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Phorochembrry
und
Photobiology
Vol.
53,
No.
4, pp. 549-553.
1991
Printed
in
Great Britain.
All
rights
rcservcd
0031-8655/91 $03.00
+O.W
Copyright
@
1991 Pergamon Prcss plc
RESEARCH
NOTE
THE PHOTODEGRADATION OF PORPHYRINS IN CELLS
CAN
BE
USED
TO
ESTIMATE THE LIFETIME
OF
SINGLET OXYGEN
JOHAN
MOAN* and KRISTIAN
BERG
Institute for Cancer Research, 0310 Montebello,
Oslo
3, Norway
(Received
25
June
1990;
accepted
3
October
1990)
Abstract-NHIK 3025 cells were incubated
with
Photofrin I1 (PII) and/or tetra (3-
hydroxypheny1)porphyrin (3THPP) and exposed to light at either 400 or 420 nm, i.e. at the wavelengths
of the maxima
of
the fluorescence excitation spectra of the two dyes. The kinetics
of
the photo-
degradation of the dyes were studied. When present separately in the cells the two dyes are photode-
graded
with
a similar quantum yield. 3THPP is degraded
2-6
times more efficiently
by
light quanta
absorbed by the fluorescent fraction of 3THPP than by light quanta absorbed
by
the fluorescent
fraction of PI1 present
in
the same cells. The distance diffused
by
the reactive intermediate, supposedly
mainly
lo2,
causing the photodegradation was estimated to be on the order of 0.01-0.02 pm, which
corresponds to a lifetime of 0.01-0.04
ps
of
the intermediate
in
the cells. PI1 has binding sites at
proteins
in
the cells as shown
by
an energy transfer
band
in
the
fluorescence excitation spectrum at
290
nm.
During light exposure this band decays faster than the Soret
band
of
PI1 under the present
conditions. Photoproducts
(lo2
etc.) generated at one binding site contribute significantly in the
destruction of remote binding sites.
INTRODUCTION
Porphyrins are degraded
or
modified by light (Cox
et
af.,
1982; Krieg and Whitten, 1984). Considerable
interest has been focused on such photodegradation
(often called photobleaching), since it can be taken
advantage
of
in photodynamic therapy of cancer
(Moan, 1986; Dougherty, 1987, Mang
et
af.,
1987).
In fact, the concept of photobleaching has been
successfully applied in human clinical trials (Mang
et
af.,
1990). The mechanisms of photodegradation
are different for porphyrins in solution and for por-
phyrins in biological systems (Krieg and Whitten,
1984). In the latter case the rate constants are sig-
nificantly larger and different products are formed.
To
a first approximation the photodegradation fol-
lows first order kinetics and the rate constants are
independent
of
the initial porphyrin concentration,
both in cells and in tissues (Mang
er
af.,
1987).
This indicates that an excited porphyrin molecule is
degraded either by products formed by itself
or
by
direct interaction with its surrounding. However,
this is a crude approximation since the kinetics devi-
ate from first order after extended (but still clinically
relevant) exposure times. Furthermore, the rate
constants for degradation
of
Photofrin
I1
(PII)? are
*To
whom correspondence should be addressed.
tAbbreviations:
HPLC, high performance liquid chroma-
tography; MEM, minimal essential medium, NCS, new-
born
calf
serum, PII, Photofrin 11; PBS, Dulbeccos
phosphate buffered saline; 3THPP, tetra (3-
hydroxyphenyl) porphyrin.
PAP
53:4-I
larger when cells are exposed to light in a
D20-
buffer than when they are exposed in a
H20
buffer,
indicating that '02-production
plays
a role (Moan
et
af.,
1988).
In order to elucidate the photobleaching mechan-
isms further we have incubated cells with two differ-
ent porphyrins, Photofrin
I1
(PII) and tetra
(3-
hydroxylphenyl) porphyrin (3THPP). which have
different fluorescence excitation and emission spec-
tra. Thus, even when they are present in the
same
cell they can be excited and monitored with some
selectivity. Both are negatively charged lipophilic
dyes which, according to fluorescence microscopic
studies (data not shown), localize almost identically
in the cells, presumably in membrane structures. In
such a system it is possible to study the degradation
of
one dye caused by excitation of the other dye,
and to estimate the distance travelled by reactive
intermediates before deactivation.
MATERIALS AND METHODS
Chemicals.
Photofrin I1 was obtained from Photomed-
ica, Raritan, NJ. The drug was stored frozen
in
small vials
for up to
8
months before being used in the present
experiments. It was thoroughly checked by HPLC, and by
cell survival experiments that the drug did not change in
its characteristics during the time of storage. The pro-
cedures of HPLC and cell survival experiments are
described elsewhere (Moan
et a/.,
1982; Moan and
Sommer, 1983). The 3THPP was obtained from Porphyrin
Products, Logan,
UT.
Stock solutions (0.1 mg/mL) were
prepared
in
0.05
M
NaOH as earlier described (Moan
et
al.,
1987).
Cell cultivation.
Cells of the line NHIK 3025 (derived
549
550 JOHAN MOAN and KRlsnAN BERG
from a human carcinoma
in
situ)
were cultivated in
MEM
containing
10%
NCS. For fluorescence experiments
1W
cells were incubated in 25 cm2 Falcon tissue culture dishes.
Six hours later the medium was changed to MEM with
3% NCS with either 25 pg PII/mL and/or
1
pg
3THPPhL. These concentrations were chosen on the
basis
of
pilot experiments, in view
of
optimal analysis
of
the fluorescence spectra
of
samples containing both drugs.
At these concentrations the photosensitivity and the
flu-
orescence yield were about a factor
2
larger for cells with
PI1 than
for
cells with 3THPP, when exposed to equal
fluence rates at the optimal wavelengths
(400
nm for PI1
and 420 nm for 3THPP).
After an incubation period
of
18 h with the dyes the
cells were washed five times in icecold PBS, brought into
suspension by trypsinization, washed once more with PBS
and finally suspended in PBS at a concentration
of
approx.
loh
cellshl-as determined by Biirker chamber counting.
Irradiation
of
the samples.
The light source was a
900
W
Osram high pressure xenon lamp fitted to a Bausch
&
Lomb grating monochromator. The bandwidth
of
the light
was 15 nm as measured with a second monochromator
(Jarrel Ash) with narrow slits
(Ah
=
1
nm) and a UDT
1 la detector (United Detector Technology, Santa Monica,
CA). This detector is calibrated and was
also
used
to
determine the fluence rates at the position
of
the cells:
30 mW/cm2 at both wavelengths
(400
and 420 nm). The
samples were irradiated in a fluorescence cuvette
(3
x
3
x
20
mm)
and were gently stirred during the light
exposure.
Fluorescence measurements.
Fluorescence spectra were
recorded by means of a Perkin-Elmer LS 5 spectrofluor-
imeter equipped with a Hamamatsu R928 red sensitive
photomultiplier tube. A 580 nm cut-off filter was used to
remove scattered light from the light entering the emission
slit, which was usually set to give
AX
=
5
nm. The distor-
tion
of
the spectra resulting from such slit widths were
taken into consideration in the evaluation
of
the data but
found
to
be
of
minor importance.
Using a microcuvette with a cross section
of
only
3
x
3
mm,
inner filter effects played no significant role.
RESULTS
AND
DISCUSSION
We have earlier shown that the fluorescence
quantum yields of PI1 and 3THPP in NHIK
3025
cells are similar to within 30% (Moan
et
al.,
1987).
Therefore, and since it is difficult to record the
absorption spectra of dyes in cells with accuracy, we
decided to base our calculations
on
the fluorescence
measurements.
In
fact, it is more relevant to use
fluorescence spectra than absorption spectra for
investigations
of
this type, since the aggregated frac-
tion of the dyes in the cells is both non-fluorescent
and photochemically inactive. This has been shown
by means of absorption, fluorescence and action
spectroscopy for both dyes considered in the present
work (Western and Moan, 1988; Moan and
Sommer, 1984).
Figure 1 shows the fluorescence excitation spectra
of cells with PII, 3THPP and with the mixture of
the two dyes. There is a peak in the spectra at about
290 nm, which, in the case
of
PII, is entirely due to
energy transfer from proteins containing aromatic
amino acids to porphyrin molecules (Moan
et
al.,
1988). Thus, using the known shape
of
the excitation
spectra of PI1 in liposomes or lipids with properties
ml
Loo
m
Wavelength
hm)
Figure
1.
Fluorescence excitation spectra
of
PI1 and
3THPP in NHIK 3025 cells 18 h incubation in MEM with
3% NCS containing 25 pm PII/mL and/or 2 pg/mL
3THPP. The dotted line in the spectrum
of
PI1 shows the
spectrum in methanol in the region 250-329 nm.
comparable to those of cell membranes but without
proteins, it is possible to separate the energy trans-
fer peak from the direct excitation
of
PI1 as indi-
cated by the dotted line
on
the spectrum
for
PI1
(Fig. 1, Moan
et
al.,
1988).
In
the case
of
3THPP
such a separation was not attempted, since this dye
has a peak in its own excitation spectrum in this
wavelength region due to the phenyl rings. One can
easily separate the spectra
of
samples containing
both dyes (Fig. l), both manually and by means
of
a simple computer program.
To
a first approximation and for exposure times
shorter than
6
min, the decay kinetics
of
the Soret
band is of first order for both dyes (Figs. 2 and 3).
The same is true for the energy transfer band at
290 nm (Fig. 2). However, the rate constant for the
decay of the energy transfer band is larger than that
for the decay
of
the Soret band. Thus, the decay
of
the energy transfer band is due to at least two
factors: the photodegradation
of
PI1 and the
destruction
of
binding sites at proteins. Since the
rate constant for the decay of the energy transfer
Research Note 55
1
Figure 2. The decay of the Soret band
of
PI1 in NHIK
3025 cells monitored with the excitation and emission
wavelengths set at 400 and 625 nm, respectively; and
of
the energy transfer band
(Acxc
=
290 nm,
Acm
=
625 nm),
the contribution from direct excitation being subtracted.
The wavelength
of
the exposure light was 400 nm.
band is dependent on the concentration
of
PI1 in
the cells, the binding sites at proteins can be
degraded by photoproducts generated remotely
from these binding sites (Moan
ef
al.,
1988).
From Fig. 3 rate constants
for
the first part of
the decay of the Soret band
of
the dyes can be
determined. In the calculations we have taken into
account only the two first exposure times (i.e. 3 and
Figure 3. Decay curves
for
the Soret bands
of
PI1
and
3THPP separately
or
simultaneously present in NHIK 3025
cells exposed
to
light at either
400
or
420 nm. Incubation
conditions, see legend
of
Fig.
1.
6
min) as indicated by the regression lines drawn in
Fig. 3.
Table
1
shows that the quantum yield of photo-
degradation of
PI1
by light quanta absorbed by PI1
itself is comparable to the corresponding yield for
3THPP
or
slightly lower. However, the relative
value of the quantum yield of photodegradation of
3THPP caused by quanta absorbed by 3THPP is
3-6
times larger than the relative value for photo-
degradation of the dye caused by quanta absorbed
by PII. (Only the fluorescent and photosensitizing
fraction
of
the dyes are considered). Similarly, the
relative value
of
the quantum yield of photo-
degradation of PI1 caused by light absorption in PI1
is more than twice as large as the relative value
for photodegradation caused by light absorption in
3THPP. This is in agreement with our earlier con-
clusion that the photodegradation
of
porphyrins in
cells is predominantly a first order process (Mang
et
al.,
1987). Therefore, the present results indicate
that a photoproduct, like
lo2,
generated by the
absorption of a quantum
of
light
in
a porphyrin
molecule causes damage mainly to that molecule
and not to other porphyrin molecules in the vicinity.
The average intracellular concentrations of 3THPP
can be estimated to 40
pM
under the present con-
ditions, and that of PI1 can be estimated to
80
pM
of porphyrin rings (mol. wt
-
600)
(Moan
ef
al.,
1987). In these estimations it is assumed that the
amount of dye in the nucleus is negligible (Moan
er
al.,
1989). The nucleus constitutes about 30% of
the volume of these cells (Moan and Boye, 1981).
Thus, spheres of radius 0.02 pm located randomly
in the cytoplasm contain on the average
1
fluor-
escent molecule of 3THPP. It is, of course, a very
crude approximation that the dye molecules are
homogeneously distributed in the cytoplasm, but it
may serve in a first approximative calculation of
the distance travelled by the reactive intermediates
generated by the photoexcitation of the dye mole-
cules. If we assume that the membranes constitute
about 10% of the cytoplasmic volume (White
et
al.,
1978) and that the present lipophilic dyes are
localized mainly in membrane structures, their local
concentrations are a factor of 10 larger than esti-
mated above and the radius of
a
sphere containing
on average
1
fluorescent molecule of 3THPP is
about 0.01 pm. Singlet oxygen
(lo2)
is almost cer-
tainly a reactive intermediate generated under the
present conditions (Moan
et
al.,
1987). The lifetime
rA
of
lo2
under the present conditions can be esti-
mated by use of the formula
6
=
(60
T~)”’,
where
6
is the distance diffused by
lo2
before it is
quenched and
D
is the diffusion coefficient of
‘02.
If we assume that
D
=
1.4
x
cm2
s-l
(Moan
and Boye, 1981) and
6
is within the range
0.01-0.02
p,
rA
is within the range 0.01-0.04
ps.
Such
a
short lifetime is consistent with the lack of
a
D20
effect in many photosensitized processes in
which
lo2
is very
likely
to be involved. Using the
552
JOHAN
MOAN
and
KRISTIAN BERG
Table l(a). Photodegradation of 3THPP
~ ~~~~~~
Concentrations
hCxp
WmL)
(nm) A(3THPP) A(P1I) k(min-') @(3THPP33THPP) Q(3THPP.PII)
3THPP PI1
1
0 400 0.2 0 0.014 0.07
1
25 400 0.2 2 0.07 0.022
1
0 420
1.0
0 0.10
0.10
1
25 420
I
.0 2 0. 14 0.016
~ ~~~~~
Table
l(b).
Photodegradation
of
PI1
Concentrations
Lip
()lLg/mL)
(nm) A(PI1) A(3THPP) k(min-') @(PII,PII) (D(P11.3THPP)
PI1 3THPP
25
0
400
2.0
0
0.
13
0.M5
25 0 420 2.0
n
0.11 0.055
25
1
400
2.0 0.2
0.13
25
1
420 2.0
1
.0 0.11
<
0.03'
~~ ~~~ ~~
Aexp
is the wavelength
of
the photodegrading light.
A(PI1) and A(3THPP) are relative numbers of light quanta absorbed by fluorescing and photoactive molecules
of PI1 and 3THPP in the samples, approximated in relative values by the convolution integrals
of
the
fluorescence excitation spectra and the spectra
of
the photodegrading light at
400
and
420
nm rcspcctively,
with
AA
=
15
nm. In this approximation
it
is
assumed that the fluorescence quantum yield
of
the
fluorescing molecules
of
3THPP in the cells
is
similar
to
that of fluorescing molecules
of
PII, which
is
suggested by our earlier work (Moan et
al.,
1988). @(3THPP, 3THPP) is the yield
of
degradation of
3THPP resulting from absorption
of
light by the fluorescent fraction of 3THPP. Correspondingly.
@(3THPP. PII) is the yield
of
degradation of 3THPP resulting from light absorption by thc tluorcsccnt
fraction
of
PI1
in
the cells. @(PII, PII) and @(PII, 3THPP) are defined analoguously. The yields arc
given
in
relative values.
"0.03
is the upper limit
of
@(PII, 3THPP) since the maximal error in the relative valucs
of
k
(thc rate
constants for photodegradation
of
the dyes as estimated by the fluorescence experiments) is
30%
in ihc
present data as estimated from two parallel experimental series.
value
T,,
=
0.04
ps
water will account for only 1%
of the quenching of
'02
in
a cell and no D20 effect
can be expected. This
is
consistent with experiments
indicating that
TA
<
0.6
ps
in
cells (Firey
et
al.,
1988). Thus, the time resolution
of
experiments
intended to determine
in
cells directly would
have to be improved by more than a factor of 10
from what can be achieved at present.
It
should be noted that detection
of
the
1272
nm
lo2
phosphorescence is probably the
only
way to
prove that is generated
in
a cell, since scavenger
experiments can always be questioned because of
inhomogeneity
of
the dye- and scavenger molecules
and reactions
of
most scavengers with other possible
intermediates. The possibility that scavenger mole-
cules and sensitizer molecules are present in differ-
ent micro-compartments
of
the cells should always
be considered, even
in
the case that both types
of
molecules have a similar lipophilicity. Since most
efficient scavenger molecules
are
only
weakly flu-
orescent, it is difficult to study their interacellular
localization pattern. The use
of
two different lipo-
philic, fluorescent and photosensitizing dyes may
give valuable information about the processes of
photosensitization
in
cells as indicated by the pre-
sent results. The conclusion that the main reactive
intermediated diffuse
of
the order
of
0.01-0.02 pm
in the cells
is
in
agreement with our observation
that photoexcitation of a porphyrin present at
or
outside the cell wall
of
an
Escherichia
cofi
cell,
whose thickness is approx.
0.03
pm, does not result
in
any photodamage to DNA
in
the bacteria (Boye
and Moan, 1980) and that uroporphyrin, which
produces with a high quantum yield (0.7-0.8)
when photoexcited (Blum and Grossweiner, 1985)
is inefficient in sensitizing cells when present
in
the medium outside the cells during light exposure
(Nelson
et
al.,
1986; Madslien,
K.,
unpublished
data).
Acknowledgement-The authors want to express their
thanks to professor Claude Rimington,
F.R.S.
for valuable
comments and advice during the preparation
of
the manu-
script.
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