Controlled tryptic digestion of prostaglandin H synthase. Characterization of protein fragments and enhanced rate of proteolysis of oxidatively inactivated enzyme

Cyclooxygenase reaction mechanism of PGHS--evidence for a reversible transition between a pentadienyl radical and a new tyrosyl radical by nitric oxide trapping

Incubation of prostaglandin H synthase-1 (PGHS-1) under anaerobic conditions with peroxide and arachidonic acid leads to two major radical species: a pentadienyl radical and a radical with a narrow EPR spectrum. The proportions of the two radicals are sensitive to temperature, favoring the narrow radical species at 22 °C. The EPR characteristics of this latter radical are somewhat similar to the previously reported narrow-singlet tyrosine radical NS1a and are insensitive to deuterium labeling of AA. To probe the origin and structure of this radical, we combined EPR analysis with nitric oxide (NO) trapping of tyrosine and substrate derived radicals for both PGHS-1 and -2. Formation of 3-nitrotyrosine in the proteins was analyzed by immunoblotting, whereas NO adducts to AA and AA metabolites were analyzed by mass spectrometry and by chromatography of (14)C-labeled products. The results indicate that both nitrated tyrosine residues and NO-AA adducts formed upon NO trapping. The NO-AA adduct was predominantly an oxime at C11 of AA with three conjugated double bonds, as indicated by absorption at 275 nm and by mass spectral analysis. This adduct amounted to 10% and 20% of the heme concentration of PGHS-1 and -2, respectively. For PGHS-1, the yield of NO-AA adduct matched the yield of the narrow radical signal obtained in parallel EPR experiments. High frequency EPR characterization of this narrow radical, reported in an accompanying paper, supports assignment to a new tyrosyl radical, NS1c, rather than an AA-based radical. To reconcile the results from EPR and NO-trapping studies, we propose that the NS1c is in equilibrium with an AA pentadienyl radical, and that the latter reacts preferentially with NO.

Characterization of an AM404 analogue, N-(3-hydroxyphenyl)arachidonoylamide, as a substrate and inactivator of prostaglandin endoperoxide synthase

N-(4-Hydroxyphenyl)arachidonoylamide (AM404) is an inhibitor of endocannabinoid inactivation that has been used in cellular and animal studies. AM404 is a derivative of arachidonic acid and has been reported to inhibit arachidonate oxygenation by prostaglandin endoperoxide synthase-1 and -2 (PGHS-1 and -2, respectively). While examining the structural requirements for inhibition of PGHS, we discovered that the meta isomer of AM404, N-(3-hydroxyphenyl)arachidonoylamide (3-HPAA), is a substrate for purified PGHS. PGHS-2 efficiently oxygenated 3-HPAA to prostaglandin and hydroxyeicosatetraenoate products. No oxidation of the phenolamide moiety was observed. 3-HPAA appeared to be converted by PGHS-1 in a similar manner; however, conversion was less efficient than that by PGHS-2. PGHS-2 was selectively, dose-dependently, and irreversibly inactivated in the presence of 3-HPAA. Complete inactivation of PGHS-2 was achieved with 10 μM 3-HPAA. Preliminary characterization revealed that 3-HPAA inactivation did not result from covalent modification of PGHS-2 or damage to the heme moiety. These studies provide additional insight into the structural requirements for substrate metabolism and inactivation of PGHS and report the first metabolism-dependent, selective inactivator of PGHS-2.

Structural basis of enantioselective inhibition of cyclooxygenase-1 by S-alpha-substituted indomethacin ethanolamides

