Prostaglandin H synthase: spectroscopic studies of the interaction with hydroperoxides and with indomethacin

Prostaglandin H synthase kinetics in the two-phase aqueous-micellar system

Reaction mixture for PGHS (prostaglandin-H-synthase) is a two-phase system including micellar hydrophobic phase and hydrophilic aqueous phase. Reagents added to the mixture are distributed between phases, thus concentrations of reagents dissolved in phases can differ significantly from their overall contents. Using dynamic light scattering we found that the hydrophobic phase produced by tween-20 consists of micelles, which radius (4-5nm) does not depend on either tween-20 overall content (0.1%-1% v/v) or arachidonic acid (AA) addition (10-1000μM) or PGHS addition (1μM). Tween-20 overall content changing from 0.1% to 2% v/v dramatically affected COX kinetic, but accounting AA distribution between phases allowed us to estimate "true" parameters, independent of the tween-20 overall content and the concentration of another substrate: KM(Ox) equals 9.8μM O2 in the aqueous phase or 0.0074bar in the gaseous phase, KM(AA) equals 5400μM AA in the phase of tween-20 micelles and 5400/PμM AA in the aqueous phase (P is the distribution ratio for the AA between the aqueous phase and the hydrophobic phase (P≫1000)). This approach allowed to evaluate PS, the distribution ratio for the AA between the hydrophobic phase and the PGHS active center (PS ~310). This coefficient indicates the AA selectivity toward the cyclooxygenase active center.

Characterization of a cellular denitrase activity that reverses nitration of cyclooxygenase

Protein 3-nitrotyrosine (3-NT) formation is frequently regarded as a simple biomarker of disease, an irreversible posttranslational modification that can disrupt protein structure and function. Nevertheless, evidence that protein 3-NT modifications may be site selective and reversible, thus allowing for physiological regulation of protein activity, has begun to emerge. We have previously reported that cyclooxygenase (COX)-1 undergoes heme-dependent nitration of Tyr(385), an internal and catalytically essential residue. In the present study, we demonstrate that nitrated COX-1 undergoes a rapid reversal of nitration by substrate-selective and biologically regulated denitrase activity. Using nitrated COX-1 as a substrate, denitrase activity was validated and quantified by analytic HPLC with electrochemical detection and determined to be constitutively active in murine and human endothelial cells, macrophages, and a variety of tissue samples. Smooth muscle cells, however, contained little denitrase activity. Further characterizing this denitrase activity, we found that it was inhibited by free 3-NT and may be enhanced by endogenous nitric oxide and exogenously administered carbon monoxide. Finally, we describe a purification protocol that results in significant enrichment of a discrete denitrase-containing fraction, which maintains activity throughout the purification process. These findings reveal that nitrated COX-1 is a substrate for a denitrase in cells and tissues, implying that the reciprocal processes of nitration and denitration may modulate bioactive lipid synthesis in the setting of inflammation. In addition, our data reveal that denitration is a controlled process that may have broad importance for regulating cell signaling events in nitric oxide-generating systems during oxidative/nitrosative stress.

Proton Coupled Electron Transfer and Redox Active Tyrosines: Structure and Function of the Tyrosyl Radicals in Ribonucleotide Reductase and Photosystem II

Proton coupled electron transfer (PCET) reactions are important in many biological processes. Tyrosine oxidation/reduction can play a critical role in facilitating these reactions. Two examples are photosystem II (PSII) and ribonucleotide reductase (RNR). RNR is essential in DNA synthesis in all organisms. In E. coli RNR, a tyrosyl radical, Y122(•), is required as a radical initiator. Photosystem II (PSII) generates molecular oxygen from water. In PSII, an essential tyrosyl radical, YZ(•), oxidizes the oxygen evolving center. However, the mechanisms, by which the extraordinary oxidizing power of the tyrosyl radical is controlled, are not well understood. This is due to the difficulty in acquiring high-resolution structural information about the radical state. Spectroscopic approaches, such as EPR and UV resonance Raman (UVRR), can give new information. Here, we discuss EPR studies of PCET and the PSII YZ radical. We also present UVRR results, which support the conclusion that Y122 undergoes an alteration in ring and backbone dihedral angle when it is oxidized. This conformational change results in a loss of hydrogen bonding to the phenolic oxygen. Our analysis suggests that access of water is an important factor in determining tyrosyl radical lifetime and function. TOC graphic.

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

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

Proton coupled electron transfer and redox active tyrosines in Photosystem II

In this article, progress in understanding proton coupled electron transfer (PCET) in Photosystem II is reviewed. Changes in acidity/basicity may accompany oxidation/reduction reactions in biological catalysis. Alterations in the proton transfer pathway can then be used to alter the rates of the electron transfer reactions. Studies of the bioenergetic complexes have played a central role in advancing our understanding of PCET. Because oxidation of the tyrosine results in deprotonation of the phenolic oxygen, redox active tyrosines are involved in PCET reactions in several enzymes. This review focuses on PCET involving the redox active tyrosines in Photosystem II. Photosystem II catalyzes the light-driven oxidation of water and reduction of plastoquinone. Photosystem II provides a paradigm for the study of redox active tyrosines, because this photosynthetic reaction center contains two tyrosines with different roles in catalysis. The tyrosines, YZ and YD, exhibit differences in kinetics and midpoint potentials, and these differences may be due to noncovalent interactions with the protein environment. Here, studies of YD and YZ and relevant model compounds are described.

