Prostaglandin H synthase-1: evaluation of C-terminus function

Dual role of cyclooxygenase-2 during tissue repair induced by low level laser therapy: an intriguing issue

Tissue repair is an excellent example of pathophysiological model for studying the role of cyclooxygenase-2 (COX-2) on eukaryotic cells. It has been established that two COX isoforms are expressed in human tissues: constitutive or induced. COX-1 activity is constitutive, present in nearly all cell types at a constant level; COX-2 activity is normally absent from cells, and when induced, the protein levels increase and decrease in a matter of hours after a single stimulus. Thus, the purpose of this review was to describe the role of COX-2 during tissue repair induced by low level laser therapy (LLLT) in humans and experimental models. COX-2 expression has been implicated in the onset or the exacerbation of inflammation during tissue repair induced by LLLT in a number of studies, Many studies are conducted to investigate the role of COX-2 during tissue repair induced by LLLT using different experimental protocols and dosages. Therefore, this is an area that warrants investigation, since the estimation of COX-2 expression from using such important techniques in therapeutics with respect to tissue repair will be added to those already established in the literature as a way to improve health status and prevention of side effects.

Decreased cyclooxygenase inhibition by aspirin in polymorphic variants of human prostaglandin H synthase-1

OBJECTIVES

Aspirin (ASA), a major antiplatelet and cancer-preventing drug, irreversibly blocks the cyclooxygenase (COX) activity of prostaglandin H synthase-1 (PGHS-1). Considerable differences in ASA effectiveness are observed between individuals, and some of this variability may be due to PGHS-1 protein variants. Our overall aim is to determine which, if any, of the known variants in the mature PGHS-1 protein lead to functional alterations in COX catalysis or inhibition by ASA. The present study targeted four PGHS-1 variants: R53H, R108Q, L237M, and V481I.

METHODS

Wild-type human PGHS-1 and the four polymorphic variants were expressed as histidine-tagged, homodimeric proteins in insect cells using baculovirus vectors, solubilized with a detergent, and purified by affinity chromatography. The purified proteins were characterized in vitro to evaluate COX and peroxidase (POX) catalytic parameters and the kinetics of COX inhibition by ASA and NS-398.

RESULTS

Compared with the wild type, several variants showed a higher COX/POX ratio (up to 1.5-fold, for R108Q), an elevated arachidonate Km (up to 1.9-fold, for R108Q), and/or a lower ASA reactivity (up to 60% less, for R108Q). The decreased ASA reactivity in R108Q reflected both a 70% increase in the Ki for ASA and a 30% decrease in the rate constant for acetyl group transfer to the protein. Computational modeling of the brief ASA pulses experienced by PGHS-1 in circulating platelets during daily ASA dosing predicted that the 60% lower ASA reactivity in R108Q yields a 15-fold increase in surviving COX activity; smaller, approximately two-fold increases in surviving COX activity were predicted for L237M and V481I. NS-398 competitively inhibited COX catalysis of the wild type (Ki=6 µmol/l) and inhibited COX inactivation by 1.0 mmol/l ASA in both the wild type (IC50=0.8 µmol/l) and R108Q (IC50=2.1 µmol/l).

CONCLUSION

Of the four PGHS-1 variants examined, R108Q exerts the largest functional effects, with evidence for impaired interactions with a COX substrate and inhibitors. As Arg108 is located on the protein surface and not in the active site, the effects of R108Q suggest a novel, unsuspected mechanism for the modulation of the PGHS-1 active site structure. The lower intrinsic ASA reactivity of R108Q, V481I, and L237M, combined with the rapid hydrolysis of ASA in the blood, suggests that these variants decrease the antiplatelet effectiveness of the drug. These PGHS-1 variants are uncommon but ASA is used widely; hence, a considerable number of individuals could be affected. Further examination of these and other PGHS-1 variants will be needed to determine whether PGHS-1 genotyping can be used to personalize anti-COX therapy.

