A mechanistic study of self-inactivation of the peroxidase activity in prostaglandin H synthase-1

Polyunsaturated Fatty Acids: Conversion to Lipid Mediators, Roles in Inflammatory Diseases and Dietary Sources

Polyunsaturated fatty acids (PUFAs) are important components of the diet of mammals. Their role was first established when the essential fatty acids (EFAs) linoleic acid and α-linolenic acid were discovered nearly a century ago. However, most of the biochemical and physiological actions of PUFAs rely on their conversion to 20C or 22C acids and subsequent metabolism to lipid mediators. As a generalisation, lipid mediators formed from n-6 PUFAs are pro-inflammatory while those from n-3 PUFAs are anti-inflammatory or neutral. Apart from the actions of the classic eicosanoids or docosanoids, many newly discovered compounds are described as Specialised Pro-resolving Mediators (SPMs) which have been proposed to have a role in resolving inflammatory conditions such as infections and preventing them from becoming chronic. In addition, a large group of molecules, termed isoprostanes, can be generated by free radical reactions and these too have powerful properties towards inflammation. The ultimate source of n-3 and n-6 PUFAs are photosynthetic organisms which contain Δ-12 and Δ-15 desaturases, which are almost exclusively absent from animals. Moreover, the EFAs consumed from plant food are in competition with each other for conversion to lipid mediators. Thus, the relative amounts of n-3 and n-6 PUFAs in the diet are important. Furthermore, the conversion of the EFAs to 20C and 22C PUFAs in mammals is rather poor. Thus, there has been much interest recently in the use of algae, many of which make substantial quantities of long-chain PUFAs or in manipulating oil crops to make such acids. This is especially important because fish oils, which are their main source in human diets, are becoming limited. In this review, the metabolic conversion of PUFAs into different lipid mediators is described. Then, the biological roles and molecular mechanisms of such mediators in inflammatory diseases are outlined. Finally, natural sources of PUFAs (including 20 or 22 carbon compounds) are detailed, as well as recent efforts to increase their production.

Lipid nutrition: "In silico" studies and undeveloped experiments

This review examines lipids and lipid-binding sites on proteins in relation to cardiovascular disease. Lipid nutrition involves food energy from ingested fatty acids plus fatty acids formed from excess ingested carbohydrate and protein. Non-esterified fatty acids (NEFA) and lipoproteins have many detailed attributes not evident in their names. Recognizing attributes of lipid-protein interactions decreases unexpected outcomes. Details of double bond position and configuration interacting with protein binding sites have unexpected consequences in acyltransferase and cell replication events. Highly unsaturated fatty acids (HUFA) have n-3 and n-6 motifs with documented differences in intensity of destabilizing positive feedback loops amplifying pathophysiology. However, actions of NEFA have been neglected relative to cholesterol, which is co-produced from excess food. Native low-density lipoproteins (LDL) bind to a high-affinity cell surface receptor which poorly recognizes biologically modified LDLs. NEFA increase negative charge of LDL and decrease its processing by "normal" receptors while increasing processing by "scavenger" receptors. A positive feedback loop in the recruitment of monocytes and macrophages amplifies chronic inflammatory pathophysiology. Computer tools combine multiple components in lipid nutrition and predict balance of energy and n-3:n-6 HUFA. The tools help design and execute precise clinical nutrition monitoring that either supports or disproves expectations.

Mechanism of reactivation of the peroxidase catalytic activity of human cyclooxygenases by reducing cosubstrate quercetin

Our earlier studies show that the peroxidase activity of cyclooxygenase 1 and 2 (COX-1 and COX-2) can be reactivated in vitro and in vivo by the presence of certain naturally-occurring flavonoids such as quercetin and myricetin, which serve as reducing cosubstrates. These compounds can activate COX at nanomolar concentrations. In the present study, quercetin is used as a representative model compound to investigate the chemical mechanism by which the peroxidase activity of human COX-1 and COX-2 is reactivated after each catalytic cycle. Molecular docking and quantum mechanics calculations are carried out to probe the interactions of quercetin with the peroxidase sites of COX-1/2 and the reactivation mechanism. It is found that some of the partially-ionized states of quercetin can bind tightly and closely inside the peroxidase active sites of the COX enzymes and directly interact with heme Fe ion. While quercetin contains several phenolic hydroxyl groups, it is found that only the C-3'-OH group can effectively donate an electron for the reduction of heme because it not only can bind closely and tightly inside the peroxidase sites of COX-1/2, but it can also facilely donate an electron to heme Fe ion. This investigation provides a mechanistic explanation for the chemical process by which quercetin reactivates COX-1/2 peroxidases. This knowledge would aid in the rational design of drugs that can selectively target the peroxidase sites of COX-1/2 either as activators or inhibitors.

Cycle Network Model of Prostaglandin H Synthase-1

The kinetic model of Prostaglandin H Synthase-1 (PGHS-1) was developed to investigate its complex network kinetics and non-steroidal anti-inflammatory drugs (NSAIDs) efficacy in different in vitro and in vivo conditions. To correctly describe the complex mechanism of PGHS-1 catalysis, we developed a microscopic approach to modelling of intricate network dynamics of 35 intraenzyme reactions among 24 intermediate states of the enzyme. The developed model quantitatively describes interconnection between cyclooxygenase and peroxidase enzyme activities; substrate (arachidonic acid, AA) and reducing cosubstrate competitive consumption; enzyme self-inactivation; autocatalytic role of AA; enzyme activation threshold; and synthesis of intermediate prostaglandin G2 (PGG2) and final prostaglandin H2 (PGH2) products under wide experimental conditions. In the paper, we provide a detailed description of the enzyme catalytic cycle, model calibration based on a series of in vitro kinetic data, and model validation using experimental data on the regulatory properties of PGHS-1. The validated model of PGHS-1 with a unified set of kinetic parameters is applicable for in silico screening and prediction of the inhibition effects of NSAIDs and their combination on the balance of pro-thrombotic (thromboxane) and anti-thrombotic (prostacyclin) prostaglandin biosynthesis in platelets and endothelial cells expressing PGHS-1.

Mechanism for the reactivation of the peroxidase activity of human cyclooxygenases: investigation using phenol as a reducing cosubstrate

It has been known for many years that the peroxidase activity of cyclooxygenase 1 and 2 (COX-1 and COX-2) can be reactivated in vitro by the presence of phenol, which serves as a reducing compound, but the underlying mechanism is still poorly understood. In the present study, we use phenol as a model compound to investigate the mechanism by which the peroxidase activity of human COXs is reactivated after each catalytic cycle. Molecular docking and quantum mechanics calculations are carried out to probe the interaction of phenol with the peroxidase site of COXs and the reactivation mechanism. It is found that the oxygen atom associated with the Fe ion in the heme group (i.e., the complex of Fe ion and porphyrin) of COXs can be removed by addition of two protons. Following its removal, phenol can readily bind inside the peroxidase active sites of the COX enzymes, and directly interact with Fe in heme to facilitate electron transfer from phenol to heme. This investigation provides theoretical evidence for several intermediates formed in the COX peroxidase reactivation cycle, thereby unveiling mechanistic details that would aid in future rational design of drugs that target the peroxidase site.