The modification of the nonselective nonsteroidal anti-inflammatory drug, indomethacin, by amidation presents a promising strategy for designing novel cyclooxygenase (COX)-2-selective inhibitors. A series of alpha-substituted indomethacin ethanolamides, which exist as R/S-enantiomeric pairs, provides a means to study the impact of stereochemistry on COX inhibition. Comparative studies revealed that the R- and S-enantiomers of the alpha-substituted analogs inhibit COX-2 with almost equal efficacy, whereas COX-1 is selectively inhibited by the S-enantiomers. Mutagenesis studies have not been able to identify residues that manifest the enantioselectivity in COX-1. In an effort to understand the structural impact of chirality on COX-1 selectivity, the crystal structures of ovine COX-1 in complexes with an enantiomeric pair of these indomethacin ethanolamides were determined at resolutions between 2.75 and 2.85 A. These structures reveal unique, enantiomer-selective interactions within the COX-1 side pocket region that stabilize drug binding and account for the chiral selectivity observed with the (S)-alpha-substituted indomethacin ethanolamides. Kinetic analysis of binding demonstrates that both inhibitors bind quickly utilizing a two-step mechanism. However, the second binding step is readily reversible for the R-enantiomer, whereas for the S-enantiomer, it is not. These studies establish for the first time the structural and kinetic basis of high affinity binding of a neutral inhibitor to COX-1 and demonstrate that the side pocket of COX-1, previously thought to be sterically inaccessible, can serve as a binding pocket for inhibitor association.

Cyclooxygenase Isozymes: The Biology of Prostaglandin Synthesis and Inhibition

Nonsteroidal anti-inflammatory drugs (NSAIDs) represent one of the most highly utilized classes of pharmaceutical agents in medicine. All NSAIDs act through inhibiting prostaglandin synthesis, a catalytic activity possessed by two distinct cyclooxygenase (COX) isozymes encoded by separate genes. The discovery of COX-2 launched a new era in NSAID pharmacology, resulting in the synthesis, marketing, and widespread use of COX-2 selective drugs. These pharmaceutical agents have quickly become established as important therapeutic medications with potentially fewer side effects than traditional NSAIDs. Additionally, characterization of the two COX isozymes is allowing the discrimination of the roles each play in physiological processes such as homeostatic maintenance of the gastrointestinal tract, renal function, blood clotting, embryonic implantation, parturition, pain, and fever. Of particular importance has been the investigation of COX-1 and -2 isozymic functions in cancer, dysregulation of inflammation, and Alzheimer's disease. More recently, additional heterogeneity in COX-related proteins has been described, with the finding of variants of COX-1 and COX-2 enzymes. These variants may function in tissue-specific physiological and pathophysiological processes and may represent important new targets for drug therapy.

The structure of mammalian cyclooxygenases

Cyclooxygenases-1 and -2 (COX-1 and COX-2, also known as prostaglandin H2 synthases-1 and -2) catalyze the committed step in prostaglandin synthesis. COX-1 and -2 are of particular interest because they are the major targets of nonsteroidal antiinflammatory drugs (NSAIDs) including aspirin, ibuprofen, and the new COX-2-selective inhibitors. Inhibition of the COXs with NSAIDs acutely reduces inflammation, pain, and fever, and long-term use of these drugs reduces the incidence of fatal thrombotic events, as well as the development of colon cancer and Alzheimer's disease. In this review, we examine how the structures of COXs relate mechanistically to cyclooxygenase and peroxidase catalysis and how alternative fatty acid substrates bind within the COX active site. We further examine how NSAIDs interact with COXs and how differences in the structure of COX-2 result in enhanced selectivity toward COX-2 inhibitors.

Comparison of the properties of prostaglandin H synthase-1 and -2

Biosynthesis of prostanoid lipid signaling agents from arachidonic acid begins with prostaglandin H synthase (PGHS), a hemoprotein in the myeloperoxidase family. Vertebrates from humans to fish have two principal isoforms of PGHS, termed PGHS-1 and-2. These two isoforms are structurally quite similar, but they have very different pathophysiological roles and are regulated very differently at the level of catalysis. The focus of this review is on the structural and biochemical distinctions between PGHS-1 and-2, and how these differences relate to the functional divergence between the two isoforms.

The structures of prostaglandin endoperoxide H synthases-1 and -2

Despite the marked differences in their physiological roles, the structures and catalytic functions of the prostaglandin H2 endoperoxide synthases-1 and -2 (PGHS-1 and -2) are almost completely identical. These integral membrane proteins catalyze the conversion of arachidonic acid to PGG2 and finally to PGH2. The crystal structures of PGHS-1 and -2 provide new insights into the catalytic mechanism for fatty acid oxygenation. Moreover, a clearer picture emerges to explain how a handful of amino acid substitutions can give rise to subtle differences in ligand binding between the two isoforms. These "small" alterations of isozyme structure are sufficient to allow the design of new, isoform-selective drugs.

Peroxidase self-inactivation in prostaglandin H synthase-1 pretreated with cyclooxygenase inhibitors or substituted with mangano protoporphyrin IX

Self-inactivation imposes an upper limit on bioactive prostanoid synthesis by prostaglandin H synthase (PGHS). Inactivation of PGHS peroxidase activity has been found to begin with Intermediate II, which contains a tyrosyl radical. The structure of this radical is altered by cyclooxygenase inhibitors, such as indomethacin and flurbiprofen, and by replacement of heme by manganese protoporphyrin IX (forming MnPGHS-1). Peroxidase self-inactivation in inhibitor-treated PGHS-1 and MnPGHS-1 was characterized by stopped-flow spectroscopic techniques and by chromatographic and mass spectrometric analysis of the metalloporphyrin. The rate of peroxidase inactivation was about 0.3 s(-)1 in inhibitor-treated PGHS-1 and much slower in MnPGHS-1 (0.05 s(-)1); as with PGHS-1 itself, the peroxidase inactivation rates were independent of peroxide concentration and structure, consistent with an inactivation process beginning with Intermediate II. The changes in metalloporphyrin absorbance spectra during inactivation of inhibitor-treated PGHS-1 were similar to those observed with PGHS-1 but were rather distinct in MnPGHS-1; the kinetics of the spectral transition from Intermediate II to the next species were comparable to the inactivation kinetics in each case. In contrast to the situation with PGHS-1 itself, significant amounts of heme degradation occurred during inactivation of inhibitor-treated PGHS-1, producing iron chlorin and heme-protein adduct species. Structural perturbations at the peroxidase site (MnPGHS-1) or at the cyclooxygenase site (inhibitor-treated PGHS-1) thus can influence markedly the kinetics and the chemistry of PGHS-1 peroxidase inactivation.

Induction of prostaglandin endoperoxide synthase-1 (COX-1) in a human promonocytic cell line by treatment with the differentiating agent TPA

Prostaglandin endoperoxide synthase (PGH synthase) is responsible for converting arachdonic acid to PGH2, the common precursor of prostaglandins. It has been shown previously that phorbol ester-induced differentiation of human promonocytic leukemia cell lines is accompanied by induction of PGH synthase enzyme and enhanced capacity to produce prostaglandins. However, the identity of the isoform of PGH synthase, i.e., PGH synthase-1 or -2, that is induced under these conditions has not been established. Northern and Western analyses revealed a dramatic increase in levels of PGH synthase-1 mRNA and protein levels within 24 hr after treatment of THP-1 cells with phorbol ester. No significant increase in PGH synthase-2 mRNA or protein was observed. The increases in PGH synthase-1 were accompanied by an enhanced capacity of the cells to produce PGE2. The current findings indicate that expression of PGH synthase-1 is greatly enhanced in a promonocytic cell line by treatment with an agent that induces differentiation.

Arachidonic acid and nonsteroidal anti-inflammatory drugs induce conformational changes in the human prostaglandin endoperoxide H2 synthase-2 (cyclooxygenase-2)

