroproteção e Doenças Metabólicas,
Departamento de Bioquímica, Instituto de Ciências Básicas da
Saúde, Universidade Federal do Rio Grande do Sul (UFRGS),
Rua Ramiro Barcelos 2600-Anexo,
90035-003 Porto Alegre, RS, Brazil
e-mail: wyse@ufrgs.br
Metab Brain Dis (2011) 26:159–172
DOI 10.1007/s11011-011-9246-x

that are independent of those used by other amino acids
(Phang et al. 2001).
As describe d by Hu and colleagues (2008 ), the Pro
metabolism (Pro cycle) in mammals involves two other
amino acid, glutamate and ornithine, and five enzymes
namely delta-1-pyrroline-5-carboxylase reductase, proline
oxidase, delta-1-pyrroline-5-carboxylate dehyd rogenase,
delta-1-pyrroline-5-carboxylate synthase and ornithine ami-
notransferase (Fig. 1).
As shown in Fig. 1, ornithine and glutamate are the
precursors of Pro, with delta-1-pyrroline-5-carboxylate
(P5C) or glutamic-gamma-semialdehyde as the common
intermediate (Adams 1970; Ross et al. 1978; Smith and
Phang 1979; Strecker 1957). P5C, a precursor and the
degradation product of Pro, is found both intracellularly
and also circulating in plasma. In Pro synthesis, P5C is
released from mitochondria and is converted to Pro by
cytosolic P5C reduct ase, an enzyme found in low concen-
trations in all tissues that utilize either NADH or NADPH
as a cofactor, since it has a higher affinity for NADPH
(Phang et al. 2001). Thus, the Pro cycle, via P5C redutase,
participates and activates the metabolism of glucose
through the pentose phosphate pathway (Phang et al.
1980; Pha ng et al . 2008b). With the exception of
conversion of P5C to Pro by P5C reductase found in
cytosol, all other reactions involved in Pro synthesis occur
in the mitochondria.
The first step in proline degradation is catalyzed by
proline oxidase (POX), also named proline dehydrogenase
(PRODH), a flavoenzyme localized at the inner mitochon-
drial membr anes tha t convert proline to P5C. In this
reaction, the transfer of electrons occurs from Pro to FAD
(flavine adenine dinucleotide) and generates FADH
2
, which
delivers its electrons into the complex II of the electron
transport chain and ATP is formed by oxidative phosphor-
ylation through the subsequent transfer of these electrons,
via cytochrome c. Thus, Pro can be a direct substrate for
ATP production (Adams and Frank 1980; Hagedorn and
Phang 1983; Phang et al. 2001). The second non-enzymatic
step involves the conversion of P5C to glutamic-gamma-
semialdehyde, which is converted to ornithine in the
reversible reaction catalyzed by ornithine amino transferase
(OAT) or to glutamate by enzyme delta-1-pyrroline-5-
carboxylate dehydrogenase (P5C dehydrogenase), which
use NAD
+
(nicotinamide adenine dinucleotide) as an electron
acceptor and generate NADH, delivering electrons for
mitochondrial respiration. This reaction is a component of
the pathway connecting the urea (ornithine/arginine) and
tricarboxylic acid cycles (glutamate/alpha-ketoglutarate).
With the exception of OAT, which catalyzes a reversible
reaction, the other four enzymes catalyze irreversible reac-
tions. With the exception of proline oxidase, which is inserted
in the inner membrane, the other reactions of Pro degradation
occur primarily in matrix mitochondria. Mitochondrial P5C
can be recycled to Pro in the cytosol by P5C redutase.
Roles of L-proline
Pro has important roles in synthesis and structure of protein
and metabolism (particularly the synthesis of arginine, poly-
amines, and glutamate via P5C). Pro is one of most abundant
amino acids, being readily available from the breakdown of
the extracellular matrix, which is composed predominately of
collagen and 25% of the amino acids of this protein are Pro
and/or its derivative hydroxyproline (Li et al. 2006). Due its
predominance in collagen and milk, the requirements for Pro
are the greatest among all amino acids (Wu et al. 2010). In
addition, the cycling of P5C and Pro between mitochondria
and cytosol can transfer reducing potential, which can
contribute to ATP production (Yeh and Phang 1988). In
addition, the Pro degradative pathway can generate gluta-
Fig. 1 Schematic Proline Cycle.
Abbreviations: P5C: delta-1-
pyrroline-5-carboxylic acid;
CoQ: coenzyme Q; Cyt c:
cytochrome c; I–IV: complexes
of electron transport chain and
V: FoFi-ATP synthase;
AA: amino acid; KA:
α-ketoacid (Adapted from
Phang et al., 2001)
160 Metab Brain Dis (2011) 26:159–172

mate and alpha-ketoglutarate, which can play an anaplerotic
role in the Krebs cycle (Phang 1985). Based on this, it has
been suggested that Pro metabolism can be activated under
stress conditions providing accessory mechanisms for bio-
energetic and redox reactions (Pandhare et al. 2009).
On the other hand, Pro has also been considered as an
osmoprotectant in bacteria and also an antioxidant in plants
(Phang 1985), as well as a bioenergetic substrate for insects
during their initiation of flight (Gade and Auerswald 2002;
Micheu et al. 2000; Phang et al. 2008a). Although the role
of Pro has been recognized in a variety of animals and plants,
the mechanisms are unclear. However, it has been suggested
that this amino acid has an important role in the co-evolution
in both plant and animal species (Phang et al. 2008a).