Structural comparisons of arachidonic acid-induced radicals formed by prostaglandin H synthase-1 and -2

Cyclooxygenase catalysis by prostaglandin H synthase (PGHS)-1 and -2 involves reaction of a peroxide-induced Tyr385 radical with arachidonic acid (AA) to form an AA radical that reacts with O(2). The potential for isomeric AA radicals and formation of an alternate tyrosyl radical at Tyr504 complicate analysis of radical intermediates. We compared the EPR spectra of PGHS-1 and -2 reacted with peroxide and AA or specifically deuterated AA in anaerobic, single-turnover experiments. With peroxide-treated PGHS-2, the carbon-centered radical observed after AA addition was consistently a pentadienyl radical; a variable wide-singlet (WS) contribution from mixture of Tyr385 and Tyr504 radicals was also present. Analogous reactions with PGHS-1 produced EPR signals consistent with varying proportions of pentadienyl and tyrosyl radicals, and two additional EPR signals. One, insensitive to oxygen exposure, is the narrow singlet tyrosyl radical with clear hyperfine features found previously in inhibitor-pretreated PGHS-1. The second type of EPR signal is a narrow singlet lacking detailed hyperfine features that disappeared upon oxygen exposure. This signal was previously ascribed to an allyl radical, but high field EPR analysis indicated that ~90% of the signal originates from a novel tyrosyl radical, with a small contribution from a carbon-centered species. The radical kinetics could be resolved by global analysis of EPR spectra of samples trapped at various times during anaerobic reaction of PGHS-1 with a mixture of peroxide and AA. The improved understanding of the dynamics of AA and tyrosyl radicals in PGHS-1 and -2 will be useful for elucidating details of the cyclooxygenase mechanism, particularly the H-transfer between tyrosyl radical and AA.

Control of proton and electron transfer in de novo designed, biomimetic β hairpins

Tyrosine side chains are involved in proton coupled electron transfer reactions (PCET) in many complex proteins, including photosystem II (PSII) and ribonucleotide reductase. For example, PSII contains two redox-active tyrosines, TyrD (Y160D2) and TyrZ (Y161D1), which have different protein environments, midpoint potentials, and roles in catalysis. TyrD has a midpoint potential lower than that of TyrZ, and its protein environment is distinguished by potential π-cation interactions with arginine residues. Designed biomimetic peptides provide a system that can be used to investigate how the protein matrix controls PCET reactions. As a model for the redox-active tyrosines in PSII, we are employing a designed, 18 amino acid β hairpin peptide in which PCET reactions occur between a tyrosine (Tyr5) and a cross-strand histidine (His14). In this peptide, the single tyrosine is hydrogen-bonded to an arginine residue, Arg16, and a second arginine, Arg12, has a π-cation interaction with Tyr5. In this report, the effect of these hydrogen bonding and electrostatic interactions on the PCET reactions is investigated. Electrochemical titrations show that histidine substitutions change the nature of PCET reactions, and optical titrations show that Arg16 substitution changes the pK of Tyr5. Removal of Arg16 or Arg12 increases the midpoint potential for tyrosine oxidation. The effects of Arg12 substitution are consistent with the midpoint potential difference, which is observed for the PSII redox-active tyrosine residues. Our results demonstrate that a π-cation interaction, hydrogen bonding, and PCET reactions alter redox-active tyrosine function. These interactions can contribute equally to the control of midpoint potential and reaction rate.

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.

Peroxide-induced radical formation at TYR385 and TYR504 in human PGHS-1

Prostaglandin H synthase isoforms 1 and -2 (PGHS-1 and -2) react with peroxide to form a radical on Tyr385 that initiates the cyclooxygenase catalysis. The tyrosyl radical EPR signals of PGHS-1 and -2 change over time and are altered by cyclooxygenase inhibitor binding. We characterized the tyrosyl radical dynamics using wild type human PGHS-1 (hPGHS-1) and its Y504F, Y385F, and Y385F/Y504F mutants to determine whether the radical EPR signal changes involve Tyr504 radical formation, Tyr385 radical phenyl ring rotation, or both. Reaction of hPGHS-1 with peroxide produced a wide singlet, whereas its Y504F mutant produced only a wide doublet signal, assigned to the Tyr385 radical. The cyclooxygenase specific activity and K(M) value for arachidonate of hPGHS-1 were not affected by the Y504F mutation, but the peroxidase specific activity and the K(M) value for peroxide were increased. The Y385F and Y385F/Y504F mutants retained only a small fraction of the peroxidase activity; the former had a much-reduced yield of peroxide-induced radical and the latter essentially none. After binding of indomethacin, a cyclooxygenase inhibitor, hPGHS-1 produced a narrow singlet but the Y504F mutant did not form a tyrosyl radical. These results indicate that peroxide-induced radicals form on Tyr385 and Tyr504 of hPGHS-1, with radical primarily on Tyr504 in the wild type protein; indomethacin binding prevented radical formation on Tyr385 but allowed radical formation on Tyr504. Thus, hPGHS-1 and -2 have different distributions of peroxide-derived radical between Tyr385 and Tyr504. Y504F mutants in both hPGHS-1 and -2 significantly decreased the cyclooxygenase activation efficiency, indicating that formation of the Tyr504 radical is functionally important for both isoforms.

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.

Control of oxygenation in lipoxygenase and cyclooxygenase catalysis

Lipoxygenases (LOX) and cyclooxygenases (COX) react an achiral polyunsaturated fatty acid with oxygen to form a chiral peroxide product of high regio- and stereochemical purity. Both enzymes employ free radical chemistry reminiscent of hydrocarbon autoxidation but execute efficient control during catalysis to form a specific product over the multitude of isomers found in the nonenzymatic reaction. Exactly how both dioxygenases achieve this positional and stereo control is far from clear. We present four mechanistic models, not mutually exclusive, that could account for the specific reactions of molecular oxygen with a fatty acid in the LOX or COX active site.

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.