By using the technique of site-directed spin labeling combined with EPR spectroscopy, we have observed that binding of arachidonic acid and nonsteroidal anti-inflammatory drugs induces conformational changes in the human prostaglandin endoperoxide H(2) synthase enzyme (PGHS-2). Line shape broadening resulting from spin-spin coupling of nitroxide pairs introduced into the membrane-binding helices of PGHS-2 was used to calculate the inter-helical distances and changes in these distances that occur in response to binding various ligands. The inter-residue distances determined for the PGHS-2 holoenzyme using EPR were 1-7.9 A shorter than those of the crystal structure of the PGHS-2 holoenzyme. However, inter-helical distances calculated and determined by EPR for PGHS-2 complexed with arachidonic acid, flurbiprofen, and SC-58125 were in close agreement with those obtained from the cognate crystal structures. These results indicate that the structure of the solubilized PGHS-2 holoenzyme measured in solution differs from the crystal structure of PGHS-2 holoenzyme obtained by x-ray analysis. Furthermore, binding of ligands induces a conformational change in the holo-PGHS-2, converting it to a structure similar to those obtained by x-ray analysis. Proteolysis protection assays had previously provided circumstantial evidence that binding of heme and non-steroidal anti-inflammatory drugs alters the conformation of PGHS, but the present experiments are the first to directly measure such changes. The finding that arachidonate can also induce a conformational change in PGHS-2 was unexpected, and the magnitude of changes suggests this structural flexibility may be integral to the cyclooxygenase catalytic mechanism.

Design of selective inhibitors of cyclooxygenase-2 as nonulcerogenic anti-inflammatory agents

The discovery of a second isoform of cyclooxygenase (cyclooxygenase-2) that is expressed in inflammatory cells and the central nervous system, but not in the gastric mucosa, offers the possibility of developing anti-inflammatory and analgesic agents that lack the gastrointestinal side effects of currently available nonsteroidal anti-inflammatory drugs. Lead compounds from several different structural classes have been identified and shown to be slow, tight-binding inhibitors that express their selectivity for cyclooxygenase-2 in the time-dependent step. The determination of structures of enzyme-inhibitor co-crystals along with site-directed mutagenesis experiments reveal the molecular basis for selectivity of some, but not all, inhibitors. Preclinical and clinical studies suggest cyclooxygenase-2 inhibitors are highly promising new agents for the treatment of pain and inflammation, and for the prevention of cancer.

Comparison of prostaglandin H synthase isoform structures using limited proteolytic digestion

Prostaglandin H synthase (PGHS) catalyzes a key step in the biosynthesis of a variety of bioactive lipid mediators. The two known isoforms (PGHS-1 and -2) share about 60% amino acid identity, but exhibit distinct interactions with substrates, activators, and inhibitors. Ovine PGHS-1 has previously been shown to have a distinctive protease-sensitive site near Arg277; cleavage by trypsin, chymotrypsin, or proteinase K produces fragments of 33 and 38 kDa and loss of activity. The ovine PGHS-1 crystal structure shows Arg277 located in an exposed loop structure; homology modeling predicts similar loop structures for both human isoforms (hPGHS-1 and -2). We have used limited proteolytic digestion of recombinant hPGHS-1 and hPGHS-2 to probe their structures. Incubation of hPGHS-1 with either trypsin or proteinase K produced 33- and 38-kDa fragments and loss of activity. In contrast, incubation of hPGHS-2 with the same proteases led to cleavage of only a 2- to 3-kDa fragment, with no decrease in activity. Immunoblotting with site-specific antibodies demonstrated that the cleaved fragment originated from the hPGHS-2 C-terminus. Similar immunoblotting experiments indicated that trypsin did not attack the ovine PGHS-1 C-terminus. Mutagenesis was used to replace Pro263 of hPGHS-2 (corresponds to Arg277 of ovine PGHS-1) with arginine, inserting a potential trypsin site. Incubation of this P263R hPGHS-2 mutant with either trypsin or proteinase K resulted in cleavage near the C-terminus and retention of activity, just as with wild-type hPGHS-2. A peptide containing residues 259-268 of the P263R mutant was cleaved by trypsin at the same rate as a peptide corresponding to hPGHS-1 residues 272-281, demonstrating that the sequence differences were not responsible for the lack of tryptic cleavage at residue 263 in the hPGHS-2 mutant. Preincubation of hPGHS-2 with graded levels of guanidinium HCl before incubation with proteinase K did not produce large proteolytic fragments, indicating that the hPGHS-2 loop region was not selectively unfolding. The results point to two regions of significant structural difference between PGHS-1 and -2: the Arg277 loop, which is protease-sensitive in PGHS-1 but protease-resistant in PGHS-2, and the C-terminus, which is protease-sensitive in PGHS-2 but not in PGHS-1.