The enzymology of the human prostanoid pathway

Prostanoids are short-lived autocrine and paracrine signaling molecules involved in a wide range of biological functions. They have been shown to be intimately involved in many different disease states when their regulation becomes dysfunctional. In order to fully understand the progression of any disease state or the biological functions of the well state, a complete evaluation of the genomics, proteomics, and metabolomics of the system is necessary. This review is focused on the enzymology for the enzymes involved in the synthesis of the prostanoids (prostaglandins, prostacyclins and thromboxanes). In particular, the isolation and purification of the enzymes, their enzymatic parameters and catalytic mechanisms are presented.

Kinetics and Mechanisms of Mammalian Heme Peroxidase Reactions

The kinetics and mechanism of action of the most intensively studied mammalian peroxidases, myeloperoxidase and prostaglandin H synthase are critically reviewed. Evidence against currently favored mechanisms is presented. It is shown that myeloperoxidase has a strong defence mechanism against free hypochlorous acid, commonly thought to be its principal product in its bactericidal activity. Rather, after its two-electron oxidation of chloride ion, myeloperoxidase rapidly converts it into an enzyme-bound chlorinating intermediate, most likely a chlorinated distal imidazole ring. This species chlorinates taurine which may either be a transfer agent of Cl+ to other species or may act directly in attack on invading microorganisms. The currently favored mechanism of action of prostaglandin H synthase-1 is a branching chain mechanism in which Compound I is converted into a species containing a tyrosyl radical on the opposite side of the enzyme. Once the tyrosyl radical is formed it converts arachidonic acid into a peroxide in a cyclooxygenase reaction, independent of the peroxidase activity. This mechanism cannot explain the enhancing effect of small free radical scavengers, nor the fact that peroxidase activity continues unabated while the cyclooxygenase reaction is proceeding, nor the 2: 1 ratio of small free radical scavenger to arachidonic acid consumption. A tightly coupling of peroxidase and cycloxygenase reactions appears to be the steady state mechanism, and the branching chain mechanism, if it occurs, is confined to a burst transient state phase.

Prostaglandin-endoperoxide synthase 2 (cyclooxygenase-2), a complex target for colorectal cancer prevention and therapy

A plentiful literature has linked colorectal cancer (CRC) to inflammation and prostaglandin-endoperoxide synthase (PTGS)2 expression. Accordingly, several nonsteroidal antiinflammatory drugs (NSAIDs) have been tested often successfully in CRC chemoprevention despite their different ability to specifically target PTGS2 and the low or null expression of PTGS2 in early colon adenomas. Some observational studies showed an increased survival for patients with CRC assuming NSAIDs after diagnosis, but no clinical trial has yet demonstrated the efficacy of NSAIDs against established CRC, where PTGS2 is expressed at high levels. The major limits for the application of NSAIDs, or specific PTGS2 inhibitors, as adjuvant drugs in CRC are (1) a frequent confusion about the physiological role of PTGS1 and PTGS2, reflecting in CRC pathology and therapy; (2) the presence of unavoidable side effects linked to the intrinsic function of these enzymes; (3) the need of established criteria and markers for patient selection; and (4) the evaluation of the immunomodulatory potential of PTGS2 inhibitors as possible adjuvants for immunotherapy. This review has been written to rediscover the multifaceted potential of PTGS2 targeting, hoping it could act as a starting point for a new and more aware application of NSAIDs against CRC.

Dynamics of Radical Intermediates in Prostaglandin H Synthase-1 Cyclooxygenase Reactions is Modulated by Multiple Factors

Prostaglandin H synthase (PGHS) catalyzes the biosynthesis of PGG2 and PGH2, the precursor of all prostanoids, from arachidonic acid (AA). PGHS exhibits two enzymatic activities following a branched-chain radical mechanism: 1) a peroxidase activity (POX) that utilizes hydroperoxide through heme redox cycles to generate the critical Tyr385 tyrosyl radical for coupling both enzyme activities; 2) the cyclooxygenase (COX) activity inserting two oxygen molecules into AA to generate endoperoxide/hydroperoxide PGG2 through a series of radical intermediates. Upon the generation of Tyr385 radical, COX catalysis is initiated, with C13 pro-S hydrogen abstraction from AA by Tyr385 radical to generate arachidonyl substrate radical. Oxygen provides a large driving force for the subsequent fast steps leading to the formation of PGG2, including radical redistributions, ring formations, and rearrangements. On the other hand, if the supply of oxygen is severed, equilibrium between arachidonyl radical and tyrosyl radical(s) biases largely towards the latter. In this study, we demonstrate that such equilibrium is shifted by many factors, including temperature, chemical structures of fatty acid substrates and limited supply of oxygen. We also, for the first time, reveal that this equilibrium is significantly affected by co-substrates of POX. The presence of efficient POX co-substrates, which reduces heme to its ferric state, apparently biases the equilibrium towards arachidonyl radical. Therefore a dynamic interplay exists between the two activities of PGHS.

Substrate-selective COX-2 inhibition as a novel strategy for therapeutic endocannabinoid augmentation

Pharmacologic augmentation of endogenous cannabinoid (eCB) signaling is an emerging therapeutic approach for the treatment of a broad range of pathophysiological conditions. Thus far, pharmacological approaches have focused on inhibition of the canonical eCB inactivation pathways - fatty acid amide hydrolase (FAAH) for anandamide and monoacylglycerol lipase (MAGL) for 2-arachidonoylglycerol. We review here the experimental evidence that cyclooxygenase-2 (COX-2)-mediated eCB oxygenation represents a third mechanism for terminating eCB action at cannabinoid receptors. We describe the development, molecular mechanisms, and in vivo validation of 'substrate-selective' COX-2 inhibitors (SSCIs) that prevent eCB inactivation by COX-2 without affecting prostaglandin (PG) generation from arachidonic acid (AA). Lastly, we review recent data on the potential therapeutic applications of SSCIs with a focus on neuropsychiatric disorders.

[Direction of strategic use: a new classification of non-steroidal anti-inflammatory drugs based on reactivity with peroxidase]

  The pharmaceutical effects of non-steroidal anti-inflammatory drugs (NSAIDs) occur through the inhibition of prostaglandin H synthase (PGHS). Prostaglandin H2 is produced from arachidonic acid via peroxidase and cyclooxygenase cycles in PGHS. NSAIDs exhibit different levels of reactivity in these reaction cycles. To prevent the development of side effect while maintaining the beneficial effects of drugs, a therapeutic strategy should be used. A new classification of NSAIDs has been proposed based on reactivity to peroxidase. Class 1 includes the majority of NSAIDs, which react with horseradish peroxidase (HRP) compounds I and II. Also, their drugs exhibit spectral changes induced by PGHS peroxidase and diminished ESR signals of the tyrosyl radical of metmyoglobin. They reduce compounds I and II of HRP and scavenge tyrosyl radicals. The branched chain mechanism by which the porphyrin radical is transferred to the tyrosine residue of the protein might be blocked by these NSAIDs. Class 2 includes salicylic acid derivatives that react only with the porphyrin radical and do not react with HRP compound II (oxoferryl species). Class 3 includes aspirin, nimesulide, tolmetin, and arylpropionic acid derivatives, including ibuprofen and the coxibs such as celecoxib and rofecoxib, which are not substrates for HRP or PGHS peroxidase. It seems that the selectivity of NSAIDs to PGHS1 and PGHS2 depends on their reactivity with cyclooxygenase rather than with the peroxidase of PGHS. The best drug for each inflammatory disease should therefore be selected for therapy.

Reactivity of nonsteroidal anti-inflammatory drugs with peroxidase: a classification of nonsteroidal anti-inflammatory drugs

OBJECTIVES

To improve understanding of the essential effect of nonsteroidal anti-inflammatory drugs (NSAIDs) on prostaglandin H synthase (PGHS), the reactivity of NSAIDs with peroxidases and the tyrosyl radical derived from myoglobin was examined.

METHODS

Horseradish peroxidase and myoglobin were used as models of peroxidase and cyclooxygenase of PGHS, respectively.

KEY FINDINGS

From the results, a new classification of NSAIDs has been proposed. Class 1 includes the majority of NSAIDs, which reacted with horseradish peroxidase compound I, thus causing a spectral change by PGHS peroxidase and also including diminished electron spin resonance signals of the tyrosyl radical of myoglobin. They reduced compound I of horseradish peroxidase and scavenged the tyrosyl radical. The branched-chain mechanism by which the porphyrin radical is transferred to the tyrosine residue of the protein might be blocked by these NSAIDs. Class 2 includes salicylic acid derivatives that reacted only with the porphyrin radical and not with horseradish peroxidase compound II (oxoferryl species). Class 3 includes aspirin, nimesulide, tolmetin, and arylpropionic acid derivatives, including ibuprofen and the coxibs of celecoxib and rofecoxib, which are not substrates for horseradish peroxidase or PGHS peroxidase.

CONCLUSIONS

Understanding the essential mode of action of NSAIDs is particularly important for designing an effective therapeutic strategy against inflammatory diseases.

Prostaglandins mediate the fetal pulmonary response to intrauterine inflammation

Intra-amniotic (IA) lipopolysaccharide (LPS) induces intrauterine and fetal lung inflammation and increases lung surfactant and compliance in preterm sheep; however, the mechanisms are unknown. Prostaglandins (PGs) are inflammatory mediators, and PGE2 has established roles in fetal lung surfactant production. The aim of our first study was to determine PGE2 concentrations in response to IA LPS and pulmonary gene expression for PG synthetic [prostaglandin H synthase-2 (PGHS-2) and PGE synthase (PGES)] and PG-metabolizing [prostaglandin dehydrogenase (PGDH)] enzymes and PGE2 receptors. Our second study aimed to block LPS-induced increases in PGE2 with a PGHS-2 inhibitor (nimesulide) and determine lung inflammation and surfactant protein mRNA expression. Pregnant ewes received an IA saline or LPS injection at 118 days of gestation. In study 1, fetal plasma and amniotic fluid were sampled before and at 2, 4, 6, 12, and 24 h after injection and then daily, and fetuses were delivered 2 or 7 days later. Amniotic fluid PGE2 concentrations increased ( P < 0.05) 12 h and 3–6 days after LPS. Fetal lung PGHS-2 mRNA and PGES mRNA increased 2 ( P = 0.0084) and 7 ( P = 0.014) days after LPS, respectively. In study 2, maternal intravenous nimesulide or vehicle infusion began immediately before LPS or saline injection and continued until delivery 2 days later. Nimesulide inhibited LPS-induced increases in PGE2 and decreased fetal lung IL-1β and IL-8 mRNA ( P ≤ 0.002) without altering lung inflammatory cell infiltration. Nimesulide decreased surfactant protein (SP)-A ( P = 0.05), -B ( P = 0.05), and -D ( P = 0.0015) but increased SP-C mRNA ( P = 0.023). Thus PGHS-2 mediates, at least in part, fetal pulmonary responses to inflammation.

Structural basis for certain naturally occurring bioflavonoids to function as reducing co-substrates of cyclooxygenase I and II

Background

Recent studies showed that some of the dietary bioflavonoids can strongly stimulate the catalytic activity of cyclooxygenase (COX) I and II in vitro and in vivo, presumably by facilitating enzyme re-activation. In this study, we sought to understand the structural basis of COX activation by these dietary compounds.

Methodology/Principal Findings

A combination of molecular modeling studies, biochemical analysis and site-directed mutagenesis assay was used as research tools. Three-dimensional quantitative structure-activity relationship analysis (QSAR/CoMFA) predicted that the ability of bioflavonoids to activate COX I and II depends heavily on their B-ring structure, a moiety known to be associated with strong antioxidant ability. Using the homology modeling and docking approaches, we identified the peroxidase active site of COX I and II as the binding site for bioflavonoids. Upon binding to this site, bioflavonoid can directly interact with hematin of the COX enzyme and facilitate the electron transfer from bioflavonoid to hematin. The docking results were verified by biochemical analysis, which reveals that when the cyclooxygenase activity of COXs is inhibited by covalent modification, myricetin can still stimulate the conversion of PGG₂ to PGE₂, a reaction selectively catalyzed by the peroxidase activity. Using the site-directed mutagenesis analysis, we confirmed that Q189 at the peroxidase site of COX II is essential for bioflavonoids to bind and re-activate its catalytic activity.

Conclusions/Significance

These findings provide the structural basis for bioflavonoids to function as high-affinity reducing co-substrates of COXs through binding to the peroxidase active site, facilitating electron transfer and enzyme re-activation.

In Silico Screening of Nonsteroidal Anti-Inflammatory Drugs and Their Combined Action on Prostaglandin H Synthase-1

The detailed kinetic model of Prostaglandin H Synthase-1 (PGHS-1) was applied to in silico screening of dose-dependencies for the different types of nonsteroidal anti-inflammatory drugs (NSAIDs), such as: reversible/irreversible, nonselective/selective to PGHS-1/PGHS-2 and time dependent/independent inhibitors (aspirin, ibuprofen, celecoxib, etc.) The computational screening has shown a significant variability in the IC50s of the same drug, depending on different in vitro and in vivo experimental conditions. To study this high heterogeneity in the inhibitory effects of NSAIDs, we have developed an in silico approach to evaluate NSAID action on targets under different PGHS-1 microenvironmental conditions, such as arachidonic acid, reducing cofactor, and peroxide concentrations. The designed technique permits translating the drug IC₅₀, obtained in one experimental setting to another, and predicts in vivo inhibitory effects based on the relevant in vitro data. For the aspirin case, we elucidated the mechanism underlying the enhancement and reduction (aspirin resistance) of its efficacy, depending on PGHS-1 microenvironment in in vitro/in vivo experimental settings. We also present the results of the in silico screening of the combined action of sets of two NSAIDs (aspirin with ibuprofen, aspirin with celecoxib), and study the mechanism of the experimentally observed effect of the suppression of aspirin-mediated PGHS-1 inhibition by selective and nonselective NSAIDs. Furthermore, we discuss the applications of the obtained results to the problems of standardization of NSAID test assay, dependence of the NSAID efficacy on cellular environment of PGHS-1, drug resistance, and NSAID combination therapy.

Suicide inactivation of MauG during reaction with O(2) or H(2)O(2) in the absence of its natural protein substrate

MauG is a diheme protein that catalyzes the six-electron oxidation of a biosynthetic precursor protein of methylamine dehydrogenase (PreMADH) with partially synthesized tryptophan tryptophylquinone (TTQ) to yield the mature protein with the functional protein-derived TTQ cofactor. The biosynthetic reaction proceeds via a relatively stable high valent bis-Fe(IV) intermediate. Oxidizing equivalents ([O]) for this reaction may be provided by either O(2) plus electrons from an external donor or H(2)O(2). The presence or absence of PreMADH has no influence on the reactivity of MauG with [O]; however, it is demonstrated that MauG is inactivated when supplied with [O] in the absence of PreMADH. The mechanism of inactivation appears to differ depending on the source of [O]. Repeated reaction of diferrous MauG with O(2) leads to loss of activity but not inactivation of heme, as judged by absorption spectroscopy and pyridine hemochrome assay. Repeated reaction of diferric MauG with H(2)O(2) leads to loss of activity and inactivation of heme, as well as some covalent cross-linking of MauG molecules. None of these deleterious effects with either source of [O] are observed when PreMADH is present to react with MauG. The radical scavenger hydroxyurea and small molecule mimics of the monohydroxylated Trp residue of PreMADH also reacted with bis-Fe(IV) MauG and afforded protection against inactivation. These results demonstrate that while O(2) and H(2)O(2) readily react with MauG in the absence of PreMADH, the presence of this substrate is necessary to prevent suicide inactivation of MauG after formation of the bis-Fe(IV) intermediate.

Prostaglandin H synthase: resolved and unresolved mechanistic issues

The cyclooxygenase and peroxidase activities of prostaglandin H synthase (PGHS)-1 and -2 have complex kinetics, with the cyclooxygenase exhibiting feedback activation by product peroxide and irreversible self-inactivation, and the peroxidase undergoing an independent self-inactivation process. The mechanistic bases for these complex, non-linear steady-state kinetics have been gradually elucidated by a combination of structure/function, spectroscopic and transient kinetic analyses. It is now apparent that most aspects of PGHS-1 and -2 catalysis can be accounted for by a branched chain radical mechanism involving a classic heme-based peroxidase cycle and a radical-based cyclooxygenase cycle. The two cycles are linked by the Tyr385 radical, which originates from an oxidized peroxidase intermediate and begins the cyclooxygenase cycle by abstracting a hydrogen atom from the fatty acid substrate. Peroxidase cycle intermediates have been well characterized, and peroxidase self-inactivation has been kinetically linked to a damaging side reaction involving the oxyferryl heme oxidant in an intermediate that also contains the Tyr385 radical. The cyclooxygenase cycle intermediates are poorly characterized, with the exception of the Tyr385 radical and the initial arachidonate radical, which has a pentadiene structure involving C11-C15 of the fatty acid. Oxygen isotope effect studies suggest that formation of the arachidonate radical is reversible, a conclusion consistent with electron paramagnetic resonance spectroscopic observations, radical trapping by NO, and thermodynamic calculations, although moderate isotope selectivity was found for the H-abstraction step as well. Reaction with peroxide also produces an alternate radical at Tyr504 that is linked to cyclooxygenase activation efficiency and may serve as a reservoir of oxidizing equivalent. The interconversions among radicals on Tyr385, on Tyr504, and on arachidonate, and their relationships to regulation and inactivation of the cyclooxygenase, are still under active investigation for both PGHS isozymes.

Inhibition of cyclooxygenases 1 and 2 by the phospholipase-blocker, arachidonyl trifluoromethyl ketone

BACKGROUND AND PURPOSE

Arachidonyl trifluoromethyl ketone (ATK) is widely used as an inhibitor of cytosolic group IV phospholipase A(2) (cPLA(2)) and calcium-independent group VI phospholipase A(2) (iPLA(2)). ATK thus reduces arachidonic acid (AA) substrate for cyclooxygenase (COX; also known as prostaglandin H synthase) and attenuates prostaglandin (PG) synthesis. It has been shown previously, that ATK blocks thromboxane B(2) production induced by exogenous AA in human platelets. It remains, however, unknown whether ATK also directly modulates the activity of cyclooxygenase (COX).

EXPERIMENTAL APPROACH

Time courses for inhibition of COX by ATK was obtained using osteoblast-like MC3T3-E1 cells, with exogenous AA as substrate and the pure enzymes COX-1 and COX-2. PGE(2) was measured by GC-MS.

KEY RESULTS

ATK was a potent inhibitor of COX-1 and COX-2 with IC(50) values of 0.5 and 0.1 microM in MC3T3-E1 cells and of 1.7 and 2.6 microM using the pure enzymes. Inhibition was reversible, with slow- and tight-binding characteristics. The arachidonyl carbon chain was essential, as the saturated palmitoyl analogue had no effect.

CONCLUSIONS AND IMPLICATIONS

Attenuation of PG synthesis by ATK is taken to be the consequence of PLA(2) inhibition and the findings of many studies are interpreted on that basis. If there are, however, alternative routes for AA liberation (such as phospholipase C/diacyl glycerol lipase or phospholipase D), this interpretation can lead to false conclusions. As ATK is a widely used and important pharmacological tool in eicosanoid research, knowledge of its interactions with other major enzymes of the cascade is of considerable importance.

Kinetic models of cyclooxygenase and peroxidase inactivation of prostaglandin-H-synthase during catalysis

Kinetic models of inactivation of cyclooxygenase and peroxidase activities of prostaglandin-H-synthase (PGHS) during cyclooxygenase and peroxidase reactions catalyzed by the enzyme and also on preincubation with H2O2 have been developed; these models account for data obtained by the authors as well as data from the literature. Being rather simple, these models simultaneously describe the processes of cyclooxygenase and peroxidase inactivation of PGHS, using the minimal set of experimental parameters.

Cyclooxygenase and peroxidase inactivation of prostaglandin-H-synthase during catalysis

Prostaglandin-H-synthase (PGHS) is a bifunctional enzyme catalyzing cyclooxygenase and peroxidase reactions and undergoing irreversible inactivation during catalysis. A new method for kinetic studies of both PGHS activities in the course of cyclooxygenase as well as peroxidase reactions and also preincubation with hydroperoxides is suggested. It is shown that peroxidase activity is retained after complete cyclooxygenase inactivation and cyclooxygenase activity is retained after complete peroxidase inactivation. Two-stage cyclooxygenase inactivation occurs on preincubation of PGHS with hydrogen peroxide. Studies on inactivation under various conditions indicate that chemical mechanisms of cyclooxygenase and peroxidase inactivation are different. The data allow development of kinetic models.