Reprint of “Oxidative alterations of cyclooxygenase during atherogenesis” [Prostag. Oth. Lipid. M. 80 (2006) 1–14]

Nitric oxide (*NO) and eicosanoids are critical mediators of physiological and pathophysiological processes. They include inflammation and atherosclerosis. *NO production and eicosanoid synthesis become disrupted during atherosclerosis and thus, it is important to understand the mechanisms that may contribute to this outcome. We, and others, have shown that nitrogen oxide (NOx) species modulate cyclooxygenase (COX; also known as prostaglandin H2 synthase) activity and alter eicosanoid production. We have determined that peroxynitrite (ONOO-) has multiple effects on COX activity. ONOO- can provide the peroxide tone necessary for COX activation, such that simultaneous exposure of COX to its arachidonic acid substrate and ONOO- results in increased eicosanoid production. Alternatively, in the absence of arachidonic acid, ONOO- can modify COX through nitration of an essential tyrosine residue (Tyr385) such that it is incapable of catalysis. In this regard, we have shown that COX nitration occurs in human atherosclerotic tissue and in aortic lesions from ApoE-/- mice kept on a high fat diet. Additionally, we have demonstrated that Tyr nitration in ApoE-/- mice is dependent on the inducible form of NO synthase (iNOS). Under conditions where ONOO- persists and arachidonic acid is not immediately available, the cell may try to correct the situation by responding to ONOO- and releasing arachidonic acid via a signaling pathway to favor COX activation. Other post-translational modifications of COX by NOx species include S-nitrosation of cysteine (Cys) residues (which may have an activating effect) and Cys oxidation. The central focus of this review will include a discussion of how NOx species alter COX activity at the molecular level and how these modifications may contribute to altered eicosanoid output during atherosclerosis and lesion development.

Kinetic mechanism of the bifunctional enzyme prostaglandin-H-synthase. Effect of electron donors on the cyclooxygenase reaction

Prostaglandin-H-synthase (PGHS, EC 1.14.99.1) catalyzes the first committed step in biosynthesis of all prostaglandins, thromboxanes, and prostacyclins by converting arachidonic acid to prostaglandin H(2) (PGH(2)). PGHS exhibits two enzymatic activities: cyclooxygenase activity converting arachidonic acid to prostaglandin G(2) (PGG(2)) and peroxidase activity reducing the hydroperoxide PGG(2) to the corresponding alcohol, PGH(2). Despite the many investigations of the kinetics of PGHS, many features such as the absence of competition and mutual activation between the cyclooxygenase and peroxidase activities cannot be explained in terms of existing schemes. In this work we have studied the influence of different electron donors (N,N,N ,N -tetramethyl-p-phenylenediamine, L-epinephrine, 2,2 -azinobis(3-ethylbenzthiazoline-6-sulfonic acid), potassium ferrocyanide) on the PGHS activities. The proposed scheme describes independent but interconnected cyclooxygenase and peroxidase activities of PGHS. It also explains the experimental data obtained in the present work and known from the literature.

Oxidative alterations of cyclooxygenase during atherogenesis

Nitric oxide (*NO) and eicosanoids are critical mediators of physiological and pathophysiological processes. They include inflammation and atherosclerosis. *NO production and eicosanoid synthesis become disrupted during atherosclerosis and thus, it is important to understand the mechanisms that may contribute to this outcome. We, and others, have shown that nitrogen oxide (NO(x)) species modulate cyclooxygenase (COX; also known as prostaglandin H(2) synthase) activity and alter eicosanoid production. We have determined that peroxynitrite (ONOO(-)) has multiple effects on COX activity. ONOO(-) can provide the peroxide tone necessary for COX activation, such that simultaneous exposure of COX to its arachidonic acid substrate and ONOO(-) results in increased eicosanoid production. Alternatively, in the absence of arachidonic acid, ONOO(-) can modify COX through nitration of an essential tyrosine residue (Tyr385) such that it is incapable of catalysis. In this regard, we have shown that COX nitration occurs in human atherosclerotic tissue and in aortic lesions from ApoE(-/-) mice kept on a high fat diet. Additionally, we have demonstrated that Tyr nitration in ApoE(-/-) mice is dependent on the inducible form of NO synthase (iNOS). Under conditions where ONOO(-) persists and arachidonic acid is not immediately available, the cell may try to correct the situation by responding to ONOO(-) and releasing arachidonic acid via a signaling pathway to favor COX activation. Other post-translational modifications of COX by NO(x) species include S-nitrosation of cysteine (Cys) residues (which may have an activating effect) and Cys oxidation. The central focus of this review will include a discussion of how NO(x) species alter COX activity at the molecular level and how these modifications may contribute to altered eicosanoid output during atherosclerosis and lesion development.

Heme catalyzes tyrosine 385 nitration and inactivation of prostaglandin H2 synthase-1 by peroxynitrite

The mechanism by which the inflammatory enzyme prostaglandin H(2) synthase-1 (PGHS-1) deactivates remains undefined. This study aimed to determine the stabilizing parameters of PGHS-1 and identify factors leading to deactivation by nitric oxide species (NO(x)). Purified PGHS-1 was stabilized when solubilized in beta-octylglucoside (rather than Tween-20 or CHAPS) and when reconstituted with hemin chloride (rather than hematin). Peroxynitrite (ONOO(-)) activated the peroxidase site of PGHS-1 independently of the cyclooxygenase site. After ONOO(-) exposure, holoPGHS-1 could not metabolize arachidonic acid and was structurally compromised, whereas apoPGHS-1 retained full activity once reconstituted with heme. After incubation of holoPGHS-1 with ONOO(-), heme absorbance was diminished but to a lesser extent than the loss in enzymatic function, suggesting the contribution of more than one process to enzyme inactivation. Hydroperoxide scavengers improved enzyme activity, whereas hydroxyl radical scavengers provided no protection from the effects of ONOO(-). Mass spectral analyses revealed that tyrosine 385 (Tyr 385) is a target for nitration by ONOO(-) only when heme is present. Multimer formation was also observed and required heme but could be attenuated by arachidonic acid substrate. We conclude that the heme plays a role in catalyzing Tyr 385 nitration by ONOO(-) and the demise of PGHS-1.

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.

Determination of the structural environment of the tyrosyl radical in prostaglandin H2 synthase-1: a high frequency ENDOR/EPR study

The catalytically active tyrosyl radical which gives rise to the "wide doublet" (WD1) signal in ovine Prostaglandin H2 Synthase-1 has been studied using high frequency (HF) pulsed ENDOR and EPR. A hydrogen-bonded deuteron was directly detected in HFENDOR (130 GHz) spectra of 1H2O/2H2O-exchanged samples. The HFENDOR spectral simulations required a distribution in hydrogen bond distances to achieve proper fits. This range of distances was consistent with that used to model the distribution in gX values detected in pulsed HFEPR spectra. Possible hydrogen-bonding partners, as well as implications regarding the mechanism of self-inactivation for PGHS, are discussed.

Reaction of haem containing proteins and enzymes with hydroperoxides: The radical view

The reaction between hydroperoxides and the haem group of proteins and enzymes is important for the function of many enzymes but has also been implicated in a number of pathological conditions where oxygen binding proteins interact with hydrogen peroxide or other peroxides. The haem group in the oxidized Fe3+ (ferric) state reacts with hydroperoxides with a formation of the Fe4+=O (oxoferryl) haem state and a free radical primarily located on the pi-system of the haem. The radical is then transferred to an amino acid residue of the protein and undergoes further transfer and transformation processes. The free radicals formed in this reaction are reviewed for a number of proteins and enzymes. Their previously published EPR spectra are analysed in a comparative way. The radicals directly detected in most systems are tyrosyl radicals and the peroxyl radicals formed on tryptophan and possibly cysteine. The locations of the radicals in the proteins have been reported as follows: Tyr133 in soybean leghaemoglobin; alphaTyr42, alphaTrp14, betaTrp15, betaCys93, (alphaTyr24-alphaHis20), all in the alpha- and beta-subunits of human haemoglobin; Tyr103, Tyr151 and Trp14 in sperm whale myoglobin; Tyr103, Tyr146 and Trp14 in horse myoglobin; Trp14, Tyr103 and Cys110 in human Mb. The sequence of events leading to radical formation, transformation and transfer, both intra- and intermolecularly, is considered. The free radicals induced by peroxides in the enzymes are reviewed. Those include: lignin peroxidase, cytochrome c peroxidase, cytochrome c oxidase, turnip isoperoxidase 7, bovine catalase, two isoforms of prostaglandin H synthase, Mycobacterium tuberculosis and Synechocystis PCC6803 catalase-peroxidases.

A new method of identifying the site of tyrosyl radicals in proteins

Protein-bound tyrosyl radicals catalyze many important enzymatic reactions. They can also initiate oxidative damage to cells. Here we report a new method of computer simulation of tyrosyl radical electron paramagnetic resonance spectra. The method enables the determination of the rotational conformation of the phenoxyl ring in a radical with unprecedented accuracy (approximately 2 degrees ). When coupled with a new online database, all tyrosine residues in a protein can be screened for that particular conformation. For the first time we show relationships between the spin density on atom C1 (rho(C1)) and the principal g-factors measured by electron paramagnetic resonance spectroscopy (rho(C1) on g(x) is shown to be linear). The new method enables the accurate determination of rho(C1) in all known tyrosyl radicals, evaluates the likelihood of a hydrogen bond, and determines the possibility of a rho(C1) distribution in the radicals. This information, together with the accurately determined rotational conformation, is frequently sufficient to allow for an unambiguous identification of the site of radical formation. The possibility of a similar relationship between rho(C) and g(x) in other radicals, e.g., tryptophanyl, is discussed.

Characterization of the heme environment in Arabidopsis thaliana fatty acid alpha-dioxygenase-1

Plant alpha-dioxygenases (PADOX) are hemoproteins in the myeloperoxidase family. We have used a variety of spectroscopic, mutagenic, and kinetic approaches to characterize the heme environment in Arabidopsis thaliana PADOX-1. Recombinant PADOX-1 purified to homogeneity contained 1 mol of heme bound tightly but noncovalently per protein monomer. Electronic absorbance, electron paramagnetic resonance, and magnetic circular dichroism spectra showed a high spin ferric heme that could be reduced to the ferrous state by dithionite. Cyanide bound relatively weakly in the ferric PADOX-1 heme vicinity (K(d) approximately 10 mm) but did not shift the heme to the low spin state. Cyanide was a very strong inhibitor of the fatty acid oxygenase activity (K(i) approximately 5 microm) and increased the K(m) value for oxygen but not that for fatty acid. Spectroscopic analyses indicated that carbon monoxide, azide, imidazole, and a variety of substituted imidazoles did not bind appreciably in the ferric PADOX-1 heme vicinity. Substitution of His-163 and His-389 with cysteine, glutamine, tyrosine, or methionine resulted in variable degrees of perturbation of the heme absorbance spectrum and oxygenase activity, consistent with His-389 serving as the proximal heme ligand and indicating that the heme has a functional role in catalysis. Overall, A. thaliana PADOX-1 resembles a b-type cytochrome, although with much more restricted access to the distal face of the heme than seen in most other myeloperoxidase family members, explaining the previously puzzling lack of peroxidase activity in the plant protein. PADOX-1 is unusual in that it has a high affinity, inhibitory cyanide-binding site distinct from the distal heme face and the fatty acid 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.

Identification and characterization of tyrosyl radical formation in Mycobacterium tuberculosis catalase-peroxidase (KatG)

The catalytic function of Mycobacterium tuberculosis catalase-peroxidase (KatG) and its role in activation of the anti-tuberculosis antibiotic isoniazid were investigated using rapid freeze-quench electron paramagnetic resonance (RFQ-EPR) experiments. The reaction of KatG with peroxyacetic acid was followed as a function of time using x-band EPR at 77 K. A doublet EPR signal appears within 6.4 ms after mixing and at time points through hundreds of milliseconds. Thereafter, a singlet signal develops and finally predominates after 1 s, with a total yield of radical approximately 0.5 spin/heme. Simulation of the spectra provided EPR parameters consistent with those for tyrosyl radicals. Changes in the hyperfine splitting and/or line width in spectra for l-3,3-[2H2]tyrosine-labeled, but not l-2,4,5,6,7-[2H5]tryptophan-labeled KatG confirmed this assignment. The initial rate of radical formation was unchanged using a 3-fold or 10-fold excess of peroxyacetic acid, consistent with a rate-determining step involving an intermediate. Although Compound I is likely to be the precursor of tyrosyl radical in KatG, neither its EPR signal nor its reduction to Compound II during formation of the radical(s) could be observed. The tyrosyl radical doublet signal was rapidly quenched by addition of isoniazid and benzoic hydrazide, but not by iproniazid, which binds poorly to KatG.

Tyrosine nitration in prostaglandin H(2) synthase

In this study, we investigated the effects of various nitrogen oxide (NO(x)) species on the extent of prostaglandin H(2) synthase-1 (PGHS-1) nitration in purified protein and in vascular smooth muscle cells. We also examined PGHS-1 activity under these conditions and found the degree of nitration to correlate inversely with enzyme activity. In addition, since NO(x) species are thought to invoke damage during the pathogenesis of atherosclerosis, we examined human atheromatous tissue for PGHS-1 nitration. Both peroxynitrite and tetranitromethane induced Tyr nitration of purified PGHS-1, whereas 1-hydroxy-2-oxo-3-(N-methyl-aminopropyl)-3-methyl-1-triazene (NOC-7; a nitric oxide-releasing compound) did not. Smooth muscle cells treated with peroxynitrite showed PGHS-1 nitration. The extent of nitration by specific NO(x) species was determined by electrospray ionization mass spectrometry. Tetranitromethane was more effective than peroxynitrite, NOC-7, and nitrogen dioxide at nitrating a tyrosine-containing peptide (12%, 5%, 1%, and <1% nitration, respectively). Nitrogen dioxide and, to a lesser extent, peroxynitrite, induced dityrosine formation. Using UV/Vis spectroscopy, it was estimated that the reaction of PGHS-1 with excess peroxynitrite yielded two nitrated tyrosines/PGHS-1 subunit. Finally, atherosclerotic tissue obtained from endarterectomy patients was shown to contain nitrated PGHS-1. Thus, prolonged exposure to elevated levels of peroxynitrite may cause oxidative damage through tyrosine nitration.

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.

Electron paramagnetic resonance and electron nuclear double resonance spectroscopic identification and characterization of the tyrosyl radicals in prostaglandin H synthase 1

The tyrosyl radicals generated in reactions of ethyl hydrogen peroxide with both native and indomethacin-pretreated prostaglandin H synthase 1 (PGHS-1) were examined by low-temperature electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopies. In the reaction of peroxide with the native enzyme at 0 degrees C, the tyrosyl radical EPR signal underwent a continuous reduction in line width and lost intensity as the incubation time increased, changing from an initial, 35-G wide doublet to a wide singlet of slightly smaller line width and finally to a 25-G narrow singlet. The 25-G narrow singlet produced by self-inactivation was distinctly broader than the 22-G narrow singlet obtained by indomethacin treatment. Analysis of the narrow singlet EPR spectra of self-inactivated and indomethacin-pretreated enzymes suggests that they reflect conformationally distinct tyrosyl radicals. ENDOR spectroscopy allowed more detailed characterization by providing hyperfine couplings for ring and methylene protons. These results establish that the wide doublet and the 22-G narrow singlet EPR signals arise from tyrosyl radicals with different side-chain conformations. The wide-singlet ENDOR spectrum, however, is best accounted for as a mixture of native wide-doublet and self-inactivated 25-G narrow-singlet species, consistent with an earlier EPR study [DeGray et al. (1992) J. Biol. Chem. 267, 23583-23588]. We conclude that a tyrosyl residue other than the catalytically essential Y385 species is most likely responsible for the indomethacin-inhibited, narrow-singlet spectrum. Thus, this inhibitor may function by redirecting radical formation to a catalytically inactive side chain. Either radical migration or conformational relaxation at Y385 produces the 25-G narrow singlet during self-inactivation. Our ENDOR data also indicate that the catalytically active, wide-doublet species is not hydrogen bonded, which may enhance its reactivity toward the fatty-acid substrate bound nearby.

Rapid kinetics of tyrosyl radical formation and heme redox state changes in prostaglandin H synthase-1 and -2

Hydroperoxide-induced tyrosyl radicals are putative intermediates in cyclooxygenase catalysis by prostaglandin H synthase (PGHS)-1 and -2. Rapid-freeze EPR and stopped-flow were used to characterize tyrosyl radical kinetics in PGHS-1 and -2 reacted with ethyl hydrogen peroxide. In PGHS-1, a wide doublet tyrosyl radical (34-35 G) was formed by 4 ms, followed by transition to a wide singlet (33-34 G); changes in total radical intensity paralleled those of Intermediate II absorbance during both formation and decay phases. In PGHS-2, some wide doublet (30 G) was present at early time points, but transition to wide singlet (29 G) was complete by 50 ms. In contrast to PGHS-1, only the formation kinetics of the PGHS-2 tyrosyl radical matched the Intermediate II absorbance kinetics. Indomethacin-treated PGHS-1 and nimesulide-treated PGHS-2 rapidly formed narrow singlet EPR (25-26 G in PGHS-1; 21 G in PGHS-2), and the same line shapes persisted throughout the reactions. Radical intensity paralleled Intermediate II absorbance throughout the indomethacin-treated PGHS-1 reaction. For nimesulide-treated PGHS-2, radical formed in concert with Intermediate II, but later persisted while Intermediate II relaxed. These results substantiate the kinetic competence of a tyrosyl radical as the catalytic intermediate for both PGHS isoforms and also indicate that the heme redox state becomes uncoupled from the tyrosyl radical in PGHS-2.

