Tyrosyl radicals and their role in hydroperoxide-dependent activation and inactivation of prostaglandin endoperoxide synthase

Kinetic evidence for the transiently shifted acidity constant of histidine linked to paramagnetic tyrosine probed by intramolecular electron transfer in oxidized peptides

The kinetics of electron transfer (ET) from tyrosine (Tyr) to short-lived histidine (His) radicals in peptides of different structures was monitored using time-resolved chemically induced dynamic nuclear polarization (CIDNP) to follow the reduction of the His radicals using NMR detection of the diamagnetic hyperpolarized reaction products. In aqueous solution over a wide pH range, His radicals were generated in situ in the photo-induced reaction with the photosensitizer, 3,3',4,4'-tetracarboxy benzophenone. Model simulations of the CIDNP kinetics provided pH-dependent rate constants of intra- and intermolecular ET, and the pH dependencies of the reaction under study were interpreted in terms of protonation states of the reactants and the product, His with either protonated or neutral imidazole. In some cases, an increase of pKa of imidazole in the presence of the short-lived radical center at a nearby Tyr residue was revealed. Interpretation of the obtained pH dependencies made is possible to quantify the degree of paramagnetic shift of the acidity constant of the imidazole of the His residue in the peptides with a Tyr residue in its paramagnetic state, and to correlate this degree with the intramolecular ET rate constant - a higher intramolecular ET rate constant corresponded to a greater acidity constant shift.

Role of water in cyclooxygenase catalysis and design of anti-inflammatory agents targeting two sites of the enzyme

While designing the anti-inflammatory agents targeting cyclooxygenase-2 (COX-2), we first identified a water loop around the heme playing critical role in the enzyme catalysis. The results of molecular dynamic studies supported by the strong hydrogen-bonding equilibria of the participating atoms, radical stabilization energies, the pK_a of the H-donor/acceptor sites and the cyclooxygenase activity of pertinent muCOX-2 ravelled the working of the water–peptide channel for coordinating the flow of H·/electron between the heme and Y385. Based on the working of H·/electron transfer channel between the 12.5 Å distant radical generation and the radical disposal sites, a series of molecules was designed and synthesized. Among this category of compounds, an appreciably potent anti-inflammatory agent exhibiting IC₅₀ 0.06 μM against COX-2 and reversing the formalin induced analgesia and carageenan induced inflammation in mice by 90% was identified. Further it was revealed that, justifying its bidentate design, the compound targets water loop (heme bound site) and the arachidonic acid binding pockets of COX-2.

Reduction of Transient Histidine Radicals by Tyrosine: Influence of the Protonation State of Reactants

The role of tyrosine radicals as mediators of electron transfer reactions in enzymes is well established, as is the involvement of histidine as a binding partner. But how environmental factors affect these reactions remains poorly explored. In the study presented here, kinetic data on the influence of the protonation state of the reactants on the reduction of transient histidine radicals by tyrosine were obtained in neutral and basic aqueous solution (pH 6-12) using time-resolved chemically induced dynamic nuclear polarization (CIDNP). The histidine radicals were generated in the photo-induced reaction with the photosensitizer 3,3',4,4'-tetracarboxy benzophenone. From model simulations of the detected CIDNP kinetics, pH dependent second-order rate constants of the reduction of histidine radicals were obtained for four possible combinations of the amino acids and their N-acetyl derivatives, and also for the systems histidine-phenylalanine dipeptide/N-acetyl tyrosine, and N-acetyl histidine/tyrosine-glutamine dipeptide. The pH dependences of the rate constant of the reduction reaction are explained accounting for the protonation states of reactants, and also protonation state of the equilibrium form of the product - reduced form of histidine radical, which is histidine with neutral or a positively charged imidazole.

Fluorination as tool to improve bioanalytical sensitivity and COX-2-selective antitumor activity of cobalt alkyne complexes

Fluorination of the lead Co-ASS yielded antitumor active cobalt alkyne complexes that exhibited both improved COX-2 selectivity and better bioanalytical sensitivity.