L-Proline metabolism and diseases
Human inherited disorders of the metabolism of Pro are
known as hyperprolinemia type I (HPI), hyperprolinemia
type II (HPII), delta-1-pyrroline-5-carboxylate synthase
deficiency, ornithine aminotransferase deficiency, hydrox-
iprolinemia and iminoglycinuria (Mitsubuchi et al. 2008).
Inherited disorders in the degradative pathways of proline
cause hyperprolinemia in humans (Phang et al. 2001). The
first report of the direct effect of the involvement of Pro in
human disease was reported by Schafer and colleagues
(Schafer et al. 1962) in a family with hyperprolinemia,
cerebral dysfunction, renal abnormalities, hereditary ne-
phropathy and deafness. From this time onwards, many
families with hyperprolinemia have been reported in the
literature (Mitsubuchi et al. 2008) and various studies have
been pe rformed in order to understand the biological
function (Phang, Hu and David Valle groups and others),
behavioural and neurochemical effects (Wyse group and
others) and physiopathology of diseases such as hyper-
prolinemias, cancer and psychiatrics (Phang, Hu, David
Valle, Campion and other groups).
Hyperprolinemia is present in two inherited metabolic
disorders: type I and type II hyperprolinemias. These
disorders are characterized by distinct biochemical and
genetic deficiencies in the catabolic pathway (Fig. 1). HPI
is a rare inherited autosomal recessive disorder of amino
acid metabolism characterized by the hepatic deficiency of
proline oxidase (also called pr oline dehydrogenase), a
flavoenzyme localized in the inner mitochondria that
converts Pro to P5C, the first step in the Pro catabolic
pathway. Tissue accumulation of Pro occurs in affected
patients, and Pro levels can range from five- to ten times
(700 to 2400 μ M) above normal values (51 to 271 μM).
Some studies show that mild hyperprolinemia (500 to
1000 μM) may be observed in HPI heterozygot es (Phang et
al. 2001). It has been shown that the gene (PRODH1) that
encodes POX is l ocalized in the 22 q11 chromosomal
region. The clinical manifestations in patients with HPI
are still not well characterized. Some phenotypes are found
in patients with HPI, such as neurological renal, auditory
defects, ocular abnormalities, mental retardation and other
neurologic alterations, whereas others are asymptomatic
(Mitsubuchi et al. 2008; Phang et al. 2001). One case report
described a patient with psychomotor delay, right hemi-
paresis and ep ilepsy (Humbertclaude et al. 2001)and
another described a 10-year-old boy with HPI, neurologic
manifestations and a bnormalities of the central nervous
system white matter (Steinlin et al. 1989). Since HPI is not
necessarily associated with clinical manifestations this
disorder has been considered a benign condition in most
individuals under most circumstances (Phang et al. 2001).
HPII is a rare inherited autosomal recessive disorder of
amino acid metabolism, characterized by the hepatic
deficiency of delta-1-pyrroline-5-carboxylic acid dehydro-
genase activity. This enzyme catalyzes the conversion of
P5C, derived from proline or ornithine, to glutamate. This
disease is biochemically characterized by accumulating Pro
and P5C in plasma, urine and cerebrospinal fluid and,
quantitatively, the major metabolite that accumulates in the
tissue of patients with HPII is proline and not P5C (Fleming
et al. 1984; Flynn et al. 1989; Phang et al. 2001). The
plasma concentrations of Pro in HPII are greater than those
of HPI, can range from ten to fifteen times (500–3700 μM)
above normal values (51 to 271 μM), where in homo-
zygotes, the plasma levels of Pro almost always exceed
1500 μM. In addition, Pro levels in cerebrospinal fluid and
urine are correspondingly greater in type II homozygotes
than in type I subjects (Phang et al. 2001). The greater Pro
concentrations in patients with HPII seem to result from the
inhibition of proline oxidase by P5C (Valle et al. 1976).
Although a symptomatic hyperprolinemic siblings have
been identified in some pedigrees (Pavone et al. 1975;
Simila and Visakorpi 1967), a considerable number of
hyperprolinemic patients, so far detected, show neurologi-
cal manifestations including seizures and mental retardation
(Di Rosa et al. 2008; Phang et al. 2001). In this context, a
relationship between a high co ncentration of Pro and
neurological symptoms has been demonstrated in patients
with HPII (Flynn et al. 1989). In contrast to HPI, there is
persuasive evidence that HPII is causally associated with
neurologic manifestations (Phang et al. 2001).
Neuropsychiatric disorders associated
with hyperprolinemia
A 22q11.2 microdeletion causes velocardiofacial syndrome
(VCFS), an autosomal dominant genetic condition (Shprintzen
et al. 1981). Most of these deletions occur spontaneously and
Metab Brain Dis (2011) 26:159–172 161

its frequency is estimated at 1/4000 live births. Patients
affected by this syndrome present symptoms that include
cognitive dysfunction wi th mild mental retardation, and
behavioral difficulties (Karayiorgou and Gogos 2004).
Among children and adolescents, at tention defici t, hy-
peractivity, obsessive compulsive, mood and autism
spectrum disorders have been reported (Baker and Skuse
2005;Fineetal.2005;Vorstmanetal.2006;Vorstmanet
al. 2009). In adults, there is an increased (30-fold) risk of
schizophrenia (Karayiorgou and Gogos 2004; Mitsubuchi
et al. 2008).