Photolabeling of prostaglandin endoperoxide H synthase-1 with 3-trifluoro-3-(m-[125I]iodophenyl)diazirine as a probe of membrane association and the cyclooxygenase active site

Previous studies of the crystal structure of the ovine prostaglandin endoperoxide H synthase-1 (PGHS-1)/S-flurbiprofen complex (Picot, D., Loll, P. J., and Garavito, R. M. (1994) Nature 367, 243-249) suggest that the enzyme is associated with membranes through a series of four amphipathic helices located between residues 70 and 117. We have used the photoactivatable, hydrophobic reagent 3-trifluoro-3-(m-[125I]iodophenyl)diazirine ([125I]TID) which partitions into membranes and other hydrophobic domains to determine which domains of microsomal PGHS-1 are subject to photolabeling. After incubation of ovine vesicular gland microsomes with [125I]TID, ovine PGHS-1 was one of the major photolabeled proteins. Proteolytic cleavage of labeled PGHS-1 at Arg277 with trypsin established that [125I]TID was incorporated into both the 33-kDa tryptic peptide containing the amino terminus and the 38-kDa tryptic peptide containing the carboxyl terminus. This pattern of photolabeling was not affected by the presence of 20 mM glutathione, indicating that the photolabeling observed for PGHS-1 was not due to the presence of [125I]TID in the aqueous phase. However, nonradioactive TID as well as two inhibitors, ibuprofen and sulindac sulfide, which bind the cyclooxygenase active site of PGHS-1, prevented the labeling of the 38-kDa carboxyl-terminal tryptic peptide. These results suggest that [125I]TID can label both the cyclooxygenase active site in the tryptic 38-kDa fragment and a membrane binding domain located in the 33-kDa fragment. Cleavage of photolabeled PGHS-1 with endoproteinase Lys-C yielded a peptide containing residues 25-166 which was labeled with [125I]TID. This peptide contains the putative membrane binding domain of ovine PGHS-1. Our results provide biochemical support for the concept developed from the crystal structure that PGHS-1 binds to membranes via four amphipathic helices located near the NH2 terminus of the protein.

Comparison of recombinant cyclooxygenase-2 to native isoforms: aspirin labeling of the active site

The search for isoform-specific enzyme inhibitors has been the focus of much recent research effort. Towards this goal, human recombinant cyclooxygenase-2 (EC 1.14.99.1, prostaglandin H synthase) was expressed in insect cells and purified to > 98% purity. Recombinant enzyme was characterized both by physical methods and activity measurements and shown to be fully active with kinetic properties similar to native COX-2 and COX-1. After detergent extraction, the enzyme had hydrodynamic properties indistinguishable from native bovine COX-1 and corresponded to the enzyme dimer as measured with size-exclusion chromatography. Peptide mapping via Lys-C protease identified a site of N-linked glycosylation and the aspirin covalent modification site. In the presence of heme, aspirin-specifically acetylated Ser-516. The enzyme will be suitable for biophysical studies and may lead to isoform-specific enzyme inhibitors.

Prostaglandin H synthase activity in the sheep placenta during cortisol-induced labor at 128-131 days of gestation and during spontaneous delivery at term

This study investigated whether the prostaglandin H synthase (PGHS) enzyme activity of sheep fetal placental cotyledon can be induced by cortisol at 128-131 days of gestation (dga) as compared to gestational age matched controls, before PGHS's normal gestational increase would occur (experiment 1). This study also investigated whether active PGHS is diminished following prostanoid synthesis in the labor process (experiment 2). A PGHS activity assay was employed in which PGE2 product was measured under initial velocity conditions. Labor was induced before term by continuous infusion of 10 mg of cortisol succinate per day (day 1) followed by 15 mg per day (days 2-4, or until delivery) of cortisol succinate administered through the fetal saphenous vein. Cotyledonary tissue was collected from sheep at term (142-145 dga), as judged by the absence of labor-type myometrial electromyogram (EMG) activity, and during cortisol induction at 128-131 dga. Tissue was also collected from term laboring animals immediately after fetal delivery while the fetus was still attached to the umbilicus and before placental delivery. At 128-131 dga, cortisol had no significant effect on PGHS activity as compared to gestational age matched saline-infused controls; thus, it is unlikely that cortisol directly induces PGHS. In experiment 2, normal progression of active spontaneous labor led to a significant diminution of PGHS activity (p < 0.05) that may be partially explained, based on thin-layer chromatography (TLC) results, as a significant decrease in PGE2 (p < 0.05) production coincident with a lesser compensatory increase in PGD2 (p = 0.06) output.

