From the Department of Bioorganic Chemistry,
Institute of Chemistry, Tallinn Technical University, Akadeemia tee
15, Tallinn 12618, Estonia and the ¶ Department of Pharmacology,
Vanderbilt University School of Medicine,
Nashville, Tennessee 37232-6602
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ABSTRACT |
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Certain corals are rich natural sources of
prostaglandins, the metabolic origin of which has remained undefined.
By analogy with the lipoxygenase/allene oxide synthase pathway to
jasmonic acid in plants, the presence of (8R)-lipoxygenase
and allene oxide synthase in the coral Plexaura homomalla
suggested a potential metabolic route to prostaglandins (Brash, A. R., Baertshi, S. W., Ingram, C.D., and Harris, T. M. (1987)
J. Biol. Chem. 262, 15829-15839). Other evidence,
from the Arctic coral Gersemia fruticosa, has indicated a
cyclooxygenase intermediate in the biosynthesis (Varvas, K., Koljak,
R., Järving, I., Pehk, T., and Samel, N. (1994) Tetrahedron
Lett. 35, 8267-8270). In the present study, active preparations
of G. fruticosa have been used to identify both types of
arachidonic acid metabolism and specific inhibitors were used to
establish the enzyme type involved in the prostaglandin biosynthesis.
The synthesis of prostaglandins and
(11R)-hydroxyeicosatetraenoic acid was inhibited by
mammalian cyclooxygenase inhibitors (indomethacin, aspirin, and
tolfenamic acid), while the formation of the products of the
8-lipoxygenase/allene oxide pathway was not affected or was increased.
The specific cyclooxygenase-2 inhibitor, nimesulide, did not inhibit
the synthesis of prostaglandins in coral. We conclude that coral uses
two parallel routes for the initial oxidation of polyenoic acids: the
cyclooxygenase route, which leads to optically active prostaglandins,
and the lipoxygenase/allene oxide synthase metabolism, the role of
which remains to be established. An enzyme related to mammalian
cyclooxygenases is the key to prostaglandin synthesis in coral. Based
on our inhibitor data, the catalytic site of this evolutionary early
cyclooxygenase appears to differ significantly from both known
mammalian cyclooxygenases.
Natural A-, E-, and F-type prostaglandins, as well as atypical
prostanoids such as clavulones and punaglandins, have been detected in
different corals (reviewed in Ref. 1). The most important
representative in the field is the Caribbean gorgonian Plexaura
homomalla, which contains the highest known concentration of
prostanoids in Nature. The content of
PGA21 and
PGE2 esters in P. homomalla amounts to 2-3% of
dry weight (2, 3). There have been proposals over the years that
biosynthesis of prostaglandins in coral may occur by a route totally
different from the vertebrate cyclooxygenase pathway (4-6). Currently, despite the intensive studies and widespread occurrence of
prostaglandins in corals, the mechanism of biosynthesis of these
compounds has remained unresolved. This is mainly attributable to the
inability of the coral preparations studied to form prostaglandins
in vitro (4-7).
P. homomalla and other Caribbean corals readily metabolize
exogenous arachidonic acid via the (8R)-lipoxygenase pathway
(6-8). The product, (8R)-HPETE, is further converted into
an allene oxide, a highly unstable epoxide (7, 9). As one of the key
properties of allene oxides is their ability to undergo cyclization to
give cyclopentenone rings, this compound appeared to be a promising intermediate in the potential lipoxygenase pathway from arachidonic acid to prostaglandins (Scheme 1,
left side) (6, 7, 10). This type of
transformation has been observed in plants where allene oxide is an
intermediate of biosynthesis of the five-membered carbon ring of the
growth hormone, jasmonic acid (11). In preparations of P. homomalla, however, the enzymatic cyclization of allene oxide to
prostaglandins has not been realized (6-8).
INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES
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Scheme 1.