Static retention of the lumenal monotopic membrane protein torsinA in the endoplasmic reticulum

TorsinA is a membrane-associated enzyme in the endoplasmic reticulum (ER) lumen that is mutated in DYT1 dystonia. How it remains in the ER has been unclear. We report that a hydrophobic N-terminal domain (NTD) directs static retention of torsinA within the ER by excluding it from ER exit sites, as has been previously reported for short transmembrane domains (TMDs). We show that despite the NTD's physicochemical similarity to TMDs, it does not traverse the membrane, defining torsinA as a lumenal monotopic membrane protein and requiring a new paradigm to explain retention. ER retention and membrane association are perturbed by a subset of nonconservative mutations to the NTD, suggesting that a helical structure with defined orientation in the membrane is required. TorsinA preferentially enriches in ER sheets, as might be expected for a lumenal monotopic membrane protein. We propose that the principle of membrane-based protein sorting extends to monotopic membrane proteins, and identify other proteins including the monotopic lumenal enzyme cyclooxygenase 1 (prostaglandin H synthase 1) that share this mechanism of retention with torsinA.

Impact of genetic polymorphisms on platelet function and aspirin resistance

Genetic polymorphisms may affect platelets' responses to the antiplatelet therapy. Our aim was to determine the role of genetic polymorphisms on aspirin resistance in patients with coronary heart disease (CHD). A total of 126 consecutive patients (35-85 years old, 32% women) with chronic stable CHD was enrolled in the study. Platelet function assays were realized by the platelet function analyzer (PFA)-100 with collagen and epinephrine (Col/Epi) and collagen and adenosine diphosphate (Col/ADP) cartridges. Aspirin resistance was defined as having a closure time of less than 186 s with Col/Epi cartridges despite regular aspirin therapy. Factor V, prothrombin, factor XIII, beta-fibrinogen, plasminogen activator inhibitor I (PAI-1), glycoprotein IIIa, methylene tetrahydrofolate reductase, ACE and ApoB gene polymorphisms were determined by three consecutive steps: isolation and amplification of DNA and reverse hybridization. We determined that 30 patients (23.8%) had aspirin resistance by the PFA-100. Mean closure time measured with the Col/ADP cartridges was 74 +/- 12 s (51-104 s). Ten of the 30 patients with aspirin resistance were women (33.3%). Genetic polymorphisms were determined in 30 aspirin-resistant and 17 aspirin-sensitive patients. No statistically significant relationship was determined between aspirin resistance and the genetic panel. In our study we did not determine a significant relationship between the aspirin resistance and factor V, prothrombin, factor XIII, beta-fibrinogen, PAI-1, glycoprotein IIIa, methylene tetrahydrofolate reductase, ACE and ApoB gene polymorphisms.

Direct evidence of the cyclooxygenase pathway of prostaglandin synthesis in arthropods: genetic and biochemical characterization of two crustacean cyclooxygenases

Prostaglandins, well-known lipid mediators in vertebrate animals, have also shown to play certain regulatory roles in insects and other arthropods acting on reproduction, immune system and ion transport. However, knowledge of their biosynthetic pathways in arthropods is lacking. In the present study, we report the cloning and expression of cyclooxygenase (COX) from amphipod crustaceans Gammarus spp and Caprella spp. The amphipod COX proteins contain key residues shown to be important for cyclooxygenase and peroxidase activities. Differently from all other known cyclooxygenases the N-terminal signal sequence of amphipod enzymes is not cleaved during protein expression in mammalian cells. The C-terminus of amphipod COX is shorter than that of mammalian isoforms and lacks the KDEL(STEL)-type endoplasmic reticulum retention/retrieval signal. Despite that, amphipod COX proteins are N-glycosylated and locate similarly to the vertebrate COX on the endoplasmic reticulum and nuclear envelope. Both amphipod COX mRNAs encode functional cyclooxygenases that catalyze the transformation of arachidonic acid into prostaglandins. Using bioinformatic analysis we identified a COX-like gene from the human body louse Pediculus humanus corporis genome that encodes a protein with about 30% sequence identity with human COX-1 and COX-2. Although the COX gene is known to be absent from genomes of Drosophila sp., Aedes aegypti, Bombyx mori, and other insects, our studies establish the existence of the COX gene in certain lineages within the insect world.