Prostaglandin Endoperoxide H Synthases

The cyclooxygenase (COX) activity of prostaglandin endoperoxide H synthases (PGHSs) converts arachidonic acid and O2 to prostaglandin G2 (PGG2). PGHS peroxidase (POX) activity reduces PGG2 to PGH2. The first step in POX catalysis is formation of an oxyferryl heme radical cation (Compound I), which undergoes intramolecular electron transfer forming Intermediate II having an oxyferryl heme and a Tyr-385 radical required for COX catalysis. PGHS POX catalyzes heterolytic cleavage of primary and secondary hydroperoxides much more readily than H2O2, but the basis for this specificity has been unresolved. Several large amino acids form a hydrophobic "dome" over part of the heme, but when these residues were mutated to alanines there was little effect on Compound I formation from H2O2 or 15-hydroperoxyeicosatetraenoic acid, a surrogate substrate for PGG2. Ab initio calculations of heterolytic bond dissociation energies of the peroxyl groups of small peroxides indicated that they are almost the same. Molecular Dynamics simulations suggest that PGG2 binds the POX site through a peroxyl-iron bond, a hydrogen bond with His-207 and van der Waals interactions involving methylene groups adjoining the carbon bearing the peroxyl group and the protoporphyrin IX. We speculate that these latter interactions, which are not possible with H2O2, are major contributors to PGHS POX specificity. The distal Gln-203 four residues removed from His-207 have been thought to be essential for Compound I formation. However, Q203V PGHS-1 and PGHS-2 mutants catalyzed heterolytic cleavage of peroxides and exhibited native COX activity. PGHSs are homodimers with each monomer having a POX site and COX site. Cross-talk occurs between the COX sites of adjoining monomers. However, no cross-talk between the POX and COX sites of monomers was detected in a PGHS-2 heterodimer comprised of a Q203R monomer having an inactive POX site and a G533A monomer with an inactive COX site.

Metabolism of Non-steroidal Anti-inflammatory Drugs by Peroxidase: Implication for Gastrointestinal Mucosal Lesions

Non-steroidal anti-inflammatory drugs (NSAIDs) are widely used to treat inflammatory diseases including rheumatoid arthritis and gout. The anti-inflammatory action of NSAIDs is due to the inhibition of prostaglandin synthesis by preventing cyclooxygenase (COX) activity of prostaglandin H synthase (PGS). However, administration of NSAIDs causes gastrointestinal mucosal lesions and a decrease of granulocytes as side effects. PGS catalyzes two distinct enzyme reactions: (1) bis-dioxygenation of arachidonic acid catalyzed by COX activity of PGS to form PGG(2); and (2) reduction of the hydroperoxide group in PGG(2) by PGS hydroperoxidase. Most NSAID are oxidized by peroxidases to produce NSAID radicals that damage biological components such as lipids and enzymes. Indomethacin, phenylbutazone, and piroxicam are more toxic under aerobic conditions than anaerobic conditions during the interaction with peroxidase. We discuss the contribution of peroxidases in the formation of gastrointestinal mucosal lesions induced by NSAIDs.

Oxyferryl heme and not tyrosyl radical is the likely culprit in prostaglandin H synthase-1 peroxidase inactivation

Prostaglandin H synthase-1 (PGHS-1) is a bifunctional heme protein catalyzing both a peroxidase reaction, in which peroxides are converted to alcohols, and a cyclooxygenase reaction, in which arachidonic acid is converted into prostaglandin G2. Reaction of PGHS-1 with peroxide forms Intermediate I, which has an oxyferryl heme and a porphyrin radical. An intramolecular electron transfer from Tyr385 to Intermediate I forms Intermediate II, which contains two oxidants: an oxyferryl heme and the Tyr385 radical required for cyclooxygenase catalysis. Self-inactivation of the peroxidase begins with Intermediate II, but it has been unclear which of the two oxidants is involved. The kinetics of tyrosyl radical, oxyferryl heme, and peroxidase inactivation were examined in reactions of PGHS-1 reconstituted with heme or mangano protoporphyrin IX with a lipid hydroperoxide, 15-hydroperoxyeicosatetraenoic acid (15-HPETE), and ethyl hydrogen peroxide (EtOOH). Tyrosyl radical formation was significantly faster with 15-HPETE than with EtOOH and roughly paralleled oxyferryl heme formation at low peroxide levels. However, the oxyferryl heme intensity decayed much more rapidly than the tyrosyl radical intensity at high peroxide levels. The rates of reactions for PGHS-1 reconstituted with MnPPIX were approximately an order of magnitude slower, and the initial species formed displayed a wide singlet (WS) radical, rather than the wide doublet radical observed with PGHS-1 reconstituted with heme. Inactivation of the peroxidase activity during the reaction of PGHS-1 with EtOOH or 15-HPETE correlated with oxyferryl heme decay, but not with changes in tyrosyl radical intensity or EPR line shape, indicating that the oxyferryl heme, and not the tyrosyl radical, is responsible for the self-destructive peroxidase side reactions. Computer modeling to a minimal mechanism was consistent with oxyferryl heme being the source of peroxidase inactivation.

Pharmacodynamic of cyclooxygenase inhibitors in humans

We provide comprehensive knowledge on the differential regulation of expression and catalysis of cyclooxygenase (COX)-1 and COX-2 in health and disease which represents an essential requirement to read out the clinical consequences of selective and nonselective inhibition of COX-isozymes in humans. Furthermore, we describe the pharmacodynamic and pharmacokinetic characteristics of major traditional nonsteroidal anti-inflammatory drugs (tNSAIDs) and coxibs (selective COX-2 inhibitors) which play a prime role in their efficacy and toxicity. Important information derived from our pharmacological studies has clarified that nonselective COX inhibitors should be considered the tNSAIDs with a balanced inhibitory effect on both COX-isozymes (exemplified by ibuprofen and naproxen). In contrast, the tNSAIDs meloxicam, nimesulide and diclofenac (which are from 18- to 29-fold more potent towards COX-2 in vitro) and coxibs (i.e. celecoxib, valdecoxib, rofecoxib, etoricoxib and lumiracoxib, which are from 30- to 433-fold more potent towards COX-2 in vitro) should be comprised into the cluster of COX-2 inhibitors. However, the dose and frequency of administration together with individual responses will drive the degree of COX-2 inhibition and selectivity achieved in vivo. The results of clinical pharmacology of COX inhibitors support the concept that the inhibition of platelet COX-1 may translate into an increased incidence of serious upper gastrointestinal bleeding but this effect on platelet COX-1 may mitigate the cardiovascular hazard associated with the profound inhibition of COX-2-dependent prostacyclin (PGI2).

