Download Cytochrome P450 monooxygenase system in echinoderms

Comparative Biochemistry and Physiology Part C 121 (1998) 139 – 146
Review
Cytochrome P450 monooxygenase system in echinoderms1
P.J. den Besten *
AquaSense Laboratory, P.O. Box 95125, 1090 HC Amsterdam, Netherlands
Received 19 February 1997; received in revised form 8 October 1997; accepted 30 October 1997
Abstract
The results of a limited number of studies on echinoderms provide evidence for the presence of a cytochrome P450
monooxygenase system in representatives of three classes of the phylum Echinodermata: the asteroids (sea stars), holothuroids
(sea cucumbers) and echinoids (sea urchins). The monooxygenase system has been demonstrated to be involved in the metabolism
of xenobiotic compounds, but is assumed to have its primary function in the metabolism of endogenous substrates, such as
steroids. Available data on P450 cofactor requirement, P450-dependent metabolism of benzo[a]pyrene, studies with classical
inhibitors of P450, specificity of P450 induction by planar compounds, and the changes in the benzo[a]pyrene metabolite profile
in induced animals suggest similarities with the MO system present in vertebrates. However, the relatively high capacity of the
monooxygenase system in sea stars to catalyse reactions with organic hydroperoxide as donor for activated oxygen, and the low
induceability during exposure to xenobiotics indicate also important differences between the echinoderm cytochrome P450
monooxygenase system and that of vertebrates. Some evidence was found for the existence of different forms of cytochrome P450
in sea stars. Catalytic functions of the cytochrome P450 monooxygenase system of sea stars in the metabolism of steroids may be
suppressed as a result of the induction of cytochrome P450 by xenobiotics. © 1998 Elsevier Science Inc. All rights reserved.
Keywords: Echinoderms; Cytochrome P450; Monooxygenase system; Benzo[a]pyrene hydroxylase; Steroid metabolism; Induction
1. Introduction
The Echinodermata is the marine invertebrate phylum most closely related to the Vertebrata (both are
deuterostomes) and therefore is an interesting group of
organisms for a comparison of the presence and characteristics of the cytochrome P450 monooxygenase (MO)
system. A limited number of studies have dealt with
MO activity in echinoderms. MO activity in sea stars2
* Present adress: Institute for Inland Water Management and
Waste Water Treatment (RIZA), P.O. Box 17, 8200 Lelystad, Netherlands. Tel.: + 31 320 298464; e-mail [email protected]
1
This article was invited by Guest Editors Dr John J. Stegeman
and Dr David R. Livingstone to be part of a special issue of CBP on
cytochrome P450 (Comp. Biochem. Physiol. 121 C, pages 1 – 412,
1998).
2
In echinoderm literature it was proposed decades ago to replace
the name ‘starfish’ with the more appropriate name ‘sea stars’.
was shown in surveys using in vitro benzo[a]pyrene
hydroxylase (BPH) activity or biphenyl hydroxylase
activity as markers [32,33,49]. Furthermore, in literature reports can be found of studies on in vitro or in
vivo P450-mediated metabolism of both endogenous
and xenobiotic substrates. For example, the in vitro
conversion of cholesterole to pregnenolone, which in
mammalian systems is cytochrome P450-dependent
[16,40], was demonstrated for the sea star Asterias
rubens [36]. Also the putative cytochrome P450-catalyzed formation of 17a-hydroxy metabolites from
pregenolone and progesterone was found in this species
[39,45]. Examples of the metabolism of xenobiotics
include the in vivo hydroxylation of biphenyl by the sea
star Ophiocomina nigra [31] and the metabolism of
tributyltin by the sea star Leptasterias polaris [30].
Examples of MO activity in other echinoderm species
0742-8413/98/$ - see front matter © 1998 Elsevier Science Inc. All rights reserved.