In 1993, we reported that the Arctic soft coral Gersemia
fruticosa (Octocorallia, Alcyonacea,
Nephtheidae) contains optically active natural
prostaglandins PGD2, PGE2, PGF2,
and 15-keto PGF2
(12). The content of prostaglandins of
the Arctic coral (0.008% of dry weight) is several orders of magnitude
lower than that of P. homomalla, but G. fruticosa
has a significant ability to convert the exogenous arachidonic acid
into optically active prostaglandins in vitro (12). Our
further work led to the isolation of the prostaglandin endoperoxide
intermediate, PGG2, from G. fruticosa
incubations (13). It is notable that it was the 15-hydroperoxy compound
PGG2, not PGH2, that accumulated in short
incubations even in the presence of peroxidase reducing substrates. The
nature of the enzymes involved in this pathway is the subject of the
present study.
In vertebrates, prostaglandins are synthesized by prostaglandin
endoperoxide synthase (PGHS) known also as cyclooxygenase. PGHS is a
hemoprotein with two distinct catalytic activities: the cyclooxygenase
activity involved in the formation of PGG2 from arachidonic
acid, and the peroxidase activity, which catalyzes the reduction of
PGG2 to PGH2 (14). There are two PGHS isozymes called PGHS-1 (constitutive) and PGHS-2 (inducible) (15). NSAIDs, such
as aspirin, ibuprofen, and indomethacin, directly target cyclooxygenase
activity of both isozymes (14, 16). Several selective PGHS-2 inhibitors
have been developed to reduce the unfavored gastrointestinal side
effects associated with the inhibition of PGHS-1 by traditional NSAIDs
(17, 18). Cyclooxygenase type enzymes have not been identified in
nonvertebrate species, despite the detection of prostaglandins in
various organisms. The existence of prostaglandins in plant tissue has
also been reported (reviewed in Ref. 19), but there is evidence for the
catalytic oxidation of arachidonic acid into PGF2 by one
of the plant lipoxygenases (a non-heme iron dioxygenase) (20).
It therefore remains unclear whether the lipoxygenase- or
cyclooxygenase-related enzyme catalyzes prostaglandin synthesis in
G. fruticosa. In the course of the present study, we
discovered that preparations of G. fruticosa metabolize
arachidonic acid to a number of additional metabolites, including those
of the (8R)-lipoxygenase/allene oxide pathway. This enabled
us to elucidate the possible relation of these arachidonic acid
metabolites to the production of optically active prostaglandins in coral.
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EXPERIMENTAL PROCEDURES |
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Materials and Reagents
Unlabeled arachidonic acid and biosynthetic prostaglandin
standards were obtained as a generous gift from Kevelt (Tallinn, Estonia). [14C]Arachidonic acid was purchased from
Amersham Pharmacia Biotech; it was diluted with unlabeled arachidonic
acid to the specific activity needed. Racemic HETE standards were
prepared by vitamin E-controlled autooxidation (21). The
(8R,15S)-diHETE standard was prepared by
incubation of arachidonic acid with the acetone powder of the coral
P. homomalla followed by incubation of the resulting
(8R)-HETE with soybean lipoxygenase (6). Tolfenamic acid and
nimesulide were bought from Cayman Chemical. Aspirin, indomethacin and
SnCl2 were purchased from Sigma.
N,O-Bis-(trimethylsilyl)trifluoroacetamide, methoxylamine hydrochloride, and pyridine were obtained from Serva. The
coral G. fruticosa was collected in the Gulf of Kandalaksha at a depth of 25-30 m and water temperature about 4 °C. The
collected coral was frozen in liquid nitrogen within 5 min, transported on dry ice, and stored below 70 °C.
Enzyme Preparation
The acetone powder of G. fruticosa was prepared by
homogenization of 20 g of frozen coral with a Warning blender for
1 min at full speed in cold acetone (20 °C). The mixture was
centrifuged at 3000 × g at 4 °C for 5 min, the
supernatant was decanted, and the procedure was repeated three times on
the residual solids. The skeletal elements were removed from the
mixture by swirling and decanting with cold acetone. The fine solids
obtained were filtered and dried in a stream of argon at room
temperature. The yield was 1-1.5 g of powder. The acetone powder was
divided into aliquots and stored under argon at
70 °C. The enzyme
preparations remained active for several months.