Oxidation kinetics of caffeic acid by prostaglandin H synthase: potential role in regulation of prostaglandin biosynthesis

The naturally occurring catechol derivative caffeic acid is a moderate stimulator of prostaglandin H synthase cyclooxygenase activity and a good reducing substrate for prostaglandin H synthase-compounds I and II. The discrepancy between the two properties is explained by a specific peroxidative mechanism that includes the formation of an inhibitory complex of caffeic acid with native enzyme followed by a three-step irreversible ping-pong peroxidation. The concentration of caffeic acid necessary to produce 50% stimulation of 0.2 mM arachidonic acid oxidation is 0.8 +/- 0.1 mM. The rate constant for the reaction of prostaglandin H synthase with hydrogen peroxide, determined from steady-state results, is (5.68 +/- 0.1) x 10(5) M-1 s-1. The rate constant for the reaction of prostaglandin H synthase-compound II with caffeic acid is (1.25 +/- 0.1) x 10(6) M-1 s-1. The dissociation constant of caffeic acid from the inhibitory complex is 35 +/- 10 microM. In diluted enzyme solutions, caffeic acid binding is diminished and the enzyme exhibits higher peroxidase activity. Our results suggest that caffeic acid is not a O-demethylation product of ferulic acid degradation catalyzed by prostaglandin H synthase, nor a chelating agent for the heme iron. The oxidation of caffeic acid could be important in the regulation of both prostaglandin H synthase and lipoxygenase activities and hence prostaglandin and leukotriene biosynthesis.

Prostaglandin H synthase. Inactivation of the enzyme in the course of catalysis is accompanied by fast and dramatic changes in protein structure

Prostaglandin H synthase (PGHS) as apo-PGHS, holo-PGHS, and holo-PGHS, inactivated in the course of catalysis was studied using chemical modification with diethyl pyrocarbonate (DEPC). The exhausted reaction with DEPC corresponded to the modification of 7 histidine residues in apo-PGHS and 4 in holo-PGHS. All 18 histidine residues became accessible for modification with DEPC in the enzyme, inactivated in the course of catalysis. The velocities of tryptic cleavage of all the three forms into two fragments were fairly different but independent of modification. Based on the results we hypothesize fast and dramatic changes in the protein structure in the course of the substrate conversion.

Molecular evolution of cyclooxygenase and lipoxygenase

Four oxygenases of the arachidonic acid cascade (cyclooxygenase, 5-lipoxygenase, 12-lipoxygenase and 15-lipoxygenase) were investigated by the method of computer-assisted sequence comparison. From the calculations, some aspects of evolution and function of these enzymes were revealed. (1) The evolutionary origin of cyclooxygenase was different from that of lipoxygenases. (2) Cyclooxygenase was a distantly related member of a peroxidase family. (3) Enzymes with 12-lipoxygenase activity were created independently twice by gene duplication.

Chemical modification of prostaglandin endoperoxide synthase by N-acetylimidazole. Effect on enzymic activities and EPR spectroscopic properties