The catechol-o-methyltransferase (COMT) and the
proline dehydrogenase genes (known as PRODH) are
functional candidate genes located in the 22q11 chromo-
somal region that may be able to modify the psychiatric
phenotype of people with 22q11 deletion syndrome and
psychiatric disease, including schizophrenia. COMT is an
enzyme that inactivates biologically-active catechols,
including the important neurotransmitters dopamine,
noradrenaline and adrenaline. These neurotransmitters
seem to be involved in numerous physiological and
physiopatological processes, including psychiatric disor-
ders (Chen et al. 2004;Levy2009; Tan et al. 2009).
As described above in Pro metabolism, proline
oxidase (POX) is a mitochondrial i nner membrane, also
known as pro line dehydr o gen ase that conver ts Pro to
P5C. The PRODH gene is widely expressed in brain and
other tissues (Gogos et al. 1999). Also, it has been
established that P5C can be converted to glutamate and
GABA, two neurotransmitters implicated in the physiol-
ogy of schizophrenia and other psychiatric illnesses
(Roussos et al. 2009; Van Spronsen and Hoogenraad
2010). In addition, evidence to support the role of Pro in
brain function includes the presence of high affinity Pro
transporter molecules (Na
+
/Cl
−
-dependent proline
transporter-PROT),whichbelongtoalargesuperfamily
of neurotransmitter transporters, in a subset of glutama-
tergic neurons in the rat brain, including the hippocampus
(Schaffer collateral commissural and lateral perforant
pathways) (Cohen and Nadler 1997a; Fremeau et al.
1992) a nd corticostriatal pathways ( Renick et al. 1999).
Studies also show that mice lacking the PRODH gene
present prepulse inhibition and an impairment of learning
and memory (Paterlini et al. 2005). It has been demon-
strated that moderate hyperprolinemia is an intermediate
phenotype associated to c ertain forms of psychosis such as
schizoaffective disorder, but not with schizophrenia or
bipolar disorder (Jacquet e t al. 2005). On the other hand, a
recent study suggests that Pro metabolism is specifically
associated with schizophrenia (Oresic et al. 2011).
Interestingly, it has been also shown that urinary hydroxy-
proline and Pro concentrations are influenced by stress
and anxiety (Lee et al. 2011).
Behavioral and neurochemical impairments caused
by L-proline
Despite the different clinical and neuropathological
conditions, the pathomechanisms associated with various
diseases that affect the central nervous system (CNS)
seem to have a number of common features in their
processes. In this context, it has been suggested that
energy metabolism dysfunction, glutamate excitotoxicity,
oxidative stress, purinergic and cholinergic impairment
have an important role in the physiopathology of these
disorders, which seem to be associated with cognitive
deficits, as observ ed in Par kinson’sandAlzheimer’s
diseases, cerebral ischemia, amongst others (Abbracchio et al.
2009;Beal20 07;Dumontetal.2010; Halliwell
and Gutteridge 1985; Halliwell and Gutteridge 2007;
Kapogiannis and Mattson; Kim et al. 2010; Lees 1993;Lin
and Beal 2006; Maragakis and Rothstein 2001; Reddy and
Reddy 2011; Zhang et al. 2010). The effects of Pro will be
reviewed on some behavioral and neurochemical aspects
such as:
Behavior
With regard to Pro, behavioral studies show that animals
that bear a mutation in the gene that encodes proline
oxidase exhibit high plasma Pro levels and depressed
locomotor activity (Hayward et al. 1993; Kanwar and
Manaligod 1975; Moreira et al. 1989). Intracerebral
administration of Pro produces retrograde amnesia and
disrupts the formation of new memories in chickens; the
amnesic effect of Pro does not depend on inhibition of brain
protein synthesis, but suggests the involvement of gluta-
mate in this process (Cherkin et al. 1976, 1981;Van
Harreveld and Fifkova 1974). In addition, using an
experimental model of chronic hyperprolinemia in devel-
oping rats, it was shown that Pro impairs habituation
(Moreira et al. 1989
) and spatial memory in adult animals
(Bavaresco et al. 2005; Delwing et al. 2006a). Hystological
studies showed that rats subjected to same experimental
model of hyperprolinemia presented degenerative changes
in brain (Shanti et al. 2004).
Glutamatergic system
It is well known that glutamate is the major excitatory
neurotransmitter in the brain and is present at millimolar
concentrations in the adult CNS. It is released in milliseconds
from presynaptic nerve terminals, in a Ca
2+
dependent
manner, into the synaptic cleft where it diffuses to interact
with its corresponding receptors on the postsynaptic face of
an adjacent neuron. Glutamate receptors are divided into two
groups, ionotropic (representing ligand-gated ion channel:
162 Metab Brain Dis (2011) 26:159–172

NMDA, AMPA, kainate) and metabotropic (coupled to
protein G).
Normal excitatory neurotransmission is essential for
plastic p rocesses, which underlie memory and learning
(Reis et al. 2009), developmental (Segovia et al. 2001) and
environmental adaptation (Ozawa et al. 1998). In contrast,
an excessive g lutamate excitation caused by enhanced
release of glutamate in the synaptic cleft gives rise to
prolonged stimulation of its receptors and, via a complex
pathomechanism, may induce devastation of the postsyn-
aptic neurons. This process of glutamate toxicity was first
described by Lucas and Newhouse (1957), who showed
degeneration of the inner layers of the retina following
subcutaneous injections of glutamate in infant mice.
Approximately one decade after, Olney (1969) coined the
term “glutamate excitotoxicity”; from then on this process,
which can be thought of as normal physiological response
to a CNS insult, has been implicated in the pathogenesis of
various acute and chronic disorders (Maragakis and
Rothstein 2001; Meldrum 1994).
It has been shown that glutamatergic excitotoxicity may
be linked with mitochondrial dysfunction, because energy
impairment can lead to partial membrane depolarization,
resulting in relief of the magnesium blockage of the N-
methyl-D-aspartate (NMDA) channel. Thus, even in phys-
iological concentrations, glutamate via the NMDA receptor
increases Ca
2+
influx, which promotes many normal
intracellular signaling pathways; however excessive influx
promotes pathological signaling, contributing to cell injury
and death via production free radicals such as reactive
species of oxygen (ROS) and nitric oxide (NO), as well as
other enzymatic processes (Nakamura and Lipton 2010).
The maintenance of below neurotoxic levels of extracel-
lular glutamate concent rations at glutamatergic synapses in
the brain is an essential role of glial cells and this is
achieved through high-affinity sodium-dependent glutamate
transporters, namely GLAST and GLT-1, present mainly in
astrocytes (Anderson and Swanson 2000; Attwell 2000;
Chen and Swanson 2003; Danbolt 2001). Furthermore,
glutamate uptake is inwardly associated with transport of
sodium, resulting in an increase in the intracellular sodium
concentration (Chatton et al. 2000; Rose and Ransom 1996;
Voutsinos-Porche et al. 2003). Such sodium elevations
stimulate Na
+
,K
+
-ATPase and cause increased ATP con-
sumption and glucose uptake by astrocytes (Chatton et al.
2000; Loaiza et al. 2003; Pellerin and Magistretti 1994;
Porras e t al. 2008). Since free radicals are highly reactive
molecules and can modify proteins in many different ways,
it has been suggested that they can inhibit glutamate uptake
in astrocyte cultures (Piani et al. 1993; Sorg et al. 1997;
Volterra et al. 1994).
Excitotoxic properties have been also demonstrated for
Pro, which at higher concentrations activates NMDA and
AMPA receptors, suggesting that Pro might potentiate
glutamate transmission (Cohen and Nadler 1997b; Fremeau
et al. 1992; Nadler 1987; Nadler et al. 1992). It has also
been shown that Pro, in vitro (added to assay), decreases
glutamate uptake in the cerebral cortex and hippocampus
slices of rats. On the other hand, Pro administration at high
concentrations to plasma (similar to those found in hyper-
prolinemia) reduced glutamate uptake in the cerebral cortex
slices of rats, but did not alter this parameter in the
hippocampus slices (Delwing et al. 2007d). Knowing that
glutamate uptake by astrocytes is the main process involved
in pathophysiological neuroprotection against glutamatergic
excitotoxicity, by reducing the extracellular glutamate
concentrations below toxic levels, this inhibitory effect
caused by Pro corroborates with previous studies that
suggest that this amino acid has excitotoxic properties
(Cohen and Nadler 1997b; Fremeau et al. 1992; Nadler
1987; Nadler et al. 1992). In addition, it is possible that the
reduction in glutamate uptake is mediated by the reduction
in Na
+
,K
+
-ATPase activity caused by Pro, leading to
increased extracellular glutamate concentrations and pro-
moting excitotoxicity. Thus, a reduct ion in gluta mate
uptake and Na
+
,K
+
-ATPase activity may act synergistically
and cooperate to provoke the brain damage that is
characteristic of hyperprolinemia.
Energy metabolism
Mitochondria are responsible for the energy supply of cells;
besides playing crucial roles in other cell processes such as
signaling, calcium homeostasis, cell cycle regulation pro-
cesses, apoptosis, free radical production and thermo-
genesis, which are crucial to cell development. In
performing the primary m etabolic pathways for ATP
production, these organelles consume the greatest amount
(85–95%) of oxygen in cells to allow oxidative phosphor-
ylation, which depends on the electron transport chain
through the action of various respiratory enzyme complexes
located in the inner mitochondrial membrane. Impaired
electron transport , in turn, leads to decreased ATP produc-
tion, increased formation of toxic free radicals, and altered
Ca
2+
homeostasis. These toxic consequences of transport
chain dysfunction may sustain further mitochondrial dam-
age, including oxidation of mitochondria, DNA, protein
and lipids, and may open of the mitochondrial permeability
transition pore that, together, can lead to cell death by both
apoptotic and necrotic pathways (Dumont et al. 2010;
Mancuso et al. 2010; Reddy et al. 2008; Solaini et al. 2010;
Wallace 2005). In this context, increasing evidence sustains
the hypothesis that mitochondria energy metabolism under-
lies the pathogenesis of neurodegenerative, psychiatric and
others (Beal 2000; Beal 2007; Dumont et al. 2010; Lin and
Beal 2006; Reddy and Reddy 2011; Rezin et al. 2009;
Metab Brain Dis (2011) 26:159–172 163

Solaini et al. 2010; Sullivan and Brown 2005; Zeviani and
Carelli 2007).