Comparison of the peroxidase reaction kinetics of prostaglandin H synthase-1 and -2

Prostaglandin H synthase isoforms 1 and 2 (PGHS-1 and -2) each have a peroxidase activity and also a cyclooxygenase activity that requires initiation by hydroperoxide. The hydroperoxide initiator requirement for PGHS-2 cyclooxygenase is about 10-fold lower than for PGHS-1 cyclooxygenase, and this difference may contribute to the distinct control of cellular prostanoid synthesis by the two isoforms. We compared the kinetics of the initial peroxidase steps in PGHS-1 and -2 to quantify mechanistic differences between the isoforms that might contribute to the difference in cyclooxygenase initiation efficiency. The kinetics of formation of Intermediate I (an Fe(IV) species with a porphyrin free radical) and Intermediate II (an Fe(IV) species with a tyrosyl free radical, thought to be the crucial oxidant in cyclooxygenase catalysis) were monitored at 4 degrees c by stopped flow spectrophotometry with several hydroperoxides as substrate. With 15-hydroperoxyeicosatetraenoic acid, the rate constant for Intermediate I formation (k1) was 2.3 x 10(7) M-1 s-1 for PGHS-1 and 2.5 x 10(7) M-1 s-1 for PGHS-2, indicating that the isoforms have similar initial reactivity with this lipid hydroperoxide. For PGHS-1, the rate of conversion of Intermediate I to Intermediate II (k2) became the limiting factor when the hydroperoxide level was increased, indicating a rate constant of 10(2)-10(3) s-1 for the generation of the active cyclooxygenase species. For PGHS-2, however, the transition between Intermediates I and II was not rate-limiting even at the highest hydroperoxide concentrations tested, indicating that the k2 value for PGHS-2 was much greater than that for PGHS-1. Computer modelling predicted that faster formation of the active cyclooxygenase species (Intermediate II) or increased stability of the active species increases the resistance of the cyclooxygenase to inhibition by the intracellular hydroperoxide scavenger, glutathione peroxidase. Kinetic differences between the PGHS isoforms in forming or stabilizing the active cyclooxygenase species can thus contribute to the difference in the regulation of their cellular activities.

Carbocations in the synthesis of prostaglandins by the cyclooxygenase of PGH synthase? A radical departure!

Evidence already available is used to demonstrate that although prostaglandin G/H synthase hydroxylates arachidonic acid through radical intermediates, it effects cyclizations through a carbocation center at C-10. This is produced following migration of H to the initial radical at C-13 and a 1epsilon oxidation. Under orbital symmetry control, the cyclizations can give only the ring size and trans stereochemistry actually observed. After cyclization, the H-shift reverses to take the sequence back into current radical theory for hydroxylation at C-15. Thus 10,10-difluoroarachidonic acid cannot be cyclized, although it can be hydroxylated. Acetylation of Ser516 in the isoform synthase-2 is considered to oppose carbocation formation and/or H-migration and so prevent cyclizations while permitting hydroxylations; the associated inversion of chirality at C-15 can then readily be accommodated without the change in conformation required by other schemes. Suicide inhibition occurs when carbocations form stable bonds upon (thermal) contact with adjacent heteroatoms, etc. Because the cyclooxygenase and peroxidase functions operate simultaneously through the same heme, phenol acts as reducing cosubstrate for the cyclooxygenase, thus enabling it to promote PGG2 production and protect the enzyme from oxidative destruction.

Expression, purification, and spectroscopic characterization of human thromboxane synthase

Thromboxane A2 (TXA2) is a potent inducer of vasoconstriction and platelet aggregation. Large scale expression of TXA2 synthase (TXAS) is very useful for studies of the reaction mechanism, structural/functional relationships, and drug interactions. We report here a heterologous system for overexpression of human TXAS. The TXAS cDNA was modified by replacing the sequence encoding the first 28 amino acid residues with a CYP17 amino-terminal sequence and by adding a polyhistidine tag sequence prior to the stop codon; the cDNA was inserted into the pCW vector and co-expressed with chaperonins groES and groEL in Escherichia coli. The resulting recombinant protein was purified to electrophoretic homogeneity by affinity, ion exchange, and hydrophobic chromatography. UV-visible absorbance (UV-Vis), magnetic circular dichroism (MCD), and electron paramagnetic resonance (EPR) spectra indicate that TXAS has a typical low spin cytochrome P450 heme with an oxygen-based distal ligand. The UV-Vis and EPR spectra of recombinant TXAS were essentially identical to those of TXAS isolated from human platelets, except that a more homogenous EPR spectrum was observed for the recombinant TXAS. The recombinant protein had a heme:protein molar ratio of 0.7:1 and a specific activity of 12 micromol of TXA2/min/mg of protein at 23 degreesC. Furthermore, it catalyzed formation of TXA2, 12-hydroxy-5,8,10-heptadecatrienoic acid, and malondialdehyde in a molar ratio of 0.94:1.0:0.93. Spectral binding titrations showed that bulky heme ligands such as clotrimazole bound strongly to TXAS (Kd approximately 0.5 microM), indicating ample space at the distal face of the heme iron. Analysis of MCD and EPR spectra showed that TXAS was a typical low spin hemoprotein with a proximal thiolate ligand and had a very hydrophobic distal ligand binding domain.

Structural characterization of arachidonyl radicals formed by prostaglandin H synthase-2 and prostaglandin H synthase-1 reconstituted with mangano protoporphyrin IX

A tyrosyl radical generated in the peroxidase cycle of prostaglandin H synthase-1 (PGHS-1) can serve as the initial oxidant for arachidonic acid (AA) in the cyclooxygenase reaction. Peroxides also induce radical formation in prostaglandin H synthase-2 (PGHS-2) and in PGHS-1 reconstituted with mangano protoporphyrin IX (MnPGHS-1), but the EPR spectra of these radicals are distinct from the initial tyrosyl radical in PGHS-1. We have examined the ability of the radicals in PGHS-2 and MnPGHS-1 to oxidize AA, using single-turnover EPR studies. One wide singlet tyrosyl radical with an overall EPR line width of 29-31 gauss (G) was generated by reaction of PGHS-2 with ethyl hydroperoxide. Anaerobic addition of AA to PGHS-2 immediately after formation of this radical led to its disappearance and emergence of an AA radical (AA.) with a 7-line EPR, substantiated by experiments using octadeuterated AA. Subsequent addition of oxygen resulted in regeneration of the tyrosyl radical. In contrast, the peroxide-generated radical (a 21G narrow singlet) in a Y371F PGHS-2 mutant lacking cyclooxygenase activity failed to react with AA. The peroxide-generated radical in MnPGHS-1 exhibited a line width of 36-38G, but was also able to convert AA to an AA. with an EPR spectrum similar to that found with PGHS-2. These results indicate that the peroxide-generated radicals in PGHS-2 and MnPGHS-1 can each serve as immediate oxidants of AA to form the same carbon-centered fatty acid radical that subsequently reacts with oxygen to form a hydroperoxide. The EPR data for the AA-derived radical formed by PGHS-2 and MnPGHS-1 could be accounted for by a planar pentadienyl radical with two strongly interacting beta-protons at C10 of AA. These results support a functional role for peroxide-generated radicals in cyclooxygenase catalysis by both PGHS isoforms and provide important structural characterization of the carbon-centered AA..

Comparison of peroxidase reaction mechanisms of prostaglandin H synthase-1 containing heme and mangano protoporphyrin IX

Prostaglandin H synthase (PGHS) is a heme protein that catalyzes both the cyclooxygenase and peroxidase reactions needed to produce prostaglandins G2 and H2 from arachidonic acid. Replacement of the heme group by mangano protoporphyrin IX largely preserves the cyclooxygenase activity, but lowers the steady-state peroxidase activity by 25-fold. Thus, mangano protoporphyrin IX serves as a useful tool to evaluate the function of the heme in PGHS. A detailed kinetic analysis of the peroxidase reaction using 15-hydroperoxyeicosatetraenoic acid (15-HPETE), EtOOH, and other peroxides as substrates has been carried out to compare the characteristics of PGHS reconstituted with mangano protoporphyrin IX (Mn-PGHS) to those of the native heme enzyme (Fe-PGHS). The rate constant describing the reaction of Mn-PGHS with 15-HPETE to form the oxidized, Mn(IV) intermediate with absorption at 420 nm, exhibits saturable behavior as the 15-HPETE concentration is raised from 10 to 400 microM. This is most likely due to the presence of a second, earlier intermediate between the resting enzyme and the Mn(IV) species. Measurements at high substrate concentrations permitted resolution of the absorbance spectra of the two oxidized Mn-PGHS intermediates. The spectrum of the initial intermediate, assigned to a Mn(V) species, had a line shape similar to that of the later intermediate, assigned to a Mn(IV) species, suggesting that a porphyrin pi-cation radical is not generated in the peroxidase reaction of Mn-PGHS. The rate constant estimated for the formation of the earlier intermediate with 15-HPETE is 1.0 x 10(6) M-1 s-1 (20 degrees C, pH 7.3). A rate constant of 400 +/- 100 s-1 was estimated for the second step in the reaction. Thus, Mn-PGHS reacts considerably more slowly than Fe-PGHS with 15-HPETE to form the first high-valent intermediate, but the two enzymes appear to follow a similar overall reaction mechanism for generation of oxidized intermediates. The difference in rate constants explains the observed lower steady-state peroxidase activity of Mn-PGHS compared with Fe-PGHS.

Density functional calculations on model tyrosyl radicals

A gradient-corrected density functional theory approach (PWP86) has been applied, together with large basis sets (IGLO-III), to investigate the structure and hyperfine properties of model tyrosyl free radicals. In nature, these radicals are observed in, e.g., the charge transfer pathways in photosystem II (PSII) and in ribonucleotide reductases (RNRs). By comparing spin density distributions and proton hyperfine couplings with experimental data, it is confirmed that the tyrosyl radicals present in the proteins are neutral. It is shown that hydrogen bonding to the phenoxyl oxygen atom, when present, causes a reduction in spin density on O and a corresponding increase on C4. Calculated proton hyperfine coupling constants for the beta-protons show that the alpha-carbon is rotated 75-80 degrees out of the plane of the ring in PSII and Salmonella typhimurium RNR, but only 20-30 degrees in, e.g., Escherichia coli, mouse, herpes simplex, and bacteriophage T4-induced RNRs. Furthermore, based on the present calculations, we have revised the empirical parameters used in the experimental determination of the oxygen spin density in the tyrosyl radical in E. coli RNR and of the ring carbon spin densities, from measured hyperfine coupling constants.

Prostaglandin E2 level in gingival crevicular fluid from patients with Down syndrome

The levels of prostaglandin E2 (PGE2) and interleukin-1 beta (IL-1 beta) were determined in gingival crevicular fluid (GCF) collected from patients with gingivitis: 15 Down syndrome children and 15 controls. The mean level of PGE2 in GCF was significantly higher (P < 0.05) in the Down syndrome group (10.0 pg/microliters GCF) than in the control group (4.6 pg/microliters GCF). In GCF samples collected from sites characterized as noninflamed, the mean level of PGE2 was significantly higher (P < 0.001) in the Down syndrome group than in the controls. The mean level of PGE2 in samples from inflamed sites, on the other hand, did not differ between the two groups. The mean level of IL-1 beta was not significantly higher in the Down syndrome group than in the controls. This study shows that the level of PGE2 detected in GCF from Down syndrome patients is increased, a fact that may be of importance in the pathogenesis of the periodontal disease frequently seen in these patients.