Regulation of cyclooxygenase catalysis by hydroperoxides

Activation of cyclooxygenase catalysis in prostaglandin H synthase-1 and -2 by peroxide-dependent formation of a tyrosyl radical is emerging as an important part of regulating cellular production of bioactive prostanoids. This review discusses the mechanism of tyrosyl radical formation and the influence of peroxide, fatty acid, peroxidase cosubstrate, and protein structure on the activation process and cyclooxygenase catalysis.

ESR spin trapping of a carbon-centered free radical from agonist-stimulated human platelets

Several free radical intermediates formed during synthesis of prostaglandin H synthase (PGHS) catalyze the biosynthesis of prostaglandins from arachidonic acid (AA). We attempted to directly detect free radical intermediates of PGHS in cells. Studies were carried out using human platelets, which possess significant PGHS activity. Electron spin resonance (ESR) spectra showed a g = 2.005 signal radical, which was formed by the incubation of collagen, thrombin, AA, and a variety of peroxides with human platelets. The ESR spectra obtained using 5,5-dimethyl-1 pyrroline N-oxide (DMPO) and alpha-phenyl N-tert.-butylnitron (PBN) were typical of an immobilized nitroxide. Extensive Pronase digestion of both the DMPO and PBN adducts allowed us to deduce that it was a carbon-centered radical. The formation of this radical was inhibited by potassium cyanide and by desferroxamine. Peroxides stimulated formation of the g = 2.005 signal radical and inhibited platelet aggregation induced by AA. PGHS cosubstrates increased the intensity of the radical signal but inhibited platelet aggregation induced by AA. Both S-nitro-L-glutathione and reduced glutathione quenched the g = 2.005 radical but could not restore platelet aggregatory activity. These results suggest that the carbon-centered radical is a self-destructing free radical formed during peroxide-mediated deactivation of PGHS in human platelets.

Glycerylprostaglandin synthesis by resident peritoneal macrophages in response to a zymosan stimulus

Cyclooxygenase (COX)-2 oxygenates arachidonic acid (AA) and 2-arachidonylglycerol (2-AG) to endoperoxides, which are subsequently transformed to prostaglandins (PGs) and glycerylprostaglandins (PG-Gs). PG-G formation has not been demonstrated in intact cells treated with a physiological agonist. Resident peritoneal macrophages, which express COX-1, were pretreated with lipopolysaccharide to induce COX-2. Addition of zymosan caused release of 2-AG and production of the glyceryl esters of PGE2 and PGI2 over 60 min. The total quantity of PG-Gs (16 +/- 6 pmol/10(7) cells) was much lower than that of the corresponding PGs produced from AA (21,000 +/- 7,000 pmol/10(7) cells). The differences in PG-G and PG production were partially explained by differences in the amounts of 2-AG and AA released in response to zymosan. The selective COX-2 inhibitor, SC236, reduced PG-G and PG production by 49 and 17%, respectively, indicating a significant role for COX-1 in PG-G and especially PG synthesis. Time course studies indicated that COX-2-dependent oxygenation rapidly declined 20 min after zymosan addition. When exogenous 2-AG was added to macrophages, a substantial portion was hydrolyzed to AA and converted to PGs; 1 microm 2-AG yielded 820 +/- 200 pmol of PGs/10(7) cells and 78 +/- 41 pmol of PG-Gs/10(7) cells. SC236 reduced PG-G and PG production from exogenous 2-AG by 88 and 76%, respectively, indicating a more significant role for COX-2 in the utilization of exogenous substrate. In conclusion, lipopolysaccharide-pretreated macrophages produce PG-Gs from endogenous 2-AG during zymosan phagocytosis, but PG-G formation is limited by substrate hydrolysis and inactivation of COX-2.

Peroxynitrite provides the peroxide tone for PGHS‐2‐dependent prostacyclin synthesis in vascular smooth muscle cells

Endotoxin-treated vascular smooth muscle cells (VSMCs) were recently shown to release high amounts of prostacyclin (PGI2) dependent on the induction of prostaglandin endoperoxide synthase-2 (PGHS-2). In contrast to endothelial PGI2-synthase, for which nitration and inhibition by peroxynitrite was reported, addition of SIN-1 as a peroxynitrite-generating system did not cause inhibition but rather doubled PGI2 release by VSMC. The hypothesis of peroxynitrite supplementing an unsaturated peroxide tone for PGHS-2 was supported by H2O2 exerting the same effect. Studies performed with purified PGHS-2 revealed maximal elevation of enzyme activity in the presence of equimolar concentrations of *NO and *O2-, which together form peroxynitrite, while excessive production of either one radical was inhibitory. Most importantly, 6-keto-PGF1alpha formation by intact VSMC depended on a nearly equimolar generation of *NO and *O2- for providing the endogenous peroxide tone. These findings, together with the observation that an excess of exogenously added *NO, as well as uric acid as a scavenger of peroxynitrite potently reduced PGI2 release, underlined the role of peroxynitrite as the dominating and rate-limiting intracellular mediator of peroxide tone in VSMC. The results allow us to postulate a new cross-talk between the *NO and the prostanoid pathways with a crucial role for peroxynitrite in providing the peroxide tone for a continuous activation of PGHS-2.

Aromatic hydroxamic acids and hydrazides as inhibitors of the peroxidase activity of prostaglandin H2 synthase-2

The cyclooxygenase activity of the bifunctional enzyme prostaglandin H(2) synthase-2 (PGHS-2) is the target of non-steroidal anti-inflammatory drugs. Inhibition of the peroxidase activity of PGHS has been less studied. Using Soret absorption changes, the binding of aromatic hydroxamic acids to the peroxidase site of PGHS-2 was examined to investigate the structural determinants of inhibition. Typical of mammalian peroxidases, the K(d) for benzhydroxamic acid (42mM) is much greater than that for salicylhydroxamic acid (475microM). Binding of the hydroxamic acid tepoxalin (25microM) resulted in only minor Soret changes. However, tepoxalin is an efficient reducing cosubstrate, indicating that it is an alternative electron donor rather than an inhibitor of the peroxidase activity. Aromatic hydrazides are metabolically activated inhibitors of peroxidases. 2-Naphthoichydrazide (2-NZH) caused the time- and concentration-dependent inhibition of both PGHS-2 peroxidase and cyclooxygenase activities. H(2)O(2) was required for the inactivation of both PGHS-2 activities and indomethacin (which binds at the cyclooxygenase site) did not affect the peroxidase inhibitory potency of 2-NZH. A series of aromatic hydrazides were found to be potent inhibitors of PGHS-2 peroxidase activity with IC(50) values in the 6-100microM range for 13 of the 18 hydrazides examined. Selective inhibition of PGHS-2 over myeloperoxidase and horseradish peroxidase isozyme C was increased by certain ring substitutions. In particular, a chloro group para to the hydrazide moiety increased the PGHS-2 selectivity relative to both myeloperoxidase and horseradish peroxidase isozyme C.