Although Pro can be considered a direct substrate for
ATP production via proline oxidase/P5C dehydrogenase
and/or participate in a metabolic interlock with glucose-6-
phosphate (pentose phosphate pathways) via P5C redutase
and/or via anaplerotic reactions (alpha-ketoglutarate/Krebs
cycle) (Phang et al. 2008a, b), high levels of Pro lead to
alterations in the cell redox state, resulting in decreased
oxygen consumption and lower oxidation of the NADH
formed by the cell (Phang et al. 2001). In addition, previous
findings have demonstrated that acute and chronic Pro
administration decrease cytochrome c oxidase activity in
the cerebral cortex of r ats, indicating that Pro also
compromises the respiratory chain (Delwing et al. 2007a).
Interestingly, more recently we have shown that a single
administration of high Pro increases the activity of brain
succinate dehydroge nase ( Ferreira et al. 2010). This
phenomenon could have occurred to compensate for the
decrease in mitochondrial electron transport generated by
the inhibition of cytochrome c oxidase, which could result
in the production of free radicals.
Oxidative stress
Oxidative stress is defined as an imbalance between
formation and scavenging (neutralizing) of free radicals
and it is presumed to be involved in the physiopathology of
many diseases that affect CNS, including ischemia, epilep-
sy, and neurodegenerative and metabolic diseases (Allen
and Bayraktutan 2009; Beal 1995; Droge 2002; Halliwell
and Gutteridge 1985; Matte et al. 2006; Matte et al. 2009;
Peker et al. 2009; Wajner et al. 2007; Waldbaum and Patel
2010; Wyse et al. 2002; Zhang et al. 2007). It has been
shown that the brain is highly susceptible to oxidative stress
due to the elevated rate of oxygen consumption, presence of
high levels of polyunsaturated fatty acids and low cerebral
anti oxidant defenses compared to other tissues (Floyd
1999; Halliwell 2006), a fact that makes it more vulnerable
to reactive oxygen species. Inherently, it has been shown
that, during the Pro oxidation by proline oxidase, the
electrons from Pro can reduce oxygen to yield supero xide
(Liu et al. 2005). It has also been suggested that when the
activity of P5C dehydrogenase is decreased, P5C-Pro cycle
can transfer more electrons to the mitochon dria electron
transport chain and pro duce reactive oxyg en species
(Szabados and Savoure 2010). This phenom enon may be
explained by the increase in Pro. Interestingly, we have
shown that high Pro concentrations, similar to those found
in hyperprolinemia, induce lipoperoxidation and reduce
non-enzymatic and enzymatic antioxidant defenses in rat
brain, suggesting that Pro elicits oxidative stress (Delwing
et al. 2003).
Na
+
,K
+
-ATPase activity
Na
+
,K
+
-ATPase is a plasma membrane-embedded enzyme
responsible for the active transport of sodium and potassium
ions in the nervous system, maintaining and re-establishing,
after each depolarization, the electrochemical gradient neces-
sary for neuronal excitability and regulation of neuronal cell
volume. Because of the frequent perturbation of ion homeo-
stasis, resulting from constant neural activity, the workload of
Na
+
,K
+
-ATPase is high, consuming about 40–50% of the
ATP generated in brain (Erecinska and Silver 1994 ).
Decreased Na
+
,K
+
-ATPase is found in various neuropatho-
logical conditions, including cerebral ischemia (Wyse et al.
2000)epilepsy(Grisar1984), and neurodegenerative disor-
ders (Hattori et al. 1998;Lees1993; Pisani et al. 2006;
Vignini et al. 2007). Additionally, some psychiatric disorders
are believed to be associated with perturbation of ion
homeostasis, and earlier studies have shown that Na
+
,K
+
-
ATPase activity is decrea sed in depression and o ther
psychiatric disorders (Goldstein et al. 2006; Zugno et al.
2009). Exciting new findings have revealed additional
fundamental roles for Na
+
,K
+
-ATPase as a signal transducer
and modulator of growth, apoptosis, cell adhesion and
motility (Aperia 2007). We have shown that Pro in vitro
and in vivo (acute and chronic) decreases Na
+
,K
+
-ATPase
activity in cerebral cortex and hippocampus of rats (Pontes et
al. 2001). This inhibition may be explained by free radical
production by Pro in the brain, which damages the membrane
lipid bilayer containing Na
+
,K
+
-ATPase. Moreover, this
enzyme is known to be highly susceptible to changes in
the composition of membrane lipids (Jamme et al. 1995;
Murali et al. 2008; Rauchova et al. 1999; Zhang et al. 2007).
Besides, reduction of energy metabolism caused by Pro with
consequent decrease of ATP levels may impair the activity of
Na
+
,K
+
-ATPase and consequently the electrochemical gradi-
ent necessary for maintain neuronal excitability.
More recently, we have shown that hyperprolin emia
increases ganglioside content in the cortex and hippocam-
pus of rats , while this membrane lipid content was not
altered in the hypothalamus and cerebellum. In addition,
phospholipid and cholesterol contents were not modified in
any of the structures studied, suggesting that Pro affects in a
distinct manner different cerebral regions concerning the
lipid composition of the cell membranes, reflecting on its
distribution in the cortex membrane microdomains. Among
the consequences of these phenomena, distinct modulations
in enzymes such as Na
+
,K
+
-ATPase and synaptic transmis-
sion may be suggested (Vianna et al. 2008).
Creatine kinase activity
Creatine kinase (CK), also known as creatine phosphoki-
nase, plays a key role in energy metabolism (Eppenberger
164 Metab Brain Dis (2011) 26:159–172

et al. 1967). This enzyme catalyzes the reversible transfer
of the phosphoryl group from phosphocreatine to ADP,
to regenerate ATP. CK is especially fundamental in
tissues w ith high and fluctuating ATP consumption such
as skeletal and cardiac muscle, brain and retina, where
phophocreatine serves as an energy reservoir for the
rapid regeneration of ATP. The CK enzyme consists of
two subunits, B (brain type) and M (muscle type), which
are compartmentalized specifically in the places where
energy is produced or utilize d (Wallimann et al. 1992).
Different cells can contain several different CK isoforms,
and the isoenzyme patterns differ among organs. Two
isoforms, M-CK and ubiquitous B-CK, are cytosolic, and
two others, Mi b-CK and ubiquitous Mi a-CK, are
mitochondrial (Wallimann et al. 1998). CK is inhibited
by oxidative stress (Delwing et al. 2007b;Ferreiraetal.
2007; Zugno et al. 2007) and its activity is decreased in
neurodegenerative, metabolic and psychiatric diseases
(Aksenov et al. 2000; David et al. 1998; Delwing et al.
2007b; Zugno et al. 2007). It has been shown that in vitro
Pro and acute hyperprolinemia administration decrease
CK in the cerebral cortex of rats (Kessler et al. 2003 )and
this inhi bitory effect on the enzyme may potentially impair
energy homeostasis, since it is known that inhibition in
this enzyme can contribute to cell death (Tomimoto et al.
1993).
Acetylcholinesterase and NTPDases activities
ATP and acetylcholine serve as extracellular signaling
substanc es in the nervous sy st em and in other tiss ues .
They can even be co-stored within synaptic vesicles and
co-released from cholinergic nerves. Neither ACh nor
ATP can be directly recycled. They must first be
degraded to either choline o r adenosine and those
substances are transported back into cells. Acetylcholine
is specifically hydrolyzed by acetylcholinesterase
(AChE). This enzyme contributes to the integrity and
permeability of the synaptic membrane that occurs
during neurotransmission and conduction (Grafius et al.
1971). In addition to the classic enzymatic role, AChE
also has some non-classical properties concerning CNS
development. For instance, it is accepted that AChE has
functions a ssociated with adhesi on, neurite growth, circuit
formation and apoptosis (Johnson and Moore 2000; Layer
and Willbold 1995; Sharma and Bigbee 1998;Silmanand
Sussman 2005; Soreq and Seidman 2001;Zhangetal.
2002). In this context, it has been shown that AChE forms
a complex with amyloid precursor protein and perlecan
that seems to be involved in substratum adhesion and
polarized migration of adherent cells (Anderson e t al.
2008). This enzyme is inhibited by free radical and/or
oxidative stress and its cholinergic and non-cholinergic
actions may play a role in schizophrenia, neurodegenera-
tive and neurometabolic diseases (Arendt et al. 1992;
Cummings 2000; Henderson et al. 1996). Importantly, Pro
has been demonstrated to act as an AChE inhibitor, which
results in higher synaptic levels of acetylcholine (Delwing
et al. 2005b).
Since ATP is an unstable molecule that cannot cross
biological membranes by diffusion or active transport, its
breakdown is carried out by specific enzymes located on
the outer surface of cells, called ecto-enzymes (Plesner
1995). ATP and the other extracellular nucleoside tri- and
diphosphates can be hydrolyzed by ectonucleotidases,
including ecto-nucleoside triphosphate diphosphohydrolase
(NTPDases), which are enzymes that hydrolyze ATP and
ADP, and are present in many tissues, including the
vascular system (Ralevic and Burnstock 2003) and CNS
of several species (Sarkis et al. 1995). The AMP produced
is subsequently hydrolyzed to adenosine by an ecto-5′-
nucleotidase (CD73, EC 3.1.3.5), which constitutes the
rate-limiting step in this pathway (Battastini et al. 1995;
Zimmermann 1992). Althoug h the extracellular concentra-
tions of ATP are considerably lower than its intracellular
concentrations (3–10 mM), the extracellular ATP and its
breakdown products, ADP and adenosine, have pro-
nounced effects in a variety of biolo gical processes,
including neurotransmission, muscle contraction, cardiac
and platelet function, and vasodilatation (Agteresch et al.
1999). In addition, adenosine is an importan t endogenous
neuromodulator and an inhibitor of platelet aggregation
(Cristalli et al. 1995). On the other hand, extracellular
nucleotides may be related to the development of several
pathologies including disorders of the immune system,
epilepsy and neurodegenerative, vascular and neurometa-
bolic diseases (Bohmer et al. 2004; Bonan et al. 2000;
Bours et al. 2006; Delwing et al. 2006b, 2007c; Seye et al.
2003; Wyse et al. 1994, 1995). In regard to Pro, it has been
shown that t his amino acid does not alter nucleotide
hydrolysis when added to enzyme assays, but when
administered acutely or chronically, it decreases ATP
hydrolysis in rat cerebral cortex synaptosomes; ADP and
AMP hydrolysis are not altered by Pro administration
(Delwing et al. 2007e). Chronic hyperprolinemia decreased
ATP and ADP hydrolysis that may result in high levels of
extracellular ATP, suggestin g that this inhibition in ATP
hydrolysis can disturb a number of processes related to
brain excitability. Pro (in vitro) significantly increased ATP,
ADP and AMP hydrolysis in rat serum (Delwing et al.