PII S0742-8413(98)10034-8
140
P.J. den Besten / Comparati6e Biochemistry and Physiology, Part C 121 (1998) 139–146
are the in vivo oxidative O-demethylation of p-nitroanisole by the sea cucumber Cucumaria miniata [21], the
O-demethylation of p-nitroanisole by the sea urchin
Strongylocentrotus purpuratus [11] and the hydroxylation of 2,6-dimethyl-naphthalene by the sea urchin
Strongylocentrotus droebachiensis [28].
The presence of cytochrome P450 and other MO
system components in echinoderms was confirmed recently for three species from two classes: the asteroids
Asterias rubens and Marthasterias glacialis and the
holothurian Holothuria forskali [1]. Cytochromes P450
and b5, and the MO system-associated NADH-ferricyanide reductase, NADH-cytochrome-c reductase,
NADPH-cytochrome c reductase and benzo[a]pyrene
hydroxylase (BPH) activities were present in microsomal fractions of pyloric caeca of A. rubens and M.
glacialis and in microsomal fractions of the haemal
plexus of H. forskali, indicating that these species have
the same major MO system components as found in
other invertebrates and vertebrates (Table 1)3. For A.
rubens it was confirmed that BPH activity in pyloric
caeca microsomes is cytochrome P450-dependent (see
below). Cytochrome P450 and BaP hydroxylase activity
were not detectable in the gonads of the echinoid
Echinus esculentus [1], but in an earlier survey BPH
activity was demonstrated in the digestive gland of an
other echinoid species, the sea urchin Strongylocentrotus sp. [33]. Levels of MO activities and components in
invertebrates are usually highest in tissues associated
with the processing of food, e.g. the digestive gland in
molluscs and the intestine in nereid worms [24]. Also
for the sea star A. rubens, highest levels and activities of
the cytochrome P450 monooxygenase system were
found in the microsomal fractions of the pyloric caeca
and the stomachs, both digestive tissues [1]. The presence of cytochrome P450 in the ovaries or testes of A.
rubens was not unambiguously demonstrated in specimens collected in June, in the resting phase of gametogenesis, although cytochrome b5 and all MO-associated
activities were detected [1]. In these sea stars the
amount of gonadal tissue was small and as a consequence, the levels of cytochrome P450 may have been
too low to detect. Females collected in December had
much bigger ovaries in which cytochrome P450 could
be measured [2]. However, with microsomal fractions of
testes from male sea stars collected in December rather
unusual CO-difference spectra were found, possibly the
result of some interference of other proteins specific for
testes tissue or present at high levels (see below).
Although for the three echinoderm species A. rubens,
M. glacialis and H. forskali comparable levels of microsomal cytochrome P450 were found in the pyloric caeca
3
Recently, in experiments with microsomal fractions of pyloric
caeca from A. rubens also NADPH-dependent metabolism of lauric
acid and of oleic acid was shown.
or the haemal plexus (mean values found in the species
comparison study ranging between 40 and 76 pmol
mg − 1 microsomal protein) and comparable NADPHcytochrome c reductase activties (11–13 nmol min − 1
mg − 1 microsomal protein), considerable differences
were found for the mean BPH turnover rates: 275, 38
and 5 pmol benzo[a]pyrene metabolized min − 1 nmol − 1
cytochrome P450, respectively [1]. The turnover rate
found for A. rubens is comparable with values found
for other invertebrates (crustaceans and polychaetes)
and most (teleost) fish species, and is only slightly lower
than in rat (Table 1). However, compared to rat, the
levels and activities of MO system components in
echinoderms are much lower (Table 1).
Taken together, the available data indicate that a
cytochrome P450 monooxygenase system is present in
representatives of at least three echinoderm classes: the
asteroids (sea stars), the holothuroids (sea cucumbers)
and echinoids (sea urchins; no information available for
heart urchins or sand dollars). No observations were
made in representatives of the class of ophiuroids (brittle stars and basket stars), the class of crinoids (feather
stars and sea lilies) or the class of concentricycloids (sea
daisies). With respect to the possibility that the latter
groups may also have MO system enzymes, it is interesting to note that phylogenetic studies indicate that the
classes of holothuroids, echinoids and ophiuroids originate from a primitive ‘ophiuroid’, an echinoderm having the same ‘ancestor’ as the class of asteroids [22].