Effect of Trolox C on the oxygenation reaction of prostaglandin endoperoxide synthase with cis,cis-eicosa-11, 14-dienoic acid

Trolox C, a water-soluble derivative of alpha-tocopherol, stimulates the oxygenation of cis,cis-eicosa-11, 14-dienoic acid (AH) by prostaglandin endoperoxide synthase at lower concentrations and suppresses the stimulated reaction at higher concentrations. Surprisingly, Trolox C does not affect the stoichiometric ratio between the rate of formation of the oxygenation product 11-hydroxy-12-trans, 14-cis-eicosadienoic acid (AOH) and the rate of disappearance of molecular oxygen. The ratio of the two rates, d[AOH]/-d[O2], remains constant at 2/1 for a series of Trolox C concentrations and in the absence of Trolox C. Results indicate that AH reacts preferentially with Compound I of the enzyme and that Trolox C does not compete for Compound I. Enzyme inactivation begins with formation of an unproductive Compound I-tyrosyl radical (Compound I-X.) which has the same number of oxidizing equivalents as the conventional peroxidase Compound I. The stimulating effect of low concentrations of Trolox C can be explained by reduction of the oxyferryl heme so that Compound I-X. is reduced to a Compound II-X.species, the Compound II analog of Compound I-X.. Thus heme bleaching is prevented. A further one-electron reduction by Trolox C of Compound II-X. reforms the native enzyme, which permits enzyme recycling. Large concentrations of Trolox C inhibit reformation of native enzyme, reducing the extent of stimulation.

Reactions of prostaglandin endoperoxide synthase and its compound I with hydroperoxides

The reactions of native prostaglandin endoperoxide synthase with structurally different hydroperoxides have been investigated by using kinetic spectrophotometric scan and conventional and sequential mixing stopped-flow experiments. The second order rate constants for compound I formation are (5.9 +/- 0.1) x 10(4) M-1 s-1 using t-butyl hydroperoxide as the oxidant, (2.5 +/- 0.1) x 10(6) M-1 s-1 for ethyl hydroperoxide and (5.1 +/- 0.6) x 10(7) M-1 s-1 for m-chloroperoxybenzoic acid at pH 7.0, 6.7 +/- 0.2 degrees C, and ionic strength 0.1 M. Sequential mixing, transient state experiments show for the first time that all hydroperoxides reduce compound I in a bimolecular reaction. Ethyl hydroperoxide, t-butyl hydroperoxide, and m-chloroperoxybenzoic acid react directly with compound I. The natural substrate prostaglandin G2 forms a transient complex with compound I before the reduction step occurs. Therefore, compound I initially transforms to compound II, not to the compound I-tyrosyl radical. Second order rate constants for the reactions of compound I are (2.9 +/- 0.2) x 10(4) for t-butyl hydroperoxide, (3.5 +/- 0.5) x 10(4) for hydrogen peroxide, (4.2 +/- 0.2) x 10(4) for ethyl hydroperoxide, and (4.2 +/- 0.3) x 10(5) for m-chloroperoxybenzoic acid, all in units of M-1 s-1 and same conditions as for compound I formation. The rate of reaction of prostaglandin G2 with compound I, calculated from the ratio of kcat to Km obtained from the saturation curve, is (1.0 +/- 0.2) x 10(6) M-1 s-1 at 3.0 +/- 0.2 degrees C. Results are discussed in the context of the current state of knowledge of the mechanisms of the cyclooxygenase and peroxidase reactions of prostaglandin endoperoxide synthase.

Comparison of branched-chain and tightly coupled reaction mechanisms for prostaglandin H synthase

Two types of mechanisms have been proposed to account for the combination of peroxidase and cyclooxygenase activities in prostaglandin H synthase (PGHS). One, a branched-chain mechanism [Dietz, R., et al. (1988) Eur. J. Biochem. 171, 321-328], postulates that the cyclooxygenase reaction propagates essentially independently of peroxidase catalysis. The second, a tightly coupled mechanism [Bakovic, M., & Dunford, H. B. (1994) Biochemistry 33, 6475-6482], postulates that peroxidase catalysis is an integral part of cyclooxygenase propagation. Qualitative and quantitative predictions from the two mechanisms have been compared with several observed characteristics of the PGHS reaction with arachidonate, including the ability to accumulate PGG2 and oxidized enzyme intermediates, the stoichiometry between cosubstrate and fatty acid consumption, and the hydroperoxide activator requirement. The observed characteristics, particularly the accumulation of micromolar levels of PGG2 even in the presence of cosubstrate and the stoichiometry between cosubstrate oxidation and fatty acid oxygenation of less than 1.3 (compared to a theoretical maximum of 2.0), were largely consistent with predictions from the branched-chain mechanism, but contradicted important predictions of the tightly coupled mechanism. These results indicate that PGHS catalysis is more accurately described by the branched-chain mechanism than by the tightly coupled mechanism.

Spectroscopic evidence for reaction of prostaglandin H synthase-1 tyrosyl radical with arachidonic acid

The coupling between the peroxidase and cyclooxygenase activities of prostaglandin H synthase (PGHS) has been proposed to be mediated by a critical tyrosyl radical through a branched chain mechanism (Dietz, R., Nastainczyk, W., and Ruf, H. H. (1988) Eur. J. Biochem. 171, 321-328). In this study, we have examined the ability of PGHS isoform-1 (PGHS-1) tyrosyl radicals to react with arachidonate. Anaerobic addition of arachidonate following formation of the peroxide-induced wide doublet or wide singlet tyrosyl radical led to disappearance of the tyrosyl radicals and emergence of a new EPR signal, which is distinct from known PGHS-1 tyrosyl radicals. The new radical was clearly derived from arachidonate because its EPR line shape changed when 5,6,8,9,11,12,14,15-octadeuterated arachidonate was used. Subsequent addition of oxygen to samples containing the fatty acyl radical resulted in regeneration of tyrosyl radical EPR. In contrast, the peroxide-generated tyrosyl radical in indomethacin-treated PGHS-1 (a narrow singlet) failed to react with arachidonate, consistent with the cyclooxygenase inhibition by indomethacin. These results indicate that the peroxide-generated wide doublet and wide singlet tyrosyl radicals serve as immediate oxidants of arachidonate bound at the cyclooxygenase active site to form a carbon-centered fatty acyl radical, which reacts with oxygen to form a hydroperoxide. These observations represent the first direct evidence of chemical coupling between the peroxidase reaction and arachidonate oxygenation in PGHS-1 and support the proposed role for a tyrosyl radical in cyclooxygenase catalysis.