A review of aspirin resistance; definition, possible mechanisms, detection with platelet function tests, and its clinical outcomes

Aspirin (acetylsalicylic acid) is one of the main therapeutics in prevention of thrombo-embolic vascular events. Its efficiency is proved in the prevention of cardiovascular events. However, antiplatelet effect of aspirin is not absolute in all patients and some patients experience thrombo-embolic events despite aspirin. These patients are clinically called as aspirin resistant or aspirin non-responders. Globally, a lot of people are affected by aspirin resistance according to the high prevalence of athero-thrombotic vascular diseases. A prevalence of 5.5-45% in patients with various cardiovascular disease by different laboratory methods has been reported for aspirin resistance. Clinical outcome of aspirin resistance has been demonstrated in patients with different vascular diseases. Detection of platelet function in patients treated with aspirin may be necessary in the prediction of clinical outcome. Point of care methods, which have correlated results with the standard light transmittance aggregometry may be appropriate choices in the detection of platelets' response to antiplatelet therapy. Adequate additional therapies may reduce atherothrombotic risks and major cardiovascular events rate in aspirin resistant subjects. None of the current researches advised the cessation of aspirin therapy. There is need to investigate the efficacy of additional adenosine diphosphate receptor antagonists or newer antiplatelet agents in aspirin resistant subjects. The intent of this paper is to review the literature discussing possible mechanisms, determination techniques, and clinical effects of aspirin resistance.

Cyclooxygenases in cancer: progress and perspective

Aspirin has been used to control pain and inflammation for over a century. Epidemiological studies first associated a decreased incidence of colorectal cancer with the long-term use of aspirin in the early 1980s. Near the same time the first reports showing regression of colorectal adenomas in response to the non-steroidal anti-inflammatory drug (NSAID) sulindac were reported. In subsequent years, the use of other NSAIDs, which inhibit cyclooxygenase (COX) enzymes, was linked to reduced cancer risk in multiple tissues including those of the breast, prostate, and lung. Together these studies resulted in the identification of a new cancer preventive and/or therapeutic target-COX enzymes, especially COX-2. Meanwhile, the overexpression of COX-2, and less consistently, the upstream and downstream enzymes of the prostaglandin synthesis pathway, was demonstrated in multiple cancer types and some pre-neoplastic lesions. Direct interactions of prostaglandins with their receptors through autocrine or paracrine pathways to enhance cellular survival or stimulate angiogenesis have been proposed as the molecular mechanisms underlying the pro-carcinogenic functions of COX-2. The rapid development of safe and effective inhibitors targeting individual COX enzymes not only dramatically improved our understanding of the function of COX-2, but also resulted in discovery of COX independent functions of NSAIDs, providing important hints for future drug design. Here we review the fundamental features of COX enzymes, especially as related to carcinogenesis, their expression and function in both animal tumor models and clinical cancers and the proposed mechanisms behind their roles in cancer.

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.

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.

Aspirin resistance and genetic polymorphisms

Differences in genetic makeup or polymorphisms can affect individual drug response. Detecting genetic variation may help predict how a patient will respond to a drug and could be used as a tool to select optimal therapy, tailor dosage regimens, and improve clinical outcomes. The data are replete relative to the therapeutic efficacy of aspirin (ASA) for the prevention of ischemic events. However, there is a paucity of published data on the relationship between polymorphisms and the clinical effects on ASA. Prothrombotic genetic variations that may contribute to ASA resistance, and increased risk of cardiovascular events may involve: (1) a polymorphism on the cyclooxygenase-1 (COX-1) gene affecting Ser529; (2) overexpression of COX-2 mRNA on platelets and endothelial cells; (3) polymorphism PLA1/A2 of the gene encoding glycoprotein IIIa (GPIIIa); and (4) the homozygous 807T (873A) polymorphism allied with increased density of platelet GP Ia/IIa collagen-receptor gene. Because of the possible increased risk of ischemic vascular events, carriers of these genetic polymorphisms may be resistant to the antithrombotic effects of ASA and should be considered for additional or alternative treatment.