2. Spectral properties of P450 and cytochrome b5
For the quantification of microsomal cytochrome
P450 in sea stars, sea cucumbers and sea urchins,
CO-difference spectra were performed by adding CO
and background correction prior to the addition of
dithionite. This gave better results compared to CO-difference spectra performed by first adding dithionite and
background correction prior to the addition of CO (due
to interaction of CO with the microsomes, causing
baseline drift) [1]. The CO-difference spectra of pyloric
caeca microsomes from A. rubens and M. glacialis, and
of haemal plexus microsomes from H. forskali contained two peaks, a major peak at 448–450 nm and a
second peak at 418 nm (Fig. 1A) [1]. Both the 448–450
nm and the 418 nm peak reach their maximum size
within 5 min after the addition of dithionite. The 418
nm peak was found in the pyloric caeca of A. rubens at
comparable specific levels (per mg protein) in the cytosol and microsomal fractions, whereas cytochrome
P450 had 20–40 times lower specific levels in the cytosol compared to microsomes [1]. The 418 nm peak was
present alone (i.e. without a 448–450 nm peak) in
CO-difference spectra of microsomes from the gonads
of the sea star A. rubens collected in June and July and
3–134
89–217
140–850
970
43–910
Hepatopancreas
or digestive gland
Digestive gland
Intestine
Liver
Liver
not detectable
Ovaries
–c
40
Haemal plexus
Digestive gland
25–128 (51)
76
P450 (pmol
mg−1 prot.)
Pyloric caeca
Pyloric caeca
Tissue
4–15
1–7
15–144
152
2–29
–c
6
13
5–11 (11)
12
NADPH-cytochrome c reductase-activity (nmol
min−1 mg−1 prot.)
26–160
40–77
40–300
510
95
–c
70
79
59–76 (68)
41
Cytochrome b5
(pmol mg−1
prot.)
1–3e
2–84
2–685
407
0.5–43
detectable
not detectable
0.2
3–15 (14)
3
BaP-hydroxylase activity
(pmol min−1 mg−1 prot.)
10–37
236–385
16–1117
420
0.5–139
[25]
[25,29]
[41]
[18,19]
[17,18]
[33]
[1]
–c
–c
[1]
[1,2,4,6,12]
[1]
References
5
100–314 (275)
38
BaP-hydroxylase turnover
rate (pmol min−1 nmol−1
P450)
a
Ranges represent variation in time (range of mean values from a number of studies (including measurements in unexposed male and female sea stars in induction studies); mean value from survey
study ([1]) is given between brackets (n= 12).
b
Mean values from survey study [1] (n =3).
c
Parameter not included in this study.
d
Ranges represent variation between species.
e
BPH activity in Mytilus edulis, calculated assuming that phenols constitute 13% of total benzo[a]pyrene metabolites [23].
f
Only data from assays with microsomal fractions from teleost species.
Molluscsd
Polychaetesd
Fishd,f
Rat
Echinoderms
Sea star (A. rubens)a
Sea star (M.
glacialis)b
Sea cucumber (H.
forskali )b
Sea urchin (E. esculentus)b
Sea urchin (Strongylocentrotus sp.)
Crustaceansd
Animal group/species
Table 1
Comparison of the microsomal cytochrome P450 monooxygenase system and benzo[a]pyrene metabolism in echinoderms with the MO system in other invertebrate and vertebrate groups
P.J. den Besten / Comparati6e Biochemistry and Physiology, Part C 121 (1998) 139–146
141
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P.J. den Besten / Comparati6e Biochemistry and Physiology, Part C 121 (1998) 139–146
in CO-difference spectra from the gonads of the sea
urchin E. esculentus, also collected in June and July [1].