Protective effect of (±)α-tocopherol on brominated diphenyl ether-47-stimulated prostaglandin pathways in human extravillous trophoblasts in vitro

Brominated diphenyl ether (BDE)-47 is a prevalent flame retardant chemical found in human tissues and is linked to adverse pregnancy outcomes in humans. Because dysregulation of the prostaglandin pathway is implicated in adverse pregnancy outcomes, the present study investigates BDE-47 induction of prostaglandin synthesis in a human extravillous trophoblast cell line, HTR-8/SVneo, examining the hypothesis that BDE-47 increases generation of reactive oxygen species (ROS) to stimulate the prostaglandin response. Treatment with 20 μM BDE-47 significantly increased mRNA expression of prostaglandin-endoperoxide synthase 2 (PTGS2) at 4, 12 and 24 h, and 24-h treatment significantly increased cyclooxygenase (COX)-2 cellular protein expression and prostaglandin E2 (PGE2) concentration in culture medium. The BDE-47-stimulated PGE2 release was inhibited by the COX inhibitors indomethacin and NS398, implicating COX activity. Exposure to 20 μM BDE-47 significantly increased ROS generation as measured by carboxydichlorofluorescein fluorescence, and this response was blocked by cotreatment with the peroxyl radical scavenger (±)-α-tocopherol. (±)-α-Tocopherol cotreatment suppressed BDE-47-stimulated increases of PGE2 release without significant effects on COX-2 mRNA and protein expression, implicating a role for ROS in post-translational regulation of COX activity. Because prostaglandins regulate trophoblast functions necessary for placentation and pregnancy, further investigation is warranted of BDE-47 impacts on trophoblast responses.

Tyrosine oxidation in heme oxygenase: examination of long-range proton-coupled electron transfer

Heme oxygenase is responsible for the degradation of a histidine-ligated ferric protoporphyrin IX (Por) to biliverdin, CO, and the free ferrous ion. Described here are studies of tyrosyl radical formation reactions that occur after oxidizing Fe(III)(Por) to Fe(IV)=O(Por(·+)) in human heme oxygenase isoform-1 (hHO-1) and the structurally homologous protein from Corynebacterium diphtheriae (cdHO). Site-directed mutagenesis on hHO-1 probes the reduction of Fe(IV)=O(Por(·+)) by tyrosine residues within 11 Å of the prosthetic group. In hHO-1, Y58· is implicated as the most likely site of oxidation, based on the pH and pD dependent kinetics. The absence of solvent deuterium isotope effects in basic solutions of hHO-1 and cdHO contrasts with the behavior of these proteins in the acidic solution, suggesting that long-range proton-coupled electron transfer predominates over electron transfer.

Crystallographic, kinetic, and spectroscopic study of the first ligninolytic peroxidase presenting a catalytic tyrosine

Trametes cervina lignin peroxidase (LiP) is a unique enzyme lacking the catalytic tryptophan strictly conserved in all other LiPs and versatile peroxidases (more than 30 sequences available). Recombinant T. cervina LiP and site-directed variants were investigated by crystallographic, kinetic, and spectroscopic techniques. The crystal structure shows three substrate oxidation site candidates involving His-170, Asp-146, and Tyr-181. Steady-state kinetics for oxidation of veratryl alcohol (the typical LiP substrate) by variants at the above three residues reveals a crucial role of Tyr-181 in LiP activity. Moreover, assays with ferrocytochrome c show that its ability to oxidize large molecules (a requisite property for oxidation of the lignin polymer) originates in Tyr-181. This residue is also involved in the oxidation of 1,4-dimethoxybenzene, a reaction initiated by the one-electron abstraction with formation of substrate cation radical, as described for the well known Phanerochaete chrysosporium LiP. Detailed spectroscopic and kinetic investigations, including low temperature EPR, show that the porphyrin radical in the two-electron activated T. cervina LiP is unstable and rapidly receives one electron from Tyr-181, forming a catalytic protein radical, which is identified as an H-bonded neutral tyrosyl radical. The crystal structure reveals a partially exposed location of Tyr-181, compatible with its catalytic role, and several neighbor residues probably contributing to catalysis: (i) by enabling substrate recognition by aromatic interactions; (ii) by acting as proton acceptor/donor from Tyr-181 or H-bonding the radical form; and (iii) by providing the acidic environment that would facilitate oxidation. This is the first structure-function study of the only ligninolytic peroxidase described to date that has a catalytic tyrosine.

