Requirements for hydroperoxide by the cyclooxygenase and peroxidase activities of prostaglandin H synthase

Purified prostaglandin H synthase contains cyclooxygenase activity that forms the hydroperoxide, prostaglandin G, and peroxidase activity which removes hydroperoxides. Since hydroperoxides are necessary activators of cyclooxygenase activity, the paradoxical presence of two apparently opposing activities requires careful interpretation. Kinetic studies indicate that the concentration of hydroperoxide needed for full cyclooxygenase activity is much less than that which gives 50 percent effectiveness with the peroxidase. Thus, the peroxidase activity of the synthase is very ineffective in decreasing the hydroperoxide concentration below levels that still permit rapid cyclooxygenase action.     

Requirements for hydroperoxide by the cyclooxygenase and peroxidase activities of prostaglandin H synthase

Purified prostaglandin H synthase contains cyclooxygenase activity that forms the hydroperoxide, prostaglandin G, and peroxidase activity which removes hydroperoxides. Since hydroperoxides are necessary activators of cyclooxygenase activity, the paradoxical presence of two apparently opposing activities requires careful interpretation. Kinetic studies indicate that the concentration of hydroperoxide needed for full cyclooxygenase activity is much less than that which gives 50 percent effectiveness with the peroxidase. Thus, the peroxidase activity of the synthase is very ineffective in decreasing the hydroperoxide concentration below levels that still permit rapid cyclooxygenase action.     

The participation of hydroperoxides and oxygen radicals in the control of prostaglandin synthesis

Our results demonstrate that the organic hydroperoxide t-butyl-hydroperoxide (TBHP) influences the synthesis of prostaglandins in the human embryo lung fibroblast. TBHP inhibits or stimulates prostaglandin synthesis as a function of its concentration. Regardless of the concentration employed in these experiments however, TBHP stimulated the release of arachidonate from lipid stores. When the arachidonate release step in prostaglandin (PG) synthesis was bypassed by the addition of free arachidonate to the cell cultures, t-butyl hydroperoxide further stimulated PG synthesis, indicating that the hydroperoxide activates arachidonate conversion to prostaglandins. 4-Hydroxy-2,2,6,6-tetramethylpiperidinoxy radical, a scavenger of oxygen radicals, when added to cell cultures alone had no measurable effect on either arachidonate release or prostaglandin synthesis. When 4-hydroxy-2,2,6,6-tetramethylpiperidinoxy radical was administered to cell cultures in combination with t-butyl hydroperoxide, it increased prostaglandin synthesis while inhibiting arachidonate release by the hydroperoxide. The participation of hydroperoxides and trappers of oxygen radicals in the regulation of PG synthesis is not unique to lung fibroblasts. Endothelial cells from the vasculature as well as fibroblasts from the cornea also appear to be affected by these compounds with respect to prostaglandin synthesis.     

Constraints on prostaglandin biosynthesis in tissues

The formation of prostaglandins by prostaglandin H synthase can be limited by the availability of the fatty acid substrate or the hydroperoxide activator and also by a self-catalyzed inactivation associated with the oxygenation reaction. Each pmol of synthase appeared able to form only about 1300 pmol of prostaglandin from arachidonate before it was inactivated. This extent of synthesis was not diminished when substrate fatty acid was complexed with cytosolic proteins even though the velocity of the oxygenation reaction was greatly decreased by the lower availability of substrate acid. When the availability of hydroperoxide activator was decreased by added glutathione peroxidase, the extent of oxygenation per mol of synthase was decreased irrespective of the amount of cytosolic protein present. Approximately 65% of the total prostaglandin synthesis by homogenates was suppressed with a glutathione peroxidase to prostaglandin H synthase ratio of about 90. The remaining prostaglandin synthetic activity was more resistant, being completely suppressed only when the ratio of peroxidase to synthase exceeded 750. The overall ratio of glutathione peroxidase (peroxide-removing) capacity to prostaglandin synthetic (peroxide-forming) capacity in selected tissues ranged from over 1800 in rat liver to less than 30 in leukocytes. A comparison between the daily urinary output of prostaglandin metabolites and tissue prostaglandin synthetic capacity suggested that prostaglandin H synthase inactivation along with glutathione peroxidase suppression of the extent of prostaglandin synthase may be important in limiting prostaglandin biosynthesis within cells.     

Metabolism of paracetamol to a glutathione conjugate catalyzed by prostaglandin synthetase

Microsomes isolated from sheep seminal vesicles (SSV) were found to catalyze the metabolic activation of paracetamol as evidenced by rapid formation of paracetamol glutathione conjugate when SSV microsomes were incubated with paracetamol in the presence of arachidonic acid and GSH. The activity was inhibited by indomethacin indicating the involvement of prostaglandin synthetase in the reaction. The initial activity was very rapid, and the affinity for paracetamol in the reaction was high, since formation of the glutathione conjugate was optimal already at 0.2 mM drug concentration.It is concluded that the activation of paracetamol is due to the peroxidase activity of prostaglandin synthetase in SSV microsomes, since linolenic acid hydroperoxide was also able to support the reaction.     

Inhibition of prostaglandin delta 13 reductase activity in rabbit kidney cortex by glutathione disulfide.

t-Butyl hydroperoxide and H2O2-Fe(2+)-EDTA-glutathione system which produces hydroxyl radicals did not affect the 15-hydroxy prostaglandin dehydrogenase activity in rabbit kidney cortex. On the other hand, H2O2-Fe(2+)-EDTA-glutathione system inhibited the prostaglandin delta 13 reductase activity. Mannitol, a scavenger of hydroxyl radicals, had no effect on the inhibitory action of this system, indicating that the effect of H2O2-Fe(2+)-EDTA-glutathione system on the prostaglandin delta 13 reductase may not be due to produced hydroxyl radicals. As a result of further investigation, it was shown that glutathione disulfide, which is synthesized concomitantly with hydroxyl radicals from H2O2-Fe(2+)-EDTA-glutathione, inhibited the prostaglandin delta 13 reductase activity. These results suggest that hydroperoxides and hydroxyl radicals may not be likely candidates for the modulator of the catabolism of prostaglandins in the kidney cortex, and that glutathione disulfide has the potential to modulate the prostaglandin catabolism by affecting the prostaglandin delta 13 reductase activity.     

Biological consequences of fatty acid oxygenase reaction mechanisms

Fatty acid oxygenases catalyze the insertion of molecular oxygen into polyunsaturated fatty acids. The enzymic reactions that have been studied in detail exhibit a continuous requirement for a hydroperoxide activator and appear to proceed by a free radical chain reaction. The self-limiting nature of fatty acid oxygenase-catalyzed reactions appears to be due to enzyme self-inactivation during the reaction rather than to product inhibition. Thus “suicide” substrates are potential regulators of overall enzyme activity, although linoleate is a much weaker inactivator than the highly unsaturated fatty acids. Inhibition by added glutathione peroxidase has demonstrated the need for hydroperoxide activator in the cyclooxygenase reaction catalyzed by prostaglandin H synthase and the lipoxygenase reactions catalyzed by lung, leukocyte, and soybean enzyme preparations. The regulation of cellular hydroperoxide levels may influence the formation of prostaglandins and other autacoids by fatty acid oxygenases.     

Selective microdetermination of lipid hydroperoxides

A sensitive and selective assay for lipid hydroperoxides was developed based upon the activation by hydroperoxides of the cyclooxygenase activity of prostaglandin H synthase. The assay measures hydroperoxides directly by their stimulatory action on the cyclooxygenase and thus differs from the methods used currently which rely on the measurement of secondary products to estimate the amount of hydroperoxide. The present assay of enzymatic response was approximately linear in the range 10 to 150 pmol of added lipid hydroperoxide. This sensitivity for lipid peroxides is about 50-fold greater than that of the thiobarbiturate assay with fluorescence detection. When applied to samples of human plasma, the enzymatic assay indicated that the concentration of lipid hydroperoxides in normal subjects is 0.5 microM, more than 50-fold lower than estimated by the thiobarbiturate assay (30-50 microM). Nevertheless, the circulating concentration of 0.5 microM lipid hydroperoxide approaches that reported to have deleterious effects upon vascular prostacyclin synthase.     

Phenylbutazone-dependent epoxidation of 7,8-dihydroxy-7,8-dihydrobenzo(a)pyrene. A new mechanism for prostaglandin H synthase-catalyzed oxidations

The nonsteroidal anti-inflammatory drug phenylbutazone markedly enhances the hydroperoxide-dependent epoxidation of 7,8-dihydroxy-7,8-dihydrobenzo(a)pyrene catalyzed by microsomal and Tween-20 solubilized preparations of prostaglandin H synthase. Furthermore, phenylbutazone radically alters the hydroperoxide specificity of 7,8-dihydroxy-7,8-dihydrobenzo(a)pyrene epoxidation. In the absence of phenylbutazone, only allylic hydroperoxides are effective in initiating epoxidation, whereas in the presence of phenylbutazone the reaction can be initiated by t-butyl hydroperoxide, cumene hydroperoxide, and hydrogen peroxide. All effects are dependent on the concentration of phenylbutazone present. The primary event is the oxidation of phenylbutazone by prostaglandin H synthase. This pathway yields a peroxy radical of phenylbutazone which appears to be the epoxidizing agent. This activation of a primary substrate by a peroxidase resulting in metabolism of a secondary substrate is analogous to the halogenation reactions catalyzed by chloroperoxidase. This represents a new class of oxidation reactions catalyzed by prostaglandin H synthase.     

The mechanism for the inhibition of prostaglandin H synthase-catalyzed xenobiotic oxidation by methimazole. Reaction with free radical oxidation products

Methimazole, an irreversible, mechanism-based (suicide substrate) inhibitor of thyroid peroxidase and lactoperoxidase, also inhibits the oxidation of xenobiotics by prostaglandin hydroperoxidase. The mechanism(s) by which methimazole inhibits prostaglandin H synthase-catalyzed oxidations is not conclusively known. In studies reported here, methimazole inhibited the prostaglandin H synthase-catalyzed oxidation of benzidine, phenylbutazone, and aminopyrine in a concentration-dependent manner. Methimazole poorly supported the prostaglandin H synthase-catalyzed reduction of 5-phenyl-4-pentenyl hydroperoxide to the corresponding alcohol (5-phenyl-4-pentenyl alcohol), suggesting that methimazole is not serving as a competing reducing cosubstrate for the peroxidase. Methimazole is not a mechanism-based inhibitor of prostaglandin hydroperoxidase or horseradish peroxidase since methimazole did not inhibit the peroxidase-catalyzed, benzidine-supported reduction of 5-phenyl-4-pentenyl hydroperoxide. In contrast, methimazole inhibited the reduction of 5-phenyl-4-pentenyl hydroperoxide to 5-phenyl-4-pentenyl alcohol catalyzed by lactoperoxidase, confirming that methimazole is a mechanism-based inhibitor of that enzyme and that such inhibition can be detected by our assay. Glutathione reduces the aminopyrine cation free radical, the formation of which is catalyzed by the hydroperoxidase, back to the parent compound. Methimazole produced the same effect at concentrations equimolar to those required for glutathione. These data indicate that methimazole does not inhibit xenobiotic oxidations catalyzed by prostaglandin H synthase and horseradish peroxidase through direct interaction with the enzyme, but rather inhibits accumulation of oxidation products via reduction of a free radical-derived metabolite(s).     

Prostaglandin H synthase. Kinetics of tyrosyl radical formation and of cyclooxygenase catalysis.

Hydroperoxides are known to induce the formation of tyrosyl free radicals in prostaglandin (PG) H synthase. To evaluate the role of these radicals in cyclooxygenase catalysis we have analyzed the temporal correlation between radical formation and substrate conversion during reaction of the synthase with arachidonic acid. PGH synthase reacted with equimolar levels of arachidonic acid generated sequentially the wide doublet (34 G peak-to-trough) and wide singlet (32 G peak-to-trough) tyrosyl radical signals previously reported for reaction with hydroperoxide. The kinetics of formation and decay of the doublet signal corresponded reasonably well with those of cyclooxygenase activity. However, the wide singlet free radical signal accumulated only after prostaglandin formation had ceased, indicating that the wide singlet is not likely to be an intermediate in cyclooxygenase catalysis. When PGH synthase was reacted with 25 equivalents of arachidonic acid, the wide doublet and wide singlet radical signals were not observed. Instead, a narrower singlet (24 G peak-to-trough) tyrosyl radical was generated, similar to that found upon reaction of indomethacin-treated synthase with hydroperoxide. Only about 11 mol of prostaglandin were formed per mol of synthase before complete self-inactivation of the cyclooxygenase, far less than the 170 mol/mol synthase produced under standard assay conditions. Phenol (0.5 mM) increased the extent of cyclooxygenase reaction by only about 50%, in contrast to the 460% stimulation seen under standard assay conditions. These results indicate that the narrow singlet tyrosyl radical observed in the reaction with high levels of arachidonate in this study and by Lassmann et al. (Lassmann, G., Odenwaller, R., Curtis, J.F., DeGray, J.A., Mason, R.P., Marnett, L.J., and Eling, T.E. (1991) J. Biol. Chem. 266, 20045-20055) is associated with abnormal cyclooxygenase activity and is probably nonphysiological. In titrations of the synthase with arachidonate or with hydroperoxide, the loss of enzyme activity and destruction of heme were linear functions of the amount of titrant added. Complete inactivation of cyclooxygenase activity was found at about 10 mol of arachidonate, ethyl hydrogen peroxide, or hydrogen peroxide per mol of synthase heme; maximal bleaching of the heme Soret absorbance peak was found with 10 mol of ethyl hydroperoxide or 20 mol of either arachidonate or hydrogen peroxide per mol of synthase heme. The peak concentration of the wide doublet tyrosyl radical did not change appreciably with increased levels of ethyl hydroperoxide. In contrast, higher levels of hydroperoxide gave higher levels of the wide singlet radical species, in parallel with enzyme inactivation.(ABSTRACT TRUNCATED AT 400 WORDS)     

Phenolic anticyclooxygenase agents in antiinflammatory and analgesic therapy

This report demonstrates that a non-steroidal antiinflammatory drug, MK 447, and phenol act similarly to inhibit cyclooxygenase activity in vitro in a dose-dependent manner when the concentration of peroxide activators is decreased by glutathione peroxidase. Increasing the rate of peroxide removal with higher amounts of glutathione peroxidase increases the inhibitory potency of the phenolic agents. The results support ascribing a vital role to tissue peroxides in facilitating prostaglandin biosynthesis in vivo. They also resolve the paradoxical problem of predicting antiinflammatory activity when an agent appears to stimulate prostaglandin formation. A corollary concept is that tissues with general hyperalgesic conditions may have lower levels of hydroperoxide than are found in inflammatory conditions, and thus be more amenable to therapy with phenolic agents like MK 447.     

Amino acid substitutions in the human glutathione S-transferases confer different specificities in the prostaglandin endoperoxide conversion pathway

The human glutathione S-transferases 1-1 and 2-2, which differ from each other by 11 amino acids, have different catalytic activities against cumene hydroperoxide and t-butyl hydroperoxide. Using prostaglandin H2 as the peroxide substrate, we found that GSH S-transferase 1-1 catalyzed the transformation of prostaglandin H2 to prostaglandin F2 alpha and E2 at a 4:1 ratio whereas GSH S-transferase 2-2 produced primarily prostaglandin D2 and F2 alpha at a 4:1 ratio. Our results indicate that GSH S-transferases catalyze the reduction and isomerization of prostaglandin H2 endoperoxide in vitro. We suggest that the amino acid substitutions between these two isozymes may be responsible for the difference in catalytic specificities. We propose that these isozymes are important reagents for the biosynthesis of various prostaglandins.     

Prostaglandin H synthase: an example of enzymic symbiosis

Reaction conditions which promote the heme-dependent peroxidase activity of prostaglandin H synthase appear to stimulate the heme-dependent cyclooxygenase activity also present in the synthase, even though the cyclooxygenase requires hydroperoxide for activity. However, aspirin-treated synthase, which retains only peroxidase activity, inhibited the cyclooxygenase activity of untreated synthase in the manner observed with similar levels of glutathione peroxidase. Any stimulatory effect of the synthase peroxidase on the synthase cyclooxygenase is thus likely to involve an intramolecular mechanism. Participation of peroxidase intermediates (Compounds I and II) in the initiation of a cyclooxygenase free radical chain reaction may provide an intramolecular mechanism for stimulation of the synthase cyclooxygenase by the synthase peroxidase.     

The formation of aminopyrine cation radical by the peroxidase activity of prostaglandin H synthase and subsequent reactions of the radical

The oxidation of aminopyrine to an aminopyrine cation radical was investigated using a solubilized microsomal preparation or prostaglandin H synthase purified from ram seminal vesicles. Aminopyrine was oxidized to an aminopyrine cation radical in the presence of arachidonic acid, hydrogen peroxide, t-butyl hydroperoxide or 15-hydroperoxyarachidonic acid. Highly purified prostaglandin H synthase, which processes both cyclo-oxygenase and hydroperoxidase activity, oxidized aminopyrine to the free radical. Purified prostaglandin H synthase reconstituted with Mn2+ protoporphyrin IX, which processes only cyclo-oxygenase activity, did not catalyze the formation of the aminopyrine free radical. Aminopyrine stimulated the reduction of 15-hydroperoxy-5,8,11,13-eicosatetraenoic acid to 15-hydroxy-5,8,11-13-eicosatetraenoic acid. Approximately 1 molecule of 15-hydroperoxy-5,8,11,13-eicosatetraenoic acid was reduced for every 2 molecules of aminopyrine free radical formed, giving a stoichiometry of 1:2. The decay of the aminopyrine radical obeyed second-order kinetics. These results support the proposed mechanism in which aminopyrine is oxidized by prostaglandin H synthase hydroperoxidase to the aminopyrine free radical, which then disproportionates to the iminium cation. The iminium cation is further hydrolyzed to the demethylated amine and formaldehyde. Glutathione reduced the aminopyrine radical to aminopyrine with the concomitant oxidation of GSH to its thiyl radical as detected by ESR of the glutathione thiyl radical adduct.     

Prostaglandin H synthase and hydroperoxides: peroxidase reaction and inactivation kinetics

The reaction kinetics of the peroxidase activity of prostaglandin H synthase have been examined with 15-hydroperoxyeicosatetraenoic acid and hydrogen peroxide as substrates and tetramethylphenylenediamine as cosubstrate. The apparent Km and Vmax values for both hydroperoxides were found to increase linearly with the cosubstrate concentration. The overall reaction kinetics could be interpreted in terms of an initial reaction of the synthase with hydroperoxide to form an intermediate equivalent to horseradish peroxidase Compound I, followed by reduction of this intermediate by cosubstrate to regenerate the resting enzyme. The rate constants estimated for the generation of synthase Compound I were 7.1 X 10(7) M-1 s-1 with the lipid hydroperoxide and 9.1 X 10(4) M-1 s-1 with hydrogen peroxide. The rate constants estimated for the rate-determining step in the regeneration of resting enzyme by cosubstrate were 9.2 X 10(6) M-1 s-1 in the case of the reaction with lipid hydroperoxide and 3.5 X 10(6) M-1 s-1 in the case of reaction with hydrogen peroxide. The intrinsic affinities of the synthase peroxidase for substrate (Ks) were estimated to be on the order of 10(-8) M for lipid hydroperoxide and 10(-5) M for hydrogen peroxide. These affinities are quite similar to the reported affinities of the synthase for these hydroperoxides as activators of the cyclooxygenase. The peroxidase activity was found to be progressively inactivated during the peroxidase reaction. The rate of inactivation of the peroxidase was increased by increases in hydroperoxide level, and decreased by increases in peroxidase cosubstrate. The inactivation of the peroxidase appeared to occur by a hydroperoxide-dependent process, originating from synthase Compound I or Compound II.     

Peroxidatic oxidation of benzo[a]pyrene and prostaglandin biosynthesis.

The arachidonic acid dependent oxidation of benzo[a]pyrene to a mixture of 3,6-, 1,6-, and 6,12-quinones has been studied by using enzyme preparations from sheep seminal vesicles. Maximal oxidation is observed at 100 microM benzo[a]pyrene and 150 microM arachidonic acid. The arachidonic acid dependent oxidation is peroxidatic and utilizes prostaglandin G2 (PGG2), generated in situ from arachidonate, as the hydroperoxide substrate. 15-Hydroperoxy-5,8,11,13-eicosatetraenoic acid is equivalent to PGG2 as a hydroperoxide substrate, but hydrogen peroxide, cumene hydroperoxide, and tert-butyl hydroperoxide are much poorer substrates. Arachidonic acid dependent benzo[a]pyrene oxidation by microsomal and solubilized enzyme preparations is markedly.     

Quantitative studies of hydroperoxide reduction by prostaglandin H synthase. Reducing substrate specificity and the relationship of peroxidase to cyclooxygenase activities

The peroxidase activity of prostaglandin H (PGH) synthase catalyzes reduction of 5-phenyl-4-pentenyl hydroperoxide to 5-phenyl-4-pentenyl alcohol with a turnover number of approximately 8000 mol of 5-phenyl-4-pentenyl hydroperoxide/mol of enzyme/min. The kinetics and products of reaction establish PGH synthase as a classical heme peroxidase with catalytic efficiency similar to horseradish peroxidase. This suggests that the protein of PGH synthase evolved to facilitate peroxide heterolysis by the heme prosthetic group. Comparison of an extensive series of phenols, aromatic amines, beta-dicarbonyls, naturally occurring compounds, and nonsteroidal anti-inflammatory drugs indicates that considerable differences exist in their ability to act as reducing substrates. No correlation is observed between the ability of compounds to support peroxidatic hydroperoxide reduction and to inhibit cyclooxygenase. In addition, the resolved enantiomers of MK-410 and etodolac exhibit dramatic enantiospecific differences in their ability to inhibit cyclooxygenase but are equally potent as peroxidase-reducing substrates. This suggests that there are significant differences in the orientation of compounds at cyclooxygenase inhibitory sites and the peroxidase oxidation site(s). Comparison of 5-phenyl-4-pentenyl hydroperoxide reduction by PGH synthase and horseradish peroxidase reveals considerable differences in reducing substrate specificity. Both the cyclooxygenase and peroxidase activities of PGH synthase inactivate in the presence of low micromolar amounts of hydroperoxides and arachidonic acid. PGH synthase was most sensitive to arachidonic acid, which exhibited an I50 of 0.6 microM in the absence of all protective agents. Inactivation by hydroperoxides requires peroxidase turnover and can be prevented by reducing substrates. The I50 values for inactivation by 15-hydroperoxy-5,8,11,13-eicosatetraenoic acid are 4.0 and 92 microM, respectively, in the absence and presence of 500 microM phenol, a moderately good reducing substrate. The ability of compounds to protect against hydroperoxide-induced inactivation correlates directly with their ability to act as reducing substrates. Hydroquinone, an excellent reducing substrate, protected against hydroperoxide-induced inactivation when present in less than 3-fold molar excess over hydroperoxide. The presence of a highly efficient hydroperoxide-reducing activity appears absolutely essential for protection of the cyclooxygenase capacity of PGH synthase. The peroxidase activity is, therefore, a twin-edged sword, responsible for and protective against hydroperoxide-dependent inactivation of PGH synthase.(ABSTRACT TRUNCATED AT 400 WORDS)     

Prostaglandin H synthase: distinct binding sites for cyclooxygenase and peroxidase substrates

Prostaglandin H synthase has two distinct catalytic activities: a cyclooxygenase activity that forms prostaglandin G2 from arachidonic acid; and a peroxidase activity that reduces prostaglandin G2 to prostaglandin H2. Lipid hydroperoxides, such as prostaglandin G2, also initiate the cyclooxygenase reaction, probably via peroxidase reaction cycle enzyme intermediates. The relation between the binding sites for lipid substrates of the two activities was investigated with an analysis of the effects of arachidonic and docosahexaenoic acids on the reaction kinetics of the peroxidase activity, and their effects on the ability of a lipid hydroperoxide to initiate the cyclooxygenase reaction. The cyclooxygenase activity of pure ovine synthase was found to have an apparent Km value for arachidonate of 5.3 microM and a Ki value (competetive inhibitor) for docosahexaenoate of 5.2 microM. When present at 20 microM neither fatty acid had a significant effect on the apparent Km value of the peroxidase for 15-hydroperoxyeicosatetraenoic acid: the values were 7.6 microM in the absence of docosahexaenoic acid and 5.9 microM in its presence, and (using aspirin-treated synthase) 13.7 microM in the absence of arachidonic acid and 15.7 microM in its presence. Over a range of 1 to 110 microM the level of arachidonate had no significant effect on the initiation of the cyclooxygenase reaction by 15-hydroperoxyeicosatetraenoic acid. The inability of either arachidonic acid or docosahexaenoic acid to interfere with the interaction between the peroxidase and lipid hydroperoxides indicates that the cyclooxygenase and peroxidase activities of prostaglandin H synthase have distinct binding sites for their lipid substrates.     

Cellular regulation of prostaglandin H synthase catalysis

Prostanoids are a group of potent bioactive lipids produced by oxygenation of arachidonate or one of several related polyunsaturated fatty acids. Cellular prostaglandin biosynthesis is tightly regulated, with a large part of the control exerted at the level of cyclooxygenase catalysis by prostaglandin H synthase (PGHS). The two known isoforms of PGHS have been assigned distinct pathophysiological functions, and their cyclooxygenase activities are subject to differential cellular control. This review considers the contributions to cellular catalytic control of the two PGHS isoforms by intracellular compartmentation, accessory proteins, arachidonate levels, and availability of hydroperoxide activator.