In contrast to the sea star A. rubens, the gonads of E.
esculentus contained ripe eggs and were not in a resting
phase. This makes it unlikely that the 418 nm peak in
the latter species represents some deactivated form of
P450, as could be suggested for the resting phase gonads of A. rubens (see section above). For the gonads of
male sea stars it was observed that in December, in the
phase of testes growth, the CO-difference spectra of
testes microsomes become completely dominated by a
peak with maximal absorbance at 426 nm [1]. It is not
clear whether this peak represents a protein specific for
testes tissue.
The 418 nm peak is found also in other invertebrate
species (molluscs, crustaceans), and reported to be vary
in size with season and in some cases with exposure
[25]. The size of the 418 nm peak in the pyloric caeca of
the sea star A. rubens was found to vary thoughout the
year by a factor of about 4, showing the smallest peaks
during the summer period (see below). CO-difference
spectra of microsomes obtained from in vitro cultures
of pyloric caeca tissue (whole organs incubated in sterile sea water) showed a gradual decrease of the P450
peak over a period of 4 days (a loss of only 17% within
the first 24 h), while the 418 nm peak increased in size
rapidly (peak size more than doubled within 24 h)
(unpublished data from the author). These observations
support the idea that the 418 nm peak represents some
sort of breakdown product (and most likely not of
P450). Data of recent field trips showed an increase of
the levels of the 418 nm peak in sea stars from polluted
field sites [34]. The identity of the haemoprotein respon-
sible for the 418 nm peak in echinoderm spcies may be
similar to that in molluscs, which was suggested to
represent a normally occurring protein degradation
product, showing increased accumulation under unfavourable conditions [25].
Cytochrome b5 was measured by NADH-difference
spectra. The NADH-difference spectra were similar for
A. rubens, M. glacialis, H. forskali and E. esculentus,
with an absorbance peak at 425–426 nm (Fig. 1B) [1].
3. Sex-specific differences and seasonal variation of
cytochrome P450 in the sea star A. rubens
In pyloric caeca of sea stars collected in June and
July, no significant sex-related differences in the specific
contents and activities (per mg microsomal protein) of
MO system components were found, although the levels tended to be somewhat higher in females [1]. In the
pyloric caeca of female sea stars collected in December
and May, the cytochrome P450 level and BPH activity
were significantly higher than in male animals [2,4].
Using data from a number of studies for which female
specimen of A. rubens were collected from the Dutch
coastal zone (North Sea) in different periods of the
year, an impression can be given of the seasonal variation in the level of P450 in pyloric caeca microsomes, as
shown in Fig. 2. The high levels of P450 in August and
September were accompanied by relatively small 418
nm peaks (not shown). The period of August and
September is just before the onset of the reproductive
cycle [5,38], during which relatively high steroid synthesis activities have been found [37,46]. Steroid hormones
are supposed to play a role in the regulation of gametogenesis in the sea star, during which nutrients are
released from the pyloric caeca and taken up by the
gonads to be used for the production of gametes
[44,46]. Since different steps in steroid synthesis are
known to be cytochrome P450-dependent [16] this implies a major function of the cytochrome P450
monooxygenase in the metabolism of hormones at the
start of the reproductive cycle in sea stars.
4. Characterization of benzo[a]pyrene metabolism by
pyloric caeca microsomes of A. rubens
Fig. 1. Difference spectra of microsomal fractions of pyloric caeca of
the sea star, A. rubens. (A) CO-difference spectrum (CO and background correction prior to addition of sodium dithionite, see text);
(B) NADH-difference spectrum. Reproduced from [1].