Incubations-- 15 mg of the coral acetone powder was blended with 3 ml of 50 mM Tris buffer, pH 9, and preincubated for 5 min. The reaction was initiated with 0.4 µCi of [14C]arachidonic acid (final concentration, 100 µM) in ethanol solution. Incubations were performed at room temperature for 10 min, after which the incubation mixture was treated with SnCl2 (2 mg/ml), acidified, and the reaction products were extracted with three portions of ethyl acetate. When hydroperoxy compounds were studied, SnCl2 was omitted. The combined extracts were washed with brine, dried over potassium sulfate, and evaporated at reduced pressure to dryness. The residue was subjected to TLC analysis, and the radioactivity was monitored. The total recovery of radioactivity was about 60%.
To trap the intermediates a mild reducing agent (SnCl2; final concentration, 0.5 mM) was added to the incubation mixture directly before the substrate. Incubations in the presence of SnCl2 were performed in 50 mM Tris buffer, pH 8.
For inhibition studies, the coral acetone powder was preincubated at room temperature for 5 min with various amounts of inhibitors added in a few microliters of ethanol. An equal amount of ethanol was added to the control. The reaction was initiated with 0.3 µCi of [14C]arachidonic acid (final concentration, 50 µM). The incubation was carried out for 10 min, followed by treatment with SnCl2. The products were extracted with ethyl acetate and analyzed by TLC, and the radioactivity was monitored. All experiments were carried out in triplicate.
Chromatographic Methods
Thin Layer Chromatography-- TLC was performed using precoated plates (Silica Gel 60, 0.25 mm) from E. Merck (Darmstadt, Germany). A solvent system (A) of benzene:dioxane:acetic acid (10:5:0.25, v/v/v) for separation of polar products and a solvent system (B) of diethyl ether:hexane:acetic acid (3:3:0.05, v/v/v) for separation of nonpolar products were used. For the product quantification, the TLC plates were cut into zones and extracted with methanol, and the radioactivity was measured with a liquid scintillation counter. Authentic eicosanoid standards, used as markers, were visualized with an anisaldehyde spray reagent (22).
High Performance Liquid Chromatography-- For identification of the mono- and dihydroxy acids formed, products were separated initially by RP-HPLC using a Beckman Ultrasphere 5-µm ODS column (25 × 0.46 cm) with a solvent system of methanol:water:acetic acid (80:20:0.01, v/v/v) at a flow rate of 1 ml/min. The uv spectra and the chromatograms at 205, 220, 235, and 270 nm were recorded using a Hewlett-Packard 1040A diode array detector, and the radioactivity was monitored on-line using a Radiomatic Instruments Flo-One detector. For structural characterization, the main radiolabeled peaks were methylated with ethereal diazomethane and purified by SP-HPLC using a Beckman Ultrasphere 5-µm silica column (25 × 0.46 cm) and the solvent system of hexane:isopropyl alcohol:acetic acid (100:2:0.1, v/v/v) at a flow rate of 1 ml/min. The steric analysis of the hydroxy products was carried out with the methyl ester derivatives by chiral-phase HPLC using a Chiralcel OB column (23).
Gas Chromatography-Mass Spectrometry--
Samples were
methylated with diazomethane and purified by SP-HPLC or by TLC. The
keto groups were derivatized with a 2% solution of methoxylamine
hydrochloride in pyridine at 60 °C for 1 h. The hydroxyl groups
were silylated by treatment with
bis-(trimethylsilyl)trifluoroacetamide:pyridine (2:1, v/v) at room
temperature for 1 h, the solvent was removed, and the residue was
dissolved in hexane. Positive-ion electron impact spectra were obtained
using a Hitachi M 80-B mass spectrometer operated at 70 eV. Samples
were run on an RSL fused silica capillary column (10 m × 0.53 mm)
with temperature programming from 180 to 300 °C at
15°/min.
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RESULTS |
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Metabolism of Arachidonic Acid by G. fruticosa
[14C]Arachidonic acid was metabolized by G. fruticosa acetone powder to a complex mixture of oxygenated
compounds. The products were separated by TLC in solvent systems A
and/or B, followed by monitoring of the radioactivity. Typically, the
reaction product pattern obtained with solvent system A consisted of
the four well separated polar bands (about 19-25% of recovered
radioactivity) co-migrated with natural mammalian prostaglandin
standards and gave color reactions with anisaldehyde spray reagent
characteristic of the corresponding prostaglandins (Fig.
1A). The prostaglandins PGD2, PGE2, PGF2, and 15-keto
PGF2
formed from the exogenous arachidonic acid with the
G. fruticosa acetone powder were characterized earlier by
HPLC, GC-MS, 13C NMR, and optical rotation measurements in
comparison with authentic standards (12). The main nonpolar products
8-KETE, 8-HPETE, and 8- hydroxy-9-ketoeicosatrienoic acid (20-22%),
and unchanged arachidonic acid (30-35%) unresolved in the first
solvent system were separated using solvent system B (Fig.
1B). When [14C]arachidonic acid was incubated
with the coral acetone powder in the presence of a mild reducing agent
to trap peroxy intermediates to the corresponding hydroxy compounds,
TLC analysis with solvent system A showed three main bands (Fig.
1C). These products were completely separated by RP-HPLC
(see Fig. 2 and text below). When identical oxygenations were carried out using a heat-inactivated coral
enzyme (at 80 °C for 40 min), no detectable radioactive products
were obtained.
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Identification of Lipoxygenase Products
8-HPETE--
In order to detect hydroperoxides, the TLC plates
eluted in solvent system B were sprayed with the ferrous thiocyanate
spray reagent. One compound with RF 0.52 showed
a positive reaction. This compound was scraped from the plate,
extracted with methanol and reduced with NaBH4 to get a
slightly more polar product. The uv spectra of both unreduced and
reduced compound had max at 235 nm, which is typical of
a conjugated diene chromophore. The identity of the reduced compound as
8-HETE was confirmed by GC-MS analysis of the methyl ester
Me3Si ether derivative. The mass-spectrum had
characteristic ions at m/z 265 (base peak, represents
cleavage of the C7-C8 bond); 375 (M
31); 391 (M
15), and a molecular
ion at m/z 406.
8-Hydroxy-9-ketoeicosatrienoic Acid (-Ketol)--
This
arachidonic acid metabolite was identified in the band with
RF 0.44 in solvent system B. The compound was
scraped from the TLC plate and extracted with methanol. An aliquot of
the extract was derivatized and analyzed by GC-MS. The mass spectrum of
the methyl ester methoxime Me3Si ether derivative was
identical to that of the 8-hydroxy-9-ketoeicosa-5,11,14-trienoic acid
formed from arachidonic acid in P. homomalla incubations
(6). This mass spectrum has characteristic ion fragments at
m/z 243 (C1-C8), 310 (M
141, loss of C1-C7), 420 (M
31), 436 (M
15), and 451 (M+). The
reduction of the keto-hydroxy compound with NaBH4 led to the formation of two more polar diastereomers. The mass spectra of the
methyl ester Me3Si ether derivatives of the two
diastereomers gave the same prominent ions at m/z 243 (C1-C8), 345 (C1-C9), 355 (M
141, loss of C1-C7), 481 (M
15), and 496 (M+), which are characteristic of
the spectra of the same derivative of
8,9-dihydroxyeicosa-5,11,14-trienoic acid. The isolation of
-ketol
(the major product of nonenzymatic hydrolysis of unstable allene
oxide), which shared 7-9% of total radioactivity, is indicative of
the occurrence of an active allene oxide synthase in G. fruticosa.
8-KETE--
In addition to 8-HPETE and -ketol, one more
arachidonic acid metabolite, having about 10% of total radioactivity,
was detected in the nonpolar region (RF 0.62 in
solvent system B). The uv spectrum of this compound had
max at 280 nm, which is characteristic of the conjugated
dienone. The compound was identified as 8-KETE by its conversion into
8-HETE on treatment with NaBH4. The reduced compound
comigrated with 8-HETE standard on TLC and had a uv absorbance maximum
at 235 nm. The mass spectrum of the methyl ester Me3Si ether derivative of the reduced compound confirmed the identity as that
of 8-HETE.
Identification of Peroxy Intermediates
In order to simplify product quantification in enzyme inhibition studies, the number of labeled oxygenated metabolites was diminished by the immediate reduction of the peroxy intermediates formed with a mild reducing agent, SnCl2, to the corresponding hydroxy compounds. Additionally, the prevention of hydroperoxy acids from further transformations allowed us to isolate several minor HETEs not detectable in the absence of SnCl2. The incubation products were examined by TLC and RP-HPLC. Typical profiles of the labeled metabolites formed are shown in Figs. 1C and 2.
Prostaglandin PGF2--
The only prominent peak in
the prostaglandin region had the retention time of 5.0 min, and it was
co-chromatographed with PGF2
standard. The structural
and stereochemical identity of the coral PGF2
with the
mammalian's has been shown previously (12).
8,15-DiHETE--
The second peak had the retention time of 7.2 min
and co-chromatographed with authentic
(8R,15S)-diHETE. Furthermore, the uv spectrum of
the compound from peak 2 had max of 268 nm and shoulders
at approximately 258 and 278 nm, which were similar to those of
authentic (8R,15S)-diHETE. GC-MS analysis of the
methyl ester Me3Si ether derivatives confirmed the identity
of this compound as 8,15-diHETE. The spectra had prominent ions at 173 (cleavage of C14-C15 bond), 243 (cleavage of C8-C9 bond), 353 (cleavage of C7-C8 bond), 423 (M
71, loss of C16-C20), 404 (M
90), 463 (M
31), 479 (M
15), and 494 (M+).
(8R)-HETE, (11R)-HETE, (15S)-HETE--
The retention time of the
peak 5 was identical to that of authentic 8-HETE. Moreover, the
chromatogram showed two radioactive peaks, 3 and 4, with the retention
times of 16.5 and 17.4 min corresponding to those of authentic 15-HETE
and 11-HETE, respectively. The uv spectra of the material of peaks 3, 4, and 5 were typical of conjugated diene chromaphores and had
max near 235 nm. The identity of the products to 8-, 11-, and 15-HETE was confirmed by co-chomatography of their methyl
ester derivatives with the respective standards on SP-HPLC. The
products were subjected to GC-MS analysis as methyl ester
Me3Si ether derivatives. Peak 3 had a spectrum identical to
that of the corresponding derivative of authentic 15-HETE.
Characteristic ion fragments were observed at m/z values of
173 (C15-C20); 225 (M
181, loss of C1-C10); 335 (M
71, loss of C16-C20), and 391 (M
15). Peak 4 was identified as
11-HETE, with characteristic ion fragments at m/z values of 225 (base peak, cleavage of the C10-C11 bond), 316 (M
90), 375 (M
31), 391 (M
15), and 406 (M+). Peak 5 had a mass spectrum identical to that of the corresponding derivative
of authentic 8-HETE.
The enantiomeric composition of the products was determined by
chiral-phase HPLC of their methyl ester derivatives (23). The uv
chromatograms monitored at 235 nm showed that 15-HETE was predominantly
in the 15S conformation (93.6%), 11-HETE consisted mainly of R-enantiomer (89%), and 8-HETE was mostly
R-enantiomer (96.5%) (Fig.
3).
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Effect of Inhibitors on Arachidonic Acid Metabolism in G. fruticosa
The effect of typical mammalian cyclooxygenase inhibitors (aspirin, indomethacin, and tolfenamic acid) was tested against the product formation in coral G. fruticosa. Usually, 0.5 mM SnCl2 was included in the incubation mixture. Experiments with indomethacin and tolfenamic acid indicated that the influence of SnCl2 on the enzyme inhibition was not considerable.
Tolfenamic acid was found to be the most potent inhibitor of
prostaglandin synthesis among the drugs tested. The IC50
values were estimated to be 0.45 µM in the presence of
the reducing agent and 0.65 µM when SnCl2 was
omitted. 5 µM tolfenamic acid decreased the prostaglandin
formation by 90% in both cases. The IC50 values for
indomethacin were determined to be between 10 and 20 µM.