Prostaglandin H synthase apoprotein, without its prosthetic heme group, was inactivated by N-acetylimidazole under conditions typical for the O-acetylation of tyrosyl residues. A spontaneous reactivation occurred above pH 7.5 at 22 degrees C, which indicated spontaneous hydrolysis of acetylated residues. Below pH 7.5, where stable inactivation was observed, reactivation was achieved by reaction with hydroxylamine. Both enzymic activities of prostaglandin H synthase, cyclooxygenase and peroxidase, were inactivated and reactivated simultaneously and to the same extent. In contrast to the apoprotein, the holoenzyme with heme was not inactivated by N-acetylimidazole. The number of acetyl groups, as determined as hydroxamate after the reaction with hydroxylamine at pH 8.2, was 2.5 +/- 0.4 for the apoprotein and 1.0 +/- 0.24 for the holoenzyme. The specific binding of heme as the prosthetic group was no longer observed by EPR (signals at g = 6.7 and 5.3) when hemin was added to the N-acetylimidazole-reacted apoprotein. Treatment of N-acetylimidazole-reacted apoprotein with hydroxylamine restored the specific binding of heme. The N-acetylimidazole-reacted apoprotein supplemented with hemin and reacted with hydroperoxides, neither showed electronic absorption spectra of higher oxidation states nor an EPR doublet signal due to a tyrosyl radical. These results demonstrate that heme protects against the inactivating modification by N-acetylimidazole and that this modification prevents binding of the prosthetic heme group necessary for both enzymic activities. The absence of the prosthetic heme group explains the concomitant loss of cyclooxygenase and peroxidase activities, as well as the absence of higher oxidation states and the tyrosyl radical. We suggest that the acetylation of a residue in the heme pocket, most probably a tyrosine, although a histidine cannot be definitely disproved, exerts the inhibiting effects. This residue could be the axial ligand of the heme or in close contact to the heme. The results also show that the inhibition by N-acetylimidazole does not involve the acetylation of Ser530 which causes the inhibition by acetylsalicylic acid of cyclooxygenase. [The numbering of amino acids in ovine prostaglandin H synthase is according to DeWitt, D. L. and Smith, W. L. (1988) Proc. Natl Acad. Sci. USA 85, 1412-1416 including a signal peptide of 24 residues which is missing in the processed protein.

Prostaglandin and thromboxane biosynthesis

We describe the enzymological regulation of the formation of prostaglandin (PG) D2, PGE2, PGF2 alpha, 9 alpha, 11 beta-PGF2, PGI2 (prostacyclin), and thromboxane (Tx) A2 from arachidonic acid. We discuss the three major steps in prostanoid formation: (a) arachidonate mobilization from monophosphatidylinositol involving phospholipase C, diglyceride lipase, and monoglyceride lipase and from phosphatidylcholine involving phospholipase A2; (b) formation of prostaglandin endoperoxides (PGG2 and PGH2) catalyzed by the cyclooxygenase and peroxidase activities of PGH synthase; and (c) synthesis of PGD2, PGE2, PGF2 alpha, 9 alpha, 11 beta-PGF2, PGI2, and TxA2 from PGH2. We also include information on the roles of aspirin and other nonsteroidal anti-inflammatory drugs, dexamethasone and other anti-inflammatory steroids, platelet-derived growth factor (PDGF), and interleukin-1 in prostaglandin metabolism.

Concerted loss of cyclooxygenase and peroxidase activities from prostaglandin H synthase upon proteolytic attack

Prostaglandin H synthase has two distinct enzymatic activities: a cyclooxygenase that forms PGG2 from arachidonate and a peroxidase that can reduce hydroperoxides, such as PGG2, to the corresponding alcohols. The relative sensitivities of the two synthase activities to proteolytic attack have been examined, using trypsin, chymotrypsin, and proteinase K, all known to attack the native apoprotein in the arg 253 region. The relation between the specific activity of the synthase and the loss of the two activities and the cleavage of the synthase subunit during trypsin digestion was also examined. The cyclooxygenase and peroxidase activities declined in concert throughout room temperature digestions with each of the three proteases. There was no indication of a selective loss of either activity in any of the digestions. In separate digestions with the same preparation of synthase, 3.3% (w/w) proteinase K resulted in more extensive loss of activity (90% decrease after 90 min) than did 3% (w/w) trypsin (70% decrease after 120 min) or 5% (w/w) chymotrypsin (60% decrease after 135 min). In tryptic digestions of synthase preparations with cyclooxygenase specific activity between 16 and 125 k units/mg protein, the fractional loss of cyclooxygenase activity was, within experimental error, the same as that of peroxidase activity. The extent of cleavage of the 70 kDa synthase subunit was greater than the loss of enzymatic activity, with the discrepancy being larger for synthase preparations with lower specific activity. The presence of a variable amount of catalytically-inactive, protease-sensitive, synthase protein could account for the difference between surviving activity and intact subunit in six out of the seven synthase preparations examined. Thus, it is likely that the cyclooxygenase and peroxidase activities are destroyed together during proteolytic attack on the arg 253 region of the native synthase apoprotein.