2006b). It seems reasonable to postulate that Pro could
alter, at least in part, the responses mediated by adenine
nucleotides in the central nervous and peripheral systems of
hyperprolinemic patients.
The neurochemical effects of Pro are summarized in
Fig. 2 .
Metab Brain Dis (2011) 26:159–172 165

Possibilities for neuroprotection
The investigation of neuroprotection is one of the main
focuses of neuroscientists, since understanding the control
mechanisms of neuronal damage, caused by a neurotoxin
that is accumulated in a disorder, allows the development of
new tools for preventing it. Oxidative stress plays a critical
role in the physiopathology of most of the important neural
pathologies, including stroke, epilepsy, Parkinson’s disease,
Alzheimer’s disease and more recently neu rometabolic
disease (Behl 2005; Halliwell 1996; Zarkovic 2003 ). It is
known that, in order to defend themselves against oxidative
damage, cells develop antioxidant enzymes such as super-
oxide dismutase (SOD), catalase (CAT) and glutathione-
peroxidase (GPx). Cells also utilize non-enzymatic antiox-
idants defenses such as vitamin E (alph a-t ocop he rol ),
vitamin C (ascorbic acid) and gluthatione (GSH) (Halliwell
2006).
Since oxidative stress is an imbalance between formation
and removal of free radicals by scavengers and Pro
increases lipoperoxidation and decreases enzymatic and
non-enzymatic antioxidant defenses, strategies to prevent
brain oxidative damage seem to be adequate. In this
context, both water-soluble (vitamin C) and lipid soluble
(vitamin E) nutrients comprise an important characteristic
of the antioxidant defense system, particularly in brain cells
(Zaidi and Banu 2004). Based on this, we investigated the
effect of administration of classical antioxidants, vitamins E
and C, on the alterations in biochemical parameters namely
energy metabolism, Na
+
,K
+
-ATPase, glutamate uptake,
enzymes of cholinergic and purinergic systems, as well as
on memory deficit caused by hyperprolinemia in rats. It is
amply described in the literature that these factors seem to
be associated with the physiopathology of various diseases,
affecting CNS, at least in part, by the involvement of free
radical and/or oxidative stress.
Vitamin E, a generic term for all tocopherols and its
derivatives, is essential for normal neurological function
(Muller and Goss-Sampson 1989; Sen et al. 2004; Takada
and Suzuki 2010). Eight isomers have been found to have
vitamin E activity: alpha-, beta-, gamma- and delta-tocopherol
and alpha-, beta-, gamma- and delta-tocotrienol, which are
amply distributed in nature. Although tocopherols are
predominantly found in corn, soybean, and olive oils,
tocotrienols are found in palm, rice bran and barley oils (Sen
et al. 2004; Traber and Packer 1995; Traber and Sies 1996).
In contrast to plants, mammalian tissues contain almost
exclusively alpha-tocopherols, where the highest content of
this compound is found in adipose tissue, while erythrocytes
have a relatively low content (Azzi and Stocker 2000).
Because of its hydrophobicity, alpha-tocopherol is mainly
transported in association with lipoproteins in the plasma
compartment. It has been shown that liver, prostate and brain
tissue express a cytosolic tocopherol binding protein (Stocker
1999). Often the term, vitamin E, is synonymously used as
alpha-tocopherol.
Pro
Pro
Pro
Pro
Pro
Pro
Pro
+
_
_
_
+
+
_
_
_
ATP
ATP
ATP
ACh
ACh
Glu
ACh
ATP
Glu
ATP
Antioxidant
defenses
Postsynaptic
neuron
Lipid
peroxidation
Presynaptic
neuron
Astrocyte
lGuta
m
at
e
r
t
t
a
n
s
p
or e
r
A
R
n
C
h
C
m
A
hR
AChE
NTPDase
P2
receptor
P1
c
re
eptor
+
+
Na ,K TP
s
A
ae
NMDA
g
utamate
l
ec pt
rr
e
o
CK
+
+
NAa
a,
K
T
Pse
METC
Pro
_
ROS
5’-NT
ATP
Fig. 2 Suggested mechanisms of
neurochemical effects in hyper-
prolinemia. Accumulating proline
may exert their actions mainly by
three possible pathomechanisms,
namely oxidative stress, energy
deficit and excitotoxicity. This
amino acid may induce genera-
tion of reactive oxygen species
(ROS) and reduce tissue antioxi-
dant defences (oxidative stress).
Proline is also able to inhibit key
enzymatic activities of energy
metabolism, such as Na
+
,K
+
-
ATPase, creatine kinase and
enzymes of mitochondrial elec-
tron transfer chain (METC),
leading to diminished ATP levels
(energy failure) and increased
ROS which might cause lipid
oxidation, and protein and DNA
damage. Proline may also
decrease glutamate (Glu) uptake
in presynaptic neurons, causing
excitotoxic cell death by over-
stimulation of NMDA receptors.
NMDA N-methyl-D-aspartate
166 Metab Brain Dis (2011) 26:159–172

Vitamin E is the major lipid-soluble vitamin; its protection
against lipid peroxidation is well described, and includes
scavenging of lipid peroxyl radicals to break membrane-
damaging chain reactions (Burton et al. 1990; Sandy et al.