Dose-response curves for inhibition of coral enzymes by tolfenamic acid
and indomethacin are shown in Fig. 4,
A and B, respectively. The irreversible cyclooxygenase inhibitor, aspirin, was found to inhibit the
prostaglandin forming activity in coral with the IC50
values of 0.6 mM. As in case of mammalian cyclooxygenases,
aspirin inhibited the coral enzyme in a time-dependent
manner. For inhibition of the enzyme with 200 µM aspirin,
the half-life (t1/2) was approximately 25 min;
with 300 µM aspirin, t1/2 was
about 10 min. The dose-response curves and the time course for
inhibition of prostaglandin synthesis by aspirin are shown in Fig. 4,
C and D, respectively. Furthermore, NSAIDs
blocked the (11R)-HETE synthesis in a
dose-dependent manner to 50% of initial activity. The
amount of (15S)-HETE was too low to determine whether this
product was inhibited or not, but the accumulation of 15-HETE following
aspirin treatment of the coral enzyme was not observed. The selective
inhibitor of PGHS-2, nimesulide, was found to be a poor inhibitor of
prostaglandin catalyzing enzyme in coral and did not affect the
prostaglandin synthesis even at very high concentrations up to 1 mM. In side by side incubations, these high concentrations
of nimesulide completely blocked the activity of mammalian PGHS-1 (ram
seminal vesicle cyclooxygenase) for which it is a
less potent inhibitor than it is for PGHS-2. The
(8R)-lipoxygenase/allene oxide synthase pathway was not
blocked by NSAIDs. Otherwise, the formation of 8-HETE even increased
with increasing concentrations of NSAIDs (Fig. 4, A-C).
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DISCUSSION |
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The results of this study show that the acetone powder of G. fruticosa converts exogenous arachidonic acid into a complex mixture of oxygenation products. One dominating group of metabolites is
formed via the (8R)-lipoxygenase/allene oxide synthase and the other via the cyclooxygenase pathway. This is the first report of
both biosynthetic capabilities in the same organism. Products of the
(8R)-lipoxygenase pathway consist of (8R)-HPETE,
8-KETE, and -ketol. When incubations were performed in the presence
of a mild reducing agent, only (8R)-HETE was
formed, confirming that 8-KETE and
-ketol are formed from
8-HPETE as a common hydroperoxide intermediate. Notably, although the
co-incubation with a reducing agent blocked the allene oxide pathway
(
-ketol synthesis), it did not eliminate the prostaglandin
biosynthesis in the same incubations. We also isolated and identified
8-HETE as an endogenous component of G. fruticosa. Thus,
G. fruticosa is different from P. homomalla and
the other gorgonians where none of these lipoxygenase products have
been detected in fresh coral extracts. Otherwise, the occurrence of
8-HETE and 8-HEPE in the other Gersemia species,
Gersemia rubiformis, has been documented (24).
Unlike P. homomalla and all the other corals studied before,
G. fruticosa has a unique ability to convert exogenous
arachidonic acid into optically active prostaglandins in
vitro. The typical mammalian prostaglandins, PGE2,
PGD2, PGF2, and 15-keto PGF2
, isolated and identified in fresh coral extracts, were also synthesized from labeled arachidonic acid in a high total yield of 19-25%. Neither PGA2 nor its derivatives were detected in the
incubation mixtures or coral extracts. The esters of PGA2
are the most prominent prostaglandins in P. homomalla, but
the latter also contains esters of PGE2 and
PGF2
(25, 26), and it now seems to be much more likely
that the PGA derivatives are formed from the PGE derivatives rather
than the other way around. The isolation of PGG2 from short incubations of G. fruticosa gave a strong evidence that
prostaglandins are synthesized via the endoperoxide pathway (13).
F-type prostaglandins, the most abundant among the incubation products,
can be formed from PGG2 enzymatically or by the
non-enzymatic two-electron reduction with a natural agent. However, the
similarity of the product patterns obtained with active and thermally
inactivated coral preparations makes the latter version more favorable.