Optical spectra and kinetics of reactions of prostaglandin H synthase: effects of the substrates 13-hydroperoxyoctadeca-9,11-dienoic acid, arachidonic acid, N,N,N',N'-tetramethyl-p-phenylenediamine, and phenol and of the nonsteroidal anti-inflammatory drugs aspirin, indomethacin, phenylbutazone, and bromfenac

A combination of cyclooxygenase activity assays, rapid spectrophotometry and pre-steady-state, steady-state, and transient-state kinetics is used to characterize further the properties of prostaglandin H synthase. 13-Hydroperoxyoctadeca-9-11-dienoic acid is used as oxidizing substrate and the effects of the following compounds are examined: arachidonic acid, N,N,N',N'-tetramethyl-p-phenylenediamine, phenol, diethyldithiocarbamate, and the nonsteroidal anti-inflammatory drugs aspirin, indomethacin, phenylbutazone, and Bromfenac. The order of reactivity of four of these substrates, predominantly with compound II of prostaglandin H synthase, is N,N,N',N'-tetramethyl-p-phenylenediamine greater than phenol greater than indomethacin approximately phenylbutazone. Aspirin exhibits no effect. Arachidonic acid causes inactivation. Diethyldithiocarbamate acts as a reducing substrate for the oxidized forms of prostaglandin H synthase. Bromfenac appears to act both as a protective agent and inhibitor.

Topographic studies of microsomal and pure prostaglandin H synthase

Prostaglandin H synthase catalyzes the first step in the conversion of polyunsaturated fatty acids to prostaglandins, thromboxanes, and prostacyclins. The enzyme is normally bound to the endoplasmic reticulum membrane, but can be purified to homogeneity after solubilization with detergent. The topologies of the microsomal and the pure detergent-solubilized forms of the synthase were compared by an examination of their sensitivity to degradation by proteases, of the effect of heme on this protease sensitivity, and of the sizes of proteolytic fragments produced. For the microsomal synthase, the localization of proteolytic fragments was also determined. Analysis of the microsomal proteins after proteolytic digests involved separation by polyacrylamide gel electrophoresis and selective detection of the synthase-derived polypeptides with a polyclonal antibody against the pure synthase. With both the microsomal and the pure synthase, incubation with trypsin led to a progressive loss of cyclooxygenase activity and cleavage of the synthase subunit (70K Da) into two fragments of 38K and 33K Da. Incubation of the detergent-solubilized form of the synthase with proteinase K and chymotrypsin also produced a very similar pair of fragments (38K and 33K Da). After incubation of the microsomes with trypsin both the 38K and 33K Da fragments from the synthase remained bound to the membrane; no cyclooxygenase activity was released in soluble form from the microsomes by trypsin. Further, neither trypsin nor proteinase K released soluble radiolabeled peptides from microsomes whose synthase had been labeled with [acetyl-14C]-aspirin. With the microsomal synthase the sensitivity to protease (66% of the cyclooxygenase activity was lost after 90 min incubation with proteinase K) was enhanced by depletion of heme (84% of activity lost) and was decreased by addition of heme (only 20% of activity lost), just as had been previously demonstrated for the detergent-solubilized synthase. At each of several intervals during an incubation of the pure synthase with trypsin the extent of cleavage of the synthase polypeptide correlated reasonably well with the extent of loss of cyclooxygenase activity; a similar relation between proteolytic cleavage and loss of activity was observed in digests of the pure synthase supplemented with differing amounts of heme.(ABSTRACT TRUNCATED AT 400 WORDS)