1988). Lipid peroxyl radicals present in the plasma mem-
brane interact with alpha-tocopherol, resulting in the forma-
tion of a lipid peroxide and the alpha-tocopheroxyl radical.
Ascorbic acid (vitamin C) plays an important role together
with the lipophilic antioxidant, alpha-tocopherol, in protecting
the membrane from oxidative stress. This is, in part, because
ascorbic acid can regenerate reduced alpha-tocopherol present
in the cell membrane. During this process, alpha-tocopherol is
converted to the tocopheryl radical, requiring ascorbic acid for
its regeneration back to reduced alpha-tocopherol (Buettner
1993;CarrandFrei1999;Freietal.1990;McCay1985),
thus increasing its antioxidant activity. Ascorbic acid traps
hydroxyl and superoxide radicals (Halliwell and Gutteridge
2007). This combination of alpha-tocopherol and ascorbic
acid has proven to be effective in preventing biochemical and
behavioral deficits produced in animal models of metabolic
diseases (Wyse et al. 2002; Delwing et al. 2007a), as well as
in age-related motor and memory deficit of rats (Bickford et
al. 2000).
It has been shown that the pretreatment with alpha-
tocopherol and ascorbic acid, at ineffective doses per se,
completely prevents the spatial memory impairment caused
by Pro, supporting the notion that oxidative stress is
probably involved in this mechanism. This is in agreement
with previous studies from our laboratory reporting that the
administration of these vitamins prevents memory impair-
ment in human and animal models (Delwing et al. 2005a;
Engelhart et al. 2002 ; Monteiro et al. 2005; Reis et al. 2002;
Wengreen et al. 2007). Therefore, the imbalance between
free radical production and antioxidant defenses caused by
Pro administration could have also contributed to the spatial
navigation deficits found in rats. The se findings are in
agreement with evidence that oxidative stress and reactive
oxygen species might be involved in memory modulation
mechanisms (Abidin et al. 2004; Bickford et al. 2000 ;
Cantuti-Castelvetri et al. 2000; Silva et al. 2004). Another
line of evidence supporting the role of oxidative stress in
behavior emerges from studies showing that alpha-
tocopherol improves cognitive function of patients with
temporal lobe radionecrosis (Chan et al. 2004) and may be
beneficial in lowering the cognitive impairment in patients
with Alzheimer’s disease (Mecocci 2004). Orally supple-
mented vitamin E reaches the cerebrospinal fluid and brain
and may be an interesting approach (Vatassery 1998).
Studies also show that alpha-tocopherol provides pro-
tection to cells exposed to oxidative stress damage by
scavenging free radicals, stabilizing membranes and block-
ing the cascade of biochemical routes involved in cell death
(Kelly 1998). Interestingly, pretreatment with alpha-
tocopherol plus ascorbic acid prevents the reduction of
lipoperox id ati on, a nti oxi da nt de fen se s, Na
+
,K
+
-ATPase,
acetylcholinesterase, as well as cytochrome c oxidase in
the rat brain, caused by Pro administration (Bavaresco et al.
2003; Delwing et al. 2005a, 2006a, 2007a; Franzon et al.
2003). However, pretreatment with alpha-tocopherol and/or
ascorbic acid did not prevent the effect of Pro administra-
tion on glutamate uptake. Alpha-tocopherol per se reduced
glutamate uptake in the cerebral cortex slices of hyper-
prolinemic rats. These results reinforce the theory that the
reduction in glutamate uptake is probably not caused by
free radicals or, at least, by those scavenged by alpha-
tocopherol and ascorbic acid. Regarding the inhibitory
effect of alpha-tocopherol on glutamate uptake, no studies
are available to demonstrate such effects and new studies
should be performed to elucidate such mechanisms.
In summary, it is evident that high Pro concentrations
provoke memory deficit and/or other neurochemical e ffects,
which seem to be associated with the imbalance between
free radical production and antioxidant defenses caused by
this amino acid. Thus, it is possible that oxidative stress
could contribute to the effects of Pro on energy metabolism,
excitotoxicity, and cholinergic and purinergic systems,
which may act synergistically and cooperate, at least in
part, with the brain dysfunction that is characteristic of
hyperprolinemia. In support of this hypothesis, pretreatment
with classical antioxidants (alpha-tocopherol and ascorbic
acid) prevented various actions of Pro. We argue that
advances in the understanding of the effects of Pro in the
brain may represent a promising goal for neuroprotective
strategies for diseases that present hyperprolinemia such as
inborn errors of metabolism, schizophrenia and others.
Acknowledgements We dedicate this review to Dr. Clovis M.D.
Wannmacher and Dr. Moacir Wajner, who were supervisors of the
Master degree and of the PhD degree of Dr. Angela T.S. Wyse.
Professor Wannmacher initiated work with Inborn Errors of Metabo-
lism (IEM) in Brazil in 1970, after which Dr. Moacir Wajner and Dr.
Roberto Giugliani were incorporated and an IEM group was created in
1998, the group of IEM of the Metabolism of Department of
Biochemistry, ICBS, Federal University Federal of Rio Grande do
Sul is formed by Professors Clovis Wannmacher, Moacir Wajner,
Carlos S. Dutra-Filho and Angela T.S. Wyse.
We thank Luiz Eduardo Baggio Savio and Andréa Kurek Ferreira
for figure designs .
Conflict of interest The authors declare that they have no conflict of
interest.
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