The basis of prostaglandin synthesis in coral: molecular cloning and expression of a cyclooxygenase from the Arctic soft coral Gersemia fruticosa

In vertebrates, the synthesis of prostaglandin hormones is catalyzed by cyclooxygenase (COX)-1, a constitutively expressed enzyme with physiological functions, and COX-2, induced in inflammation and cancer. Prostaglandins have been detected in high concentrations in certain corals, and previous evidence suggested their biosynthesis through a lipoxygenase-allene oxide pathway. Here we describe the discovery of an ancestor of cyclooxygenases that is responsible for prostaglandin biosynthesis in coral. Using a homology-based polymerase chain reaction cloning strategy, the cDNA encoding a polypeptide with approximately 50% amino acid identity to both mammalian COX-1 and COX-2 was cloned and sequenced from the Arctic soft coral Gersemia fruticosa. Nearly all the amino acids essential for substrate binding and catalysis as determined in the mammalian enzymes are represented in coral COX: the arachidonate-binding Arg(120) and Tyr(355) are present, as are the heme-coordinating His(207) and His(388); the catalytic Tyr(385); and the target of aspirin attack, Ser(530). A key amino acid that determines the sensitivity to selective COX-2 inhibitors (Ile(523) in COX-1 and Val(523) in COX-2) is present in coral COX as isoleucine. The conserved Glu(524), implicated in the binding of certain COX inhibitors, is represented as alanine. Expression of the G. fruticosa cDNA afforded a functional cyclooxygenase that converted exogenous arachidonic acid to prostaglandins. The biosynthesis was inhibited by indomethacin, whereas the selective COX-2 inhibitor nimesulide was ineffective. We conclude that the cyclooxygenase occurs widely in the animal kingdom and that vertebrate COX-1 and COX-2 are evolutionary derivatives of the invertebrate precursor.

Distinct Influences of Carboxyl Terminal Segment Structure on Function in the Two Isoforms of Prostaglandin H Synthase

The cyclooxygenase activity of the two prostaglandin H synthase (PGHS) isoforms, PGHS-1 and -2, is a major control element in prostanoid biosynthesis. The two PGHS isoforms have 60% amino acid identity, with prominent differences near the C-terminus, where PGHS-2 has an additional 18-residue insert. Some mutations of the C-terminal residue in PGHS-1 and -2 have been found to disrupt catalytic activity and/or intracellular targeting of the proteins, but the relationship between C-terminal structure and function in the two isoforms has been poorly defined. Crystallographic data indicate the PGHS-1 and -2 C-termini are positioned to interact with the endoplasmic reticulum (ER) membrane, although the C-terminal segment structure was not resolved for either isoform. We constructed a series of C-terminal substitution, deletion, and insertion mutants of human PGHS-1 and -2 and evaluated the effects on cyclooxygenase activity and intracellular targeting in transfected COS-1 cells expressing the recombinant proteins. PGHS-1 cyclooxygenase activity was strongly disrupted by C-terminal substitutions and deletions, but not by elongation of the C-terminal segment, even when the ultimate residue was altered. Similar alterations to PGHS-2 had markedly less effect on cyclooxygenase activity. The results indicate that the functioning of the longer C-terminal segment in PGHS-2 is distinctly more tolerant of structural change than the shorter PGHS-1 C-terminal segment. C-Terminal substitutions or deletions did not change the subcellular localization of either isoform, even at short times after transfection, indicating that neither C-terminal segment contains indispensable intracellular targeting signals.