The high in vitro potency of prostaglandin synthesis of
G. fruticosa is all the more striking when taking into
account that its endogenous prostaglandin content is 2-3 orders lower
than that of the well studied prostaglandin-rich coral P. homomalla.
The effect of specific inhibitors on the product formation was investigated. In our earlier studies reporting prostaglandin biosynthesis in G. fruticosa, we had not detected the significant inhibition by NSAIDs (12). In retrospect, this can be attributed to the high concentrations of protein required to measure activity. In the present work, we used the highly active partially purified enzyme preparations of the freshly collected G. fruticosa. This enabled us to perform inhibition studies at lower protein concentrations, and therefore minimize the loss of inhibitors due to the absorption on membrane proteins. These studies revealed inhibition patterns similar to those obtained for mammalian PGHS. NSAIDs were found to block the production of prostaglandins in G. fruticosa, with tolfenamic acid being the most potent among the NSAIDs tested (IC50 0.45-0.65 µM). In general, the IC50 values are higher compared with those reported for purified mammalian cyclooxygenases. This may be attributed to some differences in the enzymes and also to the protein binding of the inhibitors in our experiments, which could still be a factor in the crude coral preparations employed.
The synthesis of prostaglandins in coral proved to be completely resistant to the selective PGHS-2 inhibitor, nimesulide. The highest concentrations of nimesulide (1 mM) were sufficient to cause the complete inhibition of sheep PGHS-1, yet the coral enzyme was not affected. These results indicate that the evolutionary early cyclooxygenase in coral could have significant structural differences from both mammalian PGHS-1 and PGHS-2.
While the cyclooxygenase inhibitors blocked the formation of prostaglandins, these agents actually led to an increase in the 8-lipoxygenase pathway products, presumably due to the shunting of the substrate through the lipoxygenase pathway (27). This provides another line of evidence that the (8R)-lipoxygenase/allene oxide synthase pathway is not involved in the formation of prostaglandins.
In the incubations performed in the presence of SnCl2 to
trap peroxy-intermediates by the rapid reduction into the corresponding hydroxy products, the main products were PGF2 and
(8R)-HETE. Besides these compounds, three more products,
(11R)-HETE, (15S)-HETE, and
(8R,15S)-diHETE, were detected. In principle, the
hydroperoxy precursors of monohydroxy acids, (11R)-HPETE and
(15S)-HPETE, could be synthesized by lipoxygenases (28-30)
or, alternatively, be formed as by-products of a cyclooxygenase
reaction (28, 31-34). In both cases, the oxygenation is initiated by
the pro-S hydrogen abstraction at C-13, followed by addition
of the molecular oxygen at C-11 or C-15. Biosynthesis of
(11R)-HETE has been reported in several marine invertebrates
where it was not inhibited by indomethacin (28, 35). In our
experiments, different NSAIDs caused substantial
dose-dependent inhibition of (11R)-HETE
biosynthesis, suggesting that this compound was produced as a
by-product during the cyclooxygenase-catalyzed prostaglandin
biosynthesis. The same inhibitory effect of NSAIDs on the production of
(11R)-HETE is seen with mammalian cyclooxygenases (32, 34).
The finding of NSAID inhibition of (11R)-HETE formation in
coral provides an additional support for the conclusion that the enzyme
related to mammalian cyclooxygenases accounts for the prostaglandin
biosynthesis. While the (11R)-HETE synthesis in coral was
inhibited by NSAIDs, its formation did not decrease as much as that of
the prostaglandins, leaving open the possibility that part of this
metabolite could be formed by a specific lipoxygenase. The amount of
(15S)-HETE was too small for reliable determination of the
effect of inhibitors. However, it was clear that, like mammalian
PGHS-1, aspirin treatment of coral cyclooxygenase did not result in
accumulation of 15-HETE. Whereas aspirin completely inhibits
bis-oxygenation of arachidonic acid by PGHS-1, in contrast,
aspirin-acetylated PGHS-2 metabolizes arachidonic acid primarily to
(15R)-HETE (16, 36).