Prostaglandin H synthases: members of a class of quasi-linear threshold switches

Prostaglandin H synthase (PTGS or COX) enzymes catalyze rate-limiting steps in the synthesis of potent prostanoids, including various prostaglandins, thromboxane, and prostacyclin. Mechanisms that have evolved for regulating prostanoid biosynthesis reflect a tension between achieving a rapid but measured response to cellular signals while minimizing spurious activation by signal noise. We found through mathematical modeling that the PTGS enzymes can be thought of as regulatory switches with approximately linear output above an adjustable threshold. In vivo synthesis allows continuous production while signal remains above threshold. Different isoforms show specific adaptions reflecting their physiological roles as constitutive or inducible enzymes. Mathematical modeling helps explain how these adaptations enable the PTGS1 and PTGS2 enzymes to maintain their physiological roles while avoiding potentially damaging consequences.

Intrathecal Protease-Activated Receptor Stimulation Produces Thermal Hyperalgesia through Spinal Cyclooxygenase Activity

Activation of protease-activated receptors (PARs) in non-neural tissue results in prostaglandin production. Because PARs are found in the spinal cord and increased prostaglandin release in the spinal cord causes thermal hyperalgesia, we hypothesized that activation of these spinal PARs would stimulate prostaglandin production and cause a cyclooxygenase-dependent thermal hyperalgesia. PARs were activated using either thrombin or peptide agonists derived from the four PAR subtypes, delivered to the lumbar spinal cord. Dialysis experiments were conducted in conscious, unrestrained rats using loop microdialysis probes placed in the lumbar intrathecal space. Intrathecal thrombin stimulated release of prostaglandin E (PGE)(2) but not aspartate or glutamate. Intrathecal delivery of the PAR 1-derived peptide SFLLRN-NH(2) and the PAR 2-derived peptide SLIGRL both stimulated PGE(2) release; PAR 3-derived TFRGAP and PAR 4-derived GYPGQV were inactive. Intrathecal thrombin had no effect upon formalin-induced flinching or tactile sensitivity but resulted in a thermal hyperalgesia. Intrathecal SFLLRN-NH(2) and SLIGRL both produced thermal hyperalgesia. Consistent with their effects on spinal PGE(2), hyperalgesia from these peptides was blocked by pretreatment with the cyclooxygenase inhibitor ibuprofen. SLIGRL-induced hyperalgesia was also blocked by the selective inhibitors SC 58,560 [5-(4-fluorophenyl)-1-[4-(methylsulfonyl)phenyl]-3-(trifluoromethyl)-1H-pyrazole; cyclooxygenase (COX) 1] and SC 58,125 [5-(4-chlorophenyl)-1-(4-methoxyphenyl)-3-(trifluoromethyl)-1H-pyrazole; COX 2]. These data indicate that activation of spinal PAR 2 and possibly PAR 1 results in the stimulation of the spinal cyclooxygenase cascade and a prostaglandin-dependent thermal hyperalgesia.

Role of Asn-382 and Thr-383 in Activation and Inactivation of Human Prostaglandin H Synthase Cyclooxygenase Catalysis

Cyclooxygenase catalysis by prostaglandin H synthase-1 and -2 (PGHS-1 and -2) requires activation of the normally latent enzyme by peroxide-dependent generation of a free radical at Tyr-385 (PGHS-1 numbering) in the cyclooxygenase active site; the Tyr-385 radical has also been linked to self-inactivation processes that impose an ultimate limit on cyclooxygenase catalysis. Cyclooxygenase activation is more resistant to suppression by cytosolic glutathione peroxidase in PGHS-2 than in PGHS-1. This differential response to peroxide scavenging enzymes provides a basis for the differential catalytic regulation of the two PGHS isoforms observed in vivo. We sought to identify structural differences between the isoforms, which could account for the differential cyclooxygenase activation, and used site-directed mutagenesis of recombinant human PGHS-2 to focus on one heme-vicinity residue that diverges between the two isoforms, Thr-383, and an adjacent residue that is conserved between the isoforms, Asn-382. Substitutions of Thr-383 (histidine in most PGHS-1) with histidine or aspartate decreased cyclooxygenase activation efficiency by about 40%, with little effect on cyclooxygenase specific activity or self-inactivation. Substitutions of Asn-382 with alanine, aspartate, or leucine had little effect on the cyclooxygenase specific activity or activation efficiency but almost doubled the cyclooxygenase catalytic output before self-inactivation. Asn-382 and Thr-383 mutations did not appreciably alter the Km value for arachidonate, the cyclooxygenase product profile, or the Tyr-385 radical spectroscopic characteristics, confirming the structural integrity of the cyclooxygenase site. The side chain structures of Asn-382 and Thr-383 in PGHS-2 thus selectively influence two important aspects of cyclooxygenase catalytic regulation: activation by peroxide and self-inactivation.

Cyclooxygenase Inactivation Kinetics during Reaction of Prostaglandin H Synthase-1 with Peroxide

The peroxidase and cyclooxygenase activities of prostaglandin H synthase-1 (PGHS-1) both become irreversibly inactivated during reaction with peroxide. Sequential stopped-flow absorbance measurements with a chromogenic peroxidase cosubstrate previously were used to evaluate the kinetics of peroxidase inactivation during reaction of PGHS-1 with peroxide [Wu, G., et al. (1999) J. Biol. Chem. 274, 9231-7]. This approach has now been adapted to use a chromogenic cyclooxygenase substrate to analyze the detailed kinetics of cyclooxygenase inactivation during reaction of PGHS-1 with several hydroperoxides. In the absence of added reducing cosubstrates, which maximizes the levels of oxidized enzyme intermediates expected to lead to inactivation, cyclooxygenase activity was lost as fast as, or somewhat faster than, peroxidase activity. Cyclooxygenase inactivation kinetics appeared to be sensitive to the structure of the peroxide used. The addition of reducing cosubstrate during reaction of PGHS-1 with peroxide protected the peroxidase activity to a much greater degree than the cyclooxygenase activity. The results suggest a new concept of PGHS inactivation: that distinct damage can occur at the two active sites during side reactions of Intermediate II, which forms during reaction of PGHS with peroxide and which contains two oxidants, a ferryl heme in the peroxidase site, and a tyrosyl free radical in the cyclooxygenase site.

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.