Topology of catalytic portion of prostaglandin I(2) synthase: identification by molecular modeling-guided site-specific antibodies

Prostaglandin I(2) synthase (PGIS) is an eicosanoid-synthesizing cytochrome P450, located in the endoplasmic reticulum (ER) membrane. The membrane topology of the catalytic portion of PGIS is still unknown. General models of the membrane topology of microsomal P450s have been proposed in two forms: (a) large part of the polypeptide exposed on the cytoplasmic side with an NH(2)-terminal membrane anchor to the ER membrane and (b) deep immersion of the polypeptide in the membrane, as described by J. P. Miller et al. (1996, Biochemistry 35, 1466-1474). We have characterized the membrane topology of catalytic portion of PGIS using molecular modeling-guided site-specific antibodies. A 3D working model of PGIS was constructed by homology modeling using P450(BM-3) crystal structure as a template (S. K. Shyue et al., 1997, J. Biol. Chem. 272, 3657-3662). Three hydrophilic peptides corresponding to different regions of the surface portion of PGIS with residues 109-127 (P109-127), 353-368 (P353-368), and 411-431 (P411-431) predicted from the model and an NH(2)-terminal hydrophobic peptide (residues 1-28, P1-28) were synthesized and used to prepare site-specific antibodies. All three of the hydrophilic peptide antibodies have high titer and are specifically recognized human PGIS, as shown by binding assays and Western blot analysis. In contrast, the hydrophobic NH(2)-terminal peptide has a much lower titer binding to the PGIS protein. The overall arrangement of the PGIS polypeptide with respect to the endoplasmic reticulum (ER) membrane was examined by immunocytochemistry techniques in transiently transfected COS-1 cells with recombinant human PGIS cDNA and in ECV cells expressing endogenous PGIS. The immunofluorescence staining for the cells with selective permeabilization of the plasma membrane using streptolysin O indicated that all three of the hydrophilic peptide antibodies bound to the cytoplasmic surface of the ER membrane. These results provide direct experimental evidence supporting the predicted 3D protein topological model in which the segments are located on the protein surface and the membrane topological model in which PGIS is largely exposed on the cytoplasmic side of the ER membrane. It also led us to conclude that the PGIS substrate, prostaglandin H(2) (PGH(2)), produced by prostaglandin H(2) synthase (PGHS) in the ER lumenal side must pass through the ER membrane barrier to the catalytic site of the PGIS in the cytoplasmic side of the ER membrane.

Cloning and characterization of prostaglandin endoperoxide synthase-1 and -2 from the brook trout ovary

Using a combination of reverse transcription-PCR and library screening, the cDNAs for prostaglandin endoperoxide synthase-1 (PGS-1) and 2 (PGS-2) were isolated from the brook trout ovary. The brook trout PGS-1 cDNA encodes for a 598 amino acid protein that is 69% identical to mammalian PGS-1. PGS-1 transcripts were observed in the ovary, spleen, gills, head kidney, trunk kidney, intestine, stomach, skin and heart. To our knowledge, this is the first characterization of a non-mammalian PGS-1 cDNA. The brook trout PGS-2 encodes for a 607 amino acid protein that is 69% identical to mammalian PGS-2 and was observed in the skin, gills, stomach and heart. PGS-2 transcripts were highly upregulated in the ovaries by the phorbol ester, phorbol-12-myristate-13-acetate, in combination with the calcium ionophore, A23187. However, PGS-2 was not observed in the ovary of brook trout undergoing natural oocyte maturation and ovulation.