Microsomal BPH activity of the sea star A. rubens
was shown to be dependent on the addition of NADPH
[1]. Dependence of BPH on the catalyst function of
P450 was demonstrated by the effects on BPH activity
by classical inhibitors of cytochrome P450, in particular
SKF-525A, metyrapone and ellipticine. In mammalian
systems ellipticine is thought to bind between the haem
moiety of cytochrome P450 and the hydrophobic binding site of cytochrome P450 reductase [14], so that the
P.J. den Besten / Comparati6e Biochemistry and Physiology, Part C 121 (1998) 139–146
143
Fig. 2. Seasonal variation of the level of cytochrome P450 in pyloric caeca microsomes of female sea stars, A. rubens. Data shown are mean
values 9 S.D., taken together from a number of studies in which sea stars were collected from the Dutch coastal zone, in the period 1991–1996
[4,7,12,34,35].
inhibiting effects of the compound on sea star BPH
may be another indication of the dependence of BPH
activity on electron transfer from NADPH via cytochrome P450 reductase. Later studies, however,
demonstrated that the sea star MO system has no strict
preference for NADPH. Differences between species
with regard to the MO systems have been speculated to
be related not only to the properties of cytochrome
P450s, but also to the degree of functional integration
of cytochrome P450 with cytochrome P450 reductase
[18,24,26]. In molluscan species examples have been
found of cytochrome P450dependent microsomal reactions that are independent of added NADPH [27],
whereas for both the common mussel, Mytilus edulis
(phylum: Mollusca; class: Bivalvia) [23] and the red
swamp crayfish, Procambarus clarkii (phylum:
Arthropoda; class: Crustacea) [18], NADH or organic
hydroperoxide supported microsomal oxidative xenobiotic metabolism is comparable to that in the presence
of NADPH. A comparison was made between the
HPLC benzo[a]pyrene (BaP) metabolite profiles produced after the incubation of A. rubens pyloric caeca
microsomes with BaP in the presence of NADPH,
NADH or cumene hydroperoxide (CHP) [6]. Total
rates of NADH-dependent BaP metabolism were nearly
two times higher than with NADPH. Production of
phenols, dihydrodiols and putative protein adducts was
at a rate comparable to that with NADPH. However,
quinones were the dominant metabolites formed in the
presence of NADH (61% of the total BaP metabolism,
compared to 19% in the presence of NADPH). In the
presence of 2.5 mM CHP BaP metabolism was shifted
towards the formation of quinones (39% of total
metabolism) and protein adducts (23%). This shift in
CHP supported metabolism is also observed for mammalian [8,50] and crustacean [23] microsomal systems
and is purported to result from one-electron oxidation
as the dominant mechanism of metabolism [9,10]. In
the case of the CHP-supported benzo[a]pyrene
metabolism by the sea star A. rubens, also the rates of
phenol and dihydrodiol production were about four
times higher than with NADPH [6]. This demonstrates
that the sea star MO system has no strict preference for
NADPH as a donor of reducing equivalents and that it
can also efficiently use activated oxygen from organic
hydroperoxides.
5. Characterization of P450 by Western blotting
Samples of pyloric caeca microsomes from unexposed and PCB-126 or BaP-exposed sea stars, A. rubens
(see also induction studies, below), were blotted against
antibodies to different teleost (scup) P450 subfamilies
[4]. Antibodies against scup P450 1A subfamily (mouse
monoclonal and rabbit polyclonal anti-scup P450 1A)
produced a faint band at about 54 kDa. Possibly due to
the low reactive signal, no consistent differences were
found between exposed and control samples. Sea star
microsomal proteins also cross-reacted with antibodies
against the P450 2B subfamily (rabbit polyclonal antiscup P450 2B) and antibodies against the P450 3A(?)