Effects of acetaminophen on constitutive and inducible prostanoid biosynthesis in human blood cells

1. Acetaminophen, an analgesic and antipyretic drug with weak antiinflammatory properties, has been suggested to act as a tissue-selective inhibitor of prostaglandin H synthases (PGHSs) (e.g. COX-1 and COX-2) through its reducing activity, that is influenced by the different cellular levels of peroxides. 2. We have studied the effects of acetaminophen on inducible and constitutive prostanoid biosynthesis in monocytes and platelets in vitro. To discriminate between the inhibitory effect of the drug on PGHS-isozymes vs PGE-synthases (PGESs), parallel measurements of PGE(2) and thromboxane (TX) B(2) were carried out. Since antioxidant enzymes and cofactors, present in plasma, may affect acetaminophen-dependent inhibition of prostanoids, comparative experiments in whole blood vs isolated monocytes were performed. 3. Acetaminophen inhibited LPS-induced whole blood PGE(2) and TXB(2) production, in a concentration-dependent fashion [IC(50) microM (95% confidence intervals): 44 (27-70) and 94 (79-112), respectively]. Therapeutic plasma concentrations (100 and 300 microM) of the drug more profoundly reduced PGE(2) than TXB(2) (71 +/- 3 vs 54 +/- 4 and 95 +/- 0.8 vs 78 +/- 2%, respectively, mean +/- s.e.mean, n = 6, P < 0.01). 4. Differently, in isolated monocytes stimulated with LPS, both PGE(2) and TXB(2) production was maximally reduced by only 60%. 5 At 100 and 300 microM, the drug caused a similar and incomplete inhibition of platelet PGE(2) and TXB(2) production during whole blood clotting (45 +/- 4 vs 54 +/- 4 and 75 +/- 2 vs 75 +/- 1%, respectively, mean +/- s.e.mean, n = 4). 6 In conclusion, therapeutic concentrations of acetaminophen caused an incomplete inhibition of platelet COX-1 and monocyte COX-2 but in the presence of plasma, the drug almost completely suppressed inducible PGE(2) biosynthesis through its inhibitory effects on both COX-2 and inducible PGES.

Biological significance of phospholipid hydroperoxide glutathione peroxidase (PHGPx, GPx4) in mammalian cells

Reactive oxygen species (ROS) are known mediators of intracellular signal cascades. Excessive production of ROS may lead to oxidative stress, loss of cell function, and cell death by apoptosis or necrosis. Lipid hydroperoxides are one type of ROS whose biological function has not yet been clarified. Phospholipid hydroperoxide glutathione peroxidase (PHGPx, GPx4) is a unique antioxidant enzyme that can directly reduce phospholipid hydroperoxide in mammalian cells. This contrasts with most antioxidant enzymes, which cannot reduce intracellular phospholipid hydroperoxides directly. In this review, we focus on the structure and biological functions of PHGPx in mammalian cells. Recently, molecular techniques have allowed overexpression of PHGPx in mammalian cell lines, from which it has become clear that lipid hydroperoxides also have an important function as activators of lipoxygenase and cyclooxygenase, participate in inflammation, and act as signal molecules for apoptotic cell death and receptor-mediated signal transduction at the cellular level.

In the presence of L-NAME SERCA blockade induces endothelium-dependent contraction of mouse aorta through activation of smooth muscle prostaglandin H2/thromboxane A2 receptors

1. The mechanism of transient contractions induced by the sarcoplasmic-endoplasmic reticulum calcium ATPase (SERCA) blocker cyclopiazonic acid (CPA) in the presence of L-NAME was investigated in mouse aorta. 2. The contractions elicited by 10 micro M CPA required an intact endothelium, were dependent upon external Ca(2+) and were prevented by 10 micro M indomethacin, the inhibitor of prostaglandin synthesis, or 1 micro M SQ29548, the specific prostaglandin H2/thromboxane A2 (PGH2/TXA2) receptor blocker. 3. A blocker of receptor/store operated Ca(2+) channels and voltage gated calcium channels (VGCC), SK&F 96365 (10 micro M), completely abolished the contractions, while a specific blocker of VGCC nifedipine (1 micro M) inhibited them by one third. 4. Dichlorobenzamyl hydrochloride, a blocker of Na(+)/Ca(2+) exchange effectively prevented return of tension to baseline value. 5. At higher concentrations (30-100 micro M) CPA induced indomethacin-resistant tonic contractions of mouse aorta. The CPA dose response curve for tonic contractions is shifted to the right compared to the transient contractions suggesting that smooth muscle is less sensitive to CPA than endothelium. 6. PGH2/TXA2 receptors in mouse aorta are highly sensitive to the thromboxane analogue U46619 (EC(50) : 1.93 nM). This compound stimulates contractions even in the absence of external Ca(2+), which are abolished by the Rho-kinase inhibitor HA-1077. 7. The results suggest that 10 micro M CPA induced capacitive Ca(2+) entry in endothelial cells stimulating the release of PGH2/TXA2, which subsequently caused smooth muscle contraction dependent on Ca(2+) influx and myofilament sensitization by Rho-kinase. Higher concentrations of CPA (30-100 micro M) directly induced contraction of mouse aortic smooth muscle.

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

We summarize the enzymological properties of prostaglandin endoperoxide H synthases (PGHs)-1 and -2, the enzymes that catalyze the committed step in prostaglandin biosynthesis. These isoenzymes are closely related structurally and mechanistically. Each catalyzes a peroxidase and a cyclooxygenase reaction at spatially separate but neighboring, electronically interrelated active sites. The peroxidase is necessary to activate the cyclooxygenase; oxidation of the heme group of the peroxidase by peroxide leads to oxidation of a cyclooxygenase active site tyrosine. The tyrosine radical abstracts hydrogen from arachidonic acid to form an arachidonate radical which reacts sequentially with two oxygen molecules forming the intermediate product PGG2. PGG2 is then reduced by the peroxidase activity to PGH2. Based on the crystal structure of PGHS-1 arachidonate complex, it is now possible to envision how arachidonate is bound and oxygenation occurs. Recently, it has become possible to distinguish kinetically between the cyclooxygenase and peroxidase suicide inactivation reactions.

Suicide inactivation of peroxidases and the challenge of engineering more robust enzymes

As the number of industrial applications for proteins continues to expand, the exploitation of protein engineering becomes critical. It is predicted that protein engineering can generate enzymes with new catalytic properties and create desirable, high-value, products at lower production costs. Peroxidases are ubiquitous enzymes that catalyze a variety of oxygen-transfer reactions and are thus potentially useful for industrial and biomedical applications. However, peroxidases are unstable and are readily inactivated by their substrate, hydrogen peroxide. Researchers rely on the powerful tools of molecular biology to improve the stability of these enzymes, either by protecting residues sensitive to oxidation or by devising more efficient intramolecular pathways for free-radical allocation. Here, we discuss the catalytic cycle of peroxidases and the mechanism of the suicide inactivation process to establish a broad knowledge base for future rational protein engineering.

Different suicide inactivation processes for the peroxidase and cyclooxygenase activities of prostaglandin endoperoxide H synthase-1

Prostaglandin endoperoxide H synthases (PGHSs)-1 and -2 have a cyclooxygenase (COX) activity involved in forming prostaglandin G2 (PGG2) from arachidonic acid and an associated peroxidase (POX) activity that reduces PGG2 to PGH2. Suicide inactivation processes are observed for both POX and COX reactions. Here we report COX reaction conditions for PGHS-1 under which complete COX inactivation occurs but with > or = 60% retention of POX activity. The rates of POX inactivation were compared for native oPGHS-1 versus Y385F oPGHS-1, a mutant that cannot form the Tyr385 radical of COX Intermediate II; the rates were the same for both native and Y385F oPGHS-1. Our data indicate that a COX Intermediate II/acyl or product complex is the precursor in COX inactivation. However, another species, probably an Intermediate II-like species but with a radical centered on a tyrosine other than Tyr385, is the immediate precursor for POX inactivation.

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.