Cyclooxygenases 1 and 2

Cyclooxygenase (COX), first purified in 1976 and cloned in 1988, is the key enzyme in the synthesis of prostaglandins (PGs) from arachidonic acid. In 1991, several laboratories identified a product from a second gene with COX activity and called it COX-2. However, COX-2 was inducible, and the inducing stimuli included pro-inflammatory cytokines and growth factors, implying a role for COX-2 in both inflammation and control of cell growth. The two isoforms of COX are almost identical in structure but have important differences in substrate and inhibitor selectivity and in their intracellular locations. Protective PGs, which preserve the integrity of the stomach lining and maintain normal renal function in a compromised kidney, are synthesized by COX-1. In addition to the induction of COX-2 in inflammatory lesions, it is present constitutively in the brain and spinal cord, where it may be involved in nerve transmission, particularly that for pain and fever. PGs made by COX-2 are also important in ovulation and in the birth process. The discovery of COX-2 has made possible the design of drugs that reduce inflammation without removing the protective PGs in the stomach and kidney made by COX-1. These highly selective COX-2 inhibitors may not only be anti-inflammatory but may also be active in colon cancer and Alzheimer's disease.

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.

Role of Val509 in time-dependent inhibition of human prostaglandin H synthase-2 cyclooxygenase activity by isoform-selective agents

Prostaglandin H synthase (PGHS), a key enzyme in prostanoid biosynthesis, exists as two isoforms. PGHS-1 is considered a basal enzyme; PGHS-2 is associated with inflammation and cell proliferation. A number of highly selective inhibitors for PGHS-2 cyclooxygenase activity are known. Inhibition by these agents involves an initial reversible binding, followed by a time-dependent transition to a much higher affinity enzyme-inhibitor complex, making these agents potent and poorly reversible PGHS-2 inhibitors. To investigate the PGHS-2 structural features that influence the time-dependent action of the selective inhibitors, we have constructed a three-dimensional model of human PGHS-2 by homologous modeling. Examination of the PGHS-2 model identified Val509 as a cyclooxygenase active site residue, that was not conserved in PGHS-1. Recombinant human PGHS-2 with Val509 mutated to either Ile (the corresponding residue in PGHS-1), Ala, Glu, or Lys was expressed by transient transfection of COS-1 cells to evaluate the effects of the mutations on cyclooxygenase activity and on inhibition by four agents reported to be selective for PGHS-2 (NS398, nimesulide, DuP697, and SC58125). All the recombinant proteins were of the expected mass. The mutants exhibited 45-210% of wild-type cyclooxygenase activity, with Km values for arachidonate of 2.1-7.6 microM (wild-type PGHS-2, 3.8 microM), indicating that changes in position 509 had modest effects on cyclooxygenase catalysis. Each of the agents inhibited wild-type PGHS-2 in a time-dependent fashion, and all but nimesulide did the same for the V509A mutant. In contrast, the V509E and V509I PGHS-2 mutants, like recombinant human PGHS-1, did not show time-dependent inhibition with any of the agents, and the V509K mutant responded in a time-dependent manner only to DuP697. Reversible inhibition was still observed with Val509 mutants that did not show time-dependent inhibition. Thus, the side chain structure at position 509 markedly influenced the ability of PGHS-2 to undergo the time-dependent transition without removing inhibitor or substrate binding. These results indicate that Val509 in PGHS-2 has a major role in the structural transition that underlies time-dependent inhibition by the isoform-selective agents.

What can we learn from prostaglandins and related eicosanoids in insects?

Eicosanoids are oxygenated metabolites of three C20 polyunsaturated fatty acids (20:3n-6, 20:4n-6, and 20:5n-3). While eicosanoids are very well known in mammalian systems, mostly due to their pharmaceutical interest, there is increasing recognition of the significance of these compounds in insects and other invertebrates. In this paper we consider four major concepts emerging from work on eicosanoids in invertebrates. First, the biological significance of eicosanoids extends far beyond their physiological and pathophysiological actions in human and veterinary medicine. Second, we can greatly improve our understanding of eicosanoids in insects by integrating our work on insects into ongoing studies of other invertebrates. Third, some eicosanoid actions may be fundamental to animals. Fourth, the biochemistry of eicosanoids in insects and other invertebrates can differ from expectations based on the mammalian background. Finally, we point to an uncharted frontier in insect studies--the biochemical mechanisms of eicosanoid action--by drawing attention to some of the work on eicosanoid receptors in mammalian systems.