Regarding the other by-products seen in incubation with SnCl2, the origin of (8R,15S)-diHETE is a matter of speculation. As both 8- and 15-monohydroxy acids of the correct stereochemistry are formed in the incubation, a simple explanation is that arachidonic acid is transformed to diHETE by a subsequent attack of these two lipoxygenases. This scheme is supported by the finding that indomethacin and tolfenamic acid did not affect the formation of diHETE, thus excluding the involvement of the PGHS enzyme in the oxygenation at C-15. It was shown that 15-HPETE and 15-HETE are poor substrates for the P. homomalla (8R)-lipoxygenase (37). Thus, a plausible version is that the (8R)-HETE accumulated in high concentration in the presence of SnCl2 is further oxidized by the (15S)-lipoxygenase to give the diHETE product. Alternatively, 8,15-diHETE could be formed via the double oxygenation of arachidonic acid by (8R)-lipoxygenase. This is analogous to reactions of soybean lipoxygenase (38), except that in this case both hydroxy groups would have the R-stereochemistry. The occurrence of several lipoxygenase activities with different biological functions has been reported in various marine invertebrates. P. homomalla is known to synthesize minor 11-HETE (10), but no 15-HETE formation has been found in this coral. A low 15-HETE synthesizing activity has been detected in Pseudoplexaura porosa in addition to its main (8R)-lipoxygenase activity (39).
In summary, we conclude that two distinct fatty acid metabolic routes
exist in coral, i.e. both lipoxygenase and cyclooxygenase enzymes are involved in the oxidative metabolism of arachidonic acid.
Prostaglandin biosynthesis proceeds similarly to that of mammals via
the cyclooxygenase pathway, except that the coral enzyme exhibits very
low peroxidase activity. The characterization of the coral enzyme is in
progress; in polymerase chain reaction experiments with the coral
cDNA using primers based on highly conserved mammalian PGHS
sequences, we have detected a PGHS-related sequence with about 50%
amino acid identity to each mammalian PGHS-1 and PGHS-2, confirming the
existence of an enzyme related to mammalian cyclooxygenases. The allene
oxide pathway is not involved in the formation of coral prostaglandins.
Nonetheless, there remains the possibility that some of the prostanoids
found in other species of coral such as clavulones of Clavularia
viridis, could be formed via the lipoxygenase/allene oxide
pathway. There remain many interesting possibilities for further study
of biosynthetic pathways from arachidonic acid to unusual cyclic
oxylipins in coral.
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ACKNOWLEDGEMENTS |
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The contribution of the Kartesh White Sea Biological Station of the Institute of Zoology, Russian Academy of Sciences, to the collection of G. fruticosa is gratefully acknowledged. We thank William E. Boeglin, Aleksander-Mati Müürisepp, and Milana Liiv for assistance with the mass spectrometry and Aino Vahemets for technical assistance.
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FOOTNOTES |
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* This work was supported by National Institutes of Health (NIH)/Fogarty International Research Collaboration Award Grant 11 RO3 TW00404-01, by NIH Grant GM 53638 (to A. R. B.), and by Estonian Science Foundation Grants 2153 and 3783 (to N. S.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
§ To whom correspondence should be addressed. Tel.: 372-2-526-510; Fax: 372-2-526-513; E-mail: kylli{at}boc.ic.ee.
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ABBREVIATIONS |
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The abbreviations used are:
PGA2, prostaglandin A2;
PGE2, prostaglandin
E2;
PGD2, prostaglandin D2;
PGF2, prostaglandin F2
;
PGG2, prostaglandin G2;
PGH2, prostaglandin
H2;
HETE, hydroxyeicosatetraenoic acid;
HPETE, hydroperoxyeicosatetraenoic acid;
diHETE, dihydroxyeicosatetraenoic
acid;
KETE, ketoeicosatetraenoic acid;
HEPE, hydroxyeicosapentaenoic
acid;
PGHS, prostaglandin endoperoxide synthase;
NSAID, nonsteroidal
anti-inflammatory drug;
HPLC, high performance liquid chromatography;
RP-HPLC, reverse-phase HPLC;
SP-HPLC, straight-phase HPLC;
Me3Si, trimethylsilyl;
GC-MS, gas chromatography-mass
spectroscopy.
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REFERENCES |
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