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P.J. den Besten / Comparati6e Biochemistry and Physiology, Part C 121 (1998) 139–146
Table 2
Induction of benzo[a]pyrene metabolism in pyloric caeca microsomes of female sea stars, A. rubens a
Exposure conditions
Cytochrome P450 level
(% of control)b
Clophen A50, exposure via foodchain for 3 months (S8-PCB in mussel used as food: 0.6
mg kg−1)
PCB-126 (3,3’,4,4’,5-penta-CB), 4 days after injection in coelomic cavity (dose 10 mmol
kg−1)c
PCB-153 (2,2’,4,4’,5,5’-hexa-CB), 4 days after injection in coelomic cavity (dose 10 mmol
kg−1)c
PCB-118 (2,3’,4,4’,5-penta-CB), 4 days after injection in coelomic cavity (dose 10 mmol
kg−1)c
Benzo[a]pyrene, 4 days after injection in coelomic cavity (dose 160 mmol kg−1)
BPH turnover rate
(% of control)b
58*
223*
82
186*
68
110
162
130
49
446*
a
Based on data from [2,4].
Mean cytochrome P450 levels in the control sea stars of the different studies were 36–44 pmol mg−1 microsomal protein; mean BPH turnover
rates (the amount of BaP hydroxylated per unit of time and per unit of cytochrome P450) were 100–150 pmol min−1 nmol−1 P450.
c
Highest dose used in a dose-response experiment; also the dose resulting in the largest increase in BPH activity (higher doses of PCB inhibited
BPH activity).
* Original parameter value was significantly different from corresponding control values (PB0.05).
b
subfamily (rabbit polyclonal anti-scup P450 3A). The
protein recognized by anti P450 2B was very similar in
molecular weight to scup P450 2B (about 46 kDa),
while the band recognized by anti P450 3A(?) was
substantially larger than scup P450 3A(?) (scup P450
3A(?)=53 kDa). Also a faint cross-reacting band was
found between pyloric caeca microsomes of A. rubens
and a polyclonal anti-rat-P450 1A1 antibody raised in
rabbit [4]. The fact that cross-reaction was observed
with antibodies specific for different P450 subfamilies
can be interpreted as an indication for the presence of
different isoforms of P450 in the sea star. Circumstancial evidence for the presence of diffferent forms of
cytochrome P450 was obtained in a study on the inhibiting effects of benzo[a]pyrene (BaP) on in vitro
steroid hydroxylation and vice versa, of steroids on in
vitro BaP hydroxylation in pyloric caeca microsomes.
At equimolar concentrations of inhibitor and assay
substrate, steroids did not inhibit BPH activity while in
steroid conversion assays no effect of BaP was observed. This was interpreted as evidence for the presence of at least two types of catalytic functions of P450,
the BPH having substrate preference for BaP, and the
catalyst function in steroid metabolism, with preference
for steroids [3].
6. Induceability of BPH activity in pyloric caeca
microsomes of A. rubens
Because of the relatively close phylogenetic relation
between echinoderms and vertebrates, it is also interesting to know whether the cytochrome P450 MO system
shows a similar induction response, i.e. a response only
to 3-MC-type (or mixed-type) inducers. Induction of
cytochrome P450 1A1 activity is usually detected by
measurement of the increase in ethoxyresorufin-Odeethylase (EROD) activity. In a pilot experiment
EROD activity was detected in a culture of pyloric
caeca cells from the sea star A. rubens, using ethoxyresorufin as a probe for EROD activity which was detected by confocal laser microscopy (unpublished
results). However, EROD activity appeared to be
difficult to detect in a fluorometric assay, as is the case
with most invertebrates [25]. For this reason AHH
activity, measured as benzo[a]pyrene hydroxylase
(BPH) activity, was used to study MO induction. No
increase in the microsomal level of P450 was observed
in the pyloric caeca of sea stars after short-term exposure to PCBs or to benzo[a]pyrene (Table 2). Longterm
exposure to a PCB mixture (Clophen A50) resulted in
decreased levels of cytochrome P450 and cytochrome
b5, while at the same time, BPH activity was increased
[2]. In another series of experiments sea stars were
injected with selected PCBs or BaP in order to measure
the short-term effects on the MO system [4]. BaP,
PCB-126, PCB-153 and PCB-118 were selected for their
different inductive potencies: PCB-126 and BaP as 3MC-type inducers; PCB-153 as an inducer of the PBtype and PCB-118 as a mixed-type inducer. No
dose-dependent effects were observed on the level of
microsomal P450. In most cases P450 tended to be
lower in exposed sea stars, except for sea stars injected
with the highest dose of PCB-118, in which case an
increase was indicated (Table 2). The BPH turnover
rate (i.e. the amount of BaP hydroxylated per unit of
time and per unit of cytochrome P450) was used as an
alternative parameter, to detect changes in the BPH
capacity of the cytochrome P450. Maximal increases in
MO activity were observed after 3–4 days [4]. For BaP,
dose-dependent increases in BPH activity and BPH
turnover rate were found. The maximum observed in-
P.J. den Besten / Comparati6e Biochemistry and Physiology, Part C 121 (1998) 139–146
crease in BPH turnover rate was 346% after injecting
sea stars with 160 mmol kg − 1 BaP (Table 2). Of the
three PCBs only PCB-126 caused a significant increase
of BPH activity (increase of 86% at the highest injected
concentration of 10 mmol kg − 1) (Table 2). The specificity of the MO induction in the sea star towards the type
of contaminant suggest some similarity with the MO
system of vertebrates, showing also strongest induction
with planar PCBs or PAHs [20,47].
The pattern of BaP metabolites formed during incubation of microsomes from CB-126- or BaP-injected sea
stars with BaP was shifted towards the formation of
phenols [4,6]. Also in this regard, the reaction is similar
to that in mammals and fish where the increased BaP
metabolism was similarly shown to be largely due to an
increase in hydroxylation products [42,43,48,50]. On the
other hand, major differences are indicated by the low
magnitude of the induction response in sea stars, compared to vertebrates. This may be related to the reported absence of an Ah receptor in invertebrates,
including sea stars [15]. The induction mechanism triggered by xenobiotics in sea stars (and in other invertebrates) could therefore be a primitive version of that in
vertebrates, for instance involving a different and/or
less specific receptor.
7. Relation between induction and effects on
reproduction
Evidence was obtained for the hypothesis that the
induction of the cytochrome P450 monooxygenase system is the result of a less specific response than in
vertebrates, and as a consequence could easily result in
alterations of other P450 catalyzed reactions. Exposure
of sea stars to PCBs or to BaP resulted in a reduction
in microsomal steroid hydroxylation rates [2,4]. For
PCBs it was demonstrated that longterm exposure resulted also in lowered levels of steroids in the pyloric
caeca [2]. These effects could be the result of the
reduction of the P450 level, which was indicated after
short-term exposure, and observed as a significant effect after longterm exposure [2,4]. An overall reduction
in P450 may be the result of the induction of specific
P450 isoenzymes, while other forms are suppressed [13].
This mechanism may be responsible for disturbances in
steroid metabolism in sea stars, with possible negative
consequences for reproductive success [2].
8. Conclusions
Based upon a limited number of studies it can be
concluded that a cytochrome P450 monooxygenase system is present in representatives of three classes belonging to the phylum Echinodermata. One of the natural
145
functions of the cytochrome P450 MO system is assumed to be the metabolism of steroids. The cytochrome P450 system was characterized only for the
sea star A. rubens. Available data on P450 cofactor
requirement, P450-dependent metabolism of benzo[a]pyrene, studies with classical inhibitors of P450,
specificity of P450 induction by planar compounds, and
the changes in the benzo[a]pyrene metabolite profile in
induced animals suggest similarities with the MO system present in vertebrates, indicating possible evolutionary relationships. At the same time, differences are
indicated by data from studies on the organic hydroperoxide supported benzo[a]pyrene metabolism and by the
relatively low induction capacity as a whole. It seems
worthwhile to continue research to find out whether
these differences lead to a less efficient detoxification
mechanism, and to relatively strong disturbance of the
natural functions of the cytochrome P450 monooxygenase system.
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