Physical evidence for substrate binding in preventing cyclooxygenase inactivation under nitrative stress

Prostaglandin biosynthesis is catalyzed by two spatially and functionally distinct active sites in cyclooxygenase (COX) enzymes. Despite the crucial role of COXs in biology, molecular details regarding the function and regulation of these enzymes are incompletely defined. Reactive nitrogen species, formed during oxidative stress, produce modifications that alter COX functionalities and prostaglandin biosynthesis. We previously established that COX-1 undergoes selective nitration on Tyr385 via a mechanism that requires the presence of bound heme cofactor. As this is a critical residue for COX-1 catalysis, nitration at this site results in enzyme inactivation. We now show that occupancy of the COX-1 active site with substrate protects against Tyr385 nitration and redirects nitration to alternative Tyr residues on COX-1, preserving catalytic activity. This study reveals a novel role for the substrate in protecting COX-1 from inactivation by nitration in pathophysiological settings.

Boron stabilizes peroxide mediated changes in the structure of heme proteins

Boron is reported in this study to stabilize the structure of heme proteins exposed to peroxides. The oxidized heme protein (15 microM) was treated with H(2)O(2) (10mM) in 1M glycine-NaOH buffer (pH 9.2) at 25 degrees C in absence/presence of boron, and characterized by visible absorption spectroscopy, gel exclusion chromatography, native PAGE, HPLC and DLS. Spectral analysis of exposed heme proteins revealed a decrease in absorbance in the Soret region, which was stabilized by boron. The native PAGE analysis of exposed heme proteins showed high molecular weight products; the band intensity was lesser in presence of boron. Further, elution profile of the exposed heme proteins on Sephadex G-200 column and HPLC revealed more than one peak (aggregate formation) when compared to the respective untreated proteins. DLS, which measures the hydrodynamic radius (R(H)), was used to ascertain whether the peaks correspond to monomer, dimer or aggregate forms. The R(H) of boron pretreated heme proteins was close to R(H) of the respective heme protein. Non-heme protein RNase did not show any change when exposed to peroxide. Taken together, results conclude that boron stabilizes the structure of heme proteins, which might be due to specific sites on heme proteins that can bind to borate ions.

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.

Age-associated changes in immune and inflammatory responses: impact of vitamin E intervention

Aging is associated with dysregulated immune and inflammatory responses. Declining T cell function is the most significant and best-characterized feature of immunosenescence. Intrinsic changes within T cells and extrinsic factors contribute to the age-associated decline in T cell function. T cell defect seen in aging involves multiple stages from early receptor activation events to clonal expansion. Among extrinsic factors, increased production of T cell-suppressive factor PGE(2) by macrophages (Mphi) is most recognized. Vitamin E reverses an age-associated defect in T cells, particularly naïve T cells. This effect of vitamin E is also reflected in a reduced rate of upper respiratory tract infection in the elderly and enhanced clearance of influenza infection in a rodent model. The T cell-enhancing effect of vitamin E is accomplished via its direct effect on T cells and indirectly by inhibiting PGE(2) production in Mphi. Up-regulated inflammation with aging has attracted increasing attention as a result of its implications in the pathogenesis of diseases. Increased PGE(2) production in old Mphi is a result of increased cyclooxygenase 2 (COX-2) expression, leading to higher COX enzyme activity, which in turn, is associated with the ceramide-induced up-regulation of NF-kappaB. Similar to Mphi, adipocytes from old mice have a higher expression of COX-2 as well as inflammatory cytokines IL-1beta, IL-6, and TNF-alpha, which might also be related to elevated levels of ceramide and NF-kappaB activation. This review will discuss the above age-related immune and inflammatory changes and the effect of vitamin E as nutritional intervention with a focus on the work conducted in our laboratory.

Role of Tyr residues on the protein surface of cationic cell-wall-peroxidase (CWPO-C) from poplar: potential oxidation sites for oxidative polymerization of lignin

It was previously reported that an unique peroxidase isoenzyme, cationic cell-wall-bound peroxidase (CWPO-C), from poplar callus oxidizes sinapyl alcohol, ferrocytochrome c and synthetic lignin polymers, unlike other plant peroxidases. Here, the catalytic mechanism of CWPO-C was investigated using chemical modification and homology modeling. The simulated CWPO-C structure predicts that the entrance to the heme pocket of CWPO-C is the same size as those of other plant peroxidases, suggesting that ferrocytochrome c and synthetic lignin polymers cannot interact with the heme of CWPO-C. Since Trp and Tyr residues are redox-active, such residues located on the protein surface were predicted to be active sites for CWPO-C. Modification of CWPO-C Trp residues did not suppress its oxidation activities toward guaiacol and syringaldazine. On the other hand, modification of CWPO-C Tyr residues using tetranitromethane strongly suppressed its oxidation activities toward syringaldazine and 2,6-dimethoxyphenol by 90%, respectively, and also suppressed its guaiacol oxidation activity to a lesser extent. Ferrocytochrome c was not oxidized by Tyr-modified CWPO-C. These results indicate that the Tyr residues in CWPO-C mediate its oxidation of syringyl compounds and high-molecular-weight substrates. Homology modeling indicates that Tyr-177 and Tyr-74 are located near the heme and exposed on the protein surface of CWPO-C. These results suggest that Tyr residues on the protein surface are considered to be important for the oxidation activities of CWPO-C with a wide range of substrates, and potentially unique oxidation sites for the plant peroxidase family.

Oxidation of bovine serum albumin initiated by the Fenton reaction--effect of EDTA, tert-butylhydroperoxide and tetrahydrofuran

Oxidation of bovine serum albumin (BSA) was investigated using different oxidants: The water-soluble azo-initiator 2,2'azo-bis-(2-amidinopropane) hydrochloride (AAPH), a combination of FeCl(3) and ascorbate or the Fenton oxidant consisting of FeCl(2), H(2)O(2) and EDTA. In addition, the effects of exogenous compounds such as tert-butyl hydroperoxide (tBuOOH) or solvents such as tetrahydrofuran (THF), often used in model systems, was evaluated. The extent of protein damage was studied by measuring protein carbonyl groups and protein hydroperoxides. The interaction between Fenton oxidant and EDTA, THF or tBuOOH was further characterized using spin trapping electron spin resonance (ESR) spectroscopy. The results showed that the extent of protein oxidation depended on the oxidant used. The Fenton oxidant was the most reactive of the initiators tested. However, in the absence of EDTA, the Fenton system produced protein carbonyl groups on BSA equivalent to that obtained with the other oxidants, however, significantly more protein hydroperoxide was produced. Surprisingly, it was also found that addition of tBuOOH or THF to BSA reduced protein damage when the oxidation was initiated with the Fenton oxidant. ESR investigation showed that EDTA played a key role in the generation of free radicals. It was also revealed that in an EDTA containing system both tBuOOH and THF were able to react with radicals without inducing protein damage in effect protecting BSA from oxidative damage.

A Mechanistic and Kinetic Study of the Formation of Metal Nanoparticles by Using Synthetic Tyrosine-Based Oligopeptides

Synthetic oligopeptides containing redox-active tyrosine residues have been employed to prepare gold and silver nanoparticles. In this reduction process an electron from the tyrosinate ion of the peptide is transferred to the metal ion at basic pH through the formation of a tyrosyl radical, which is eventually converted to its dityrosine form during the reaction. This reaction mechanism was confirmed from UV-visible, fluorescence, and EPR spectroscopy and was found to be pH-dependent. Transmission electron microscopy measurement shows that the average size and the monodispersity of gold nanoparticles increase as the number of tyrosine residues in the peptide increases. The kinetic study, based on spectrophotometric measurements of the surface plasmon resonance optical property, shows that the rate of formation of gold nanoparticles was much faster at higher pH than at lower pH and was also dependent on the number of tyrosine residues present in the peptide. The dityrosine form of the peptide was found to retain reducing properties like those of tyrosine in basic medium.

Structural Character and Energetics of Tyrosyl Radical Formation by Electron/Proton Transfers of a Covalently Linked Histidine-Tyrosine: A Model for Cytochrome c Oxidase

The structural, energetic, and electronic and IR spectroscopic properties for a model of the cross-linked histidine-tyrosine (His-Tyr) residues as found in cytochrome c oxidase (CcO) are investigated by ab initio methods. The formation of a His-Tyr radical is studied by two paths: proton release followed by electron release and vice versa. The energetics for the proton/electron releases of the Tyr depend modestly on the cross-linked His substituent and, more sensitively, on the charge of the cation attached to the imino N site of the His residue. Protonation of the imino N site significantly increases the electron ionization potential and decreases the proton dissociation energy, making them competitive processes. A positive charge placed at the imino N site, whose value is scanned from zero to one, shows a continuous increase in ionization potential and a decrease in proton dissociation energy, with the +1 limit agreeing well with the protonated imino N site result, indicating a dominant electrostatic effect. The charge populations and the spin density distributions of the His-Tyr model, the radical cation formed by electron ionization, the anion formed by proton dissociation, and the final His-Tyr radical depend sensitively on the substituents, implying a modulation role on the charge transfer between the phenol and imidazole rings, especially for the charged species. His-Tyr and protonated His-Tyr exhibit differences among their respective structural isomers with consequences on their IR absorptions. Small barriers between their pseudo-cis and pseudo-trans rotamers demonstrate the relative flexibility between the two rings, and these may facilitate proton release and charge transfer. The cation effect demonstrates that the cationized cross-linked His-Tyr should be the best candidate to mimic the covalently ring-linked histidine-tyrosine structure in CcO.

Vitamin E and immune response in the aged: molecular mechanisms and clinical implications

Nutritional status has been indicated as a contributing factor to age-related dysregulation of the immune response. Vitamin E, a lipid-soluble antioxidant vitamin, is important for normal function of the immune cells. The elderly are at a greater risk for vitamin E intake that is lower than recommended levels. Vitamin E supplementation above currently recommended levels has been shown to improve immune functions in the aged including delayed-type hypersensitivity skin response and antibody production in response to vaccination, which was shown to be mediated through increased production of interleukin (IL)-2, leading to enhanced proliferation of T cells, and through reduced production of prostaglandin E(2), a T-cell suppressive factor, as a result of a decreased peroxynitrite formation. Vitamin E increased both cell-dividing and IL-producing capacities of naive T cells, but not memory T cells. The vitamin E-induced enhancement of immune functions in the aged was associated with significant improvement in resistance to influenza infection in aged mice and a reduced risk of acquiring upper respiratory infections in nursing home residents. Further studies are needed to determine the signaling mechanisms involved in the upregulation of naive T-cell function by vitamin E as well as the specific mechanisms involved in reduction of risk for upper respiratory infections.

Cyclooxygenase-mediated generation of free radicals during hypoxia in the cerebral cortex of newborn piglets

Previous studies have demonstrated that free radicals are formed under hypoxic conditions in newborn piglet brain. To test the hypothesis that the cyclooxygenase pathway serves as a source of free radical generation during hypoxia studies were performed on 24 piglets divided into four groups. Six saline (group 3) and six indomethacin treated (group 4) were exposed to hypoxia (FiO2 0.05-0.07) for 60 min. Cerebral hypoxia was documented biochemically by determination of ATP and phosphocreatine. Fluorescent compounds and conjugated dienes were determined as indices of lipid peroxidation. Free radical formation was determined by using n-tert butyl phenyl nitrone (PBN) as a spin trap agent and measuring spin adduct formation in duplicate using a Varian E-109 spectrometer. Groups 1 and 2 (normoxic) showed no spin adduct formation. Group 3 showed a significant increase in spin adduct formation compared to normoxia (372+/-125 vs. 63+/-15, P<0.001). Hypoxic animals pretreated with indomethacin had a spin adduct level of 197+/-132 and were similar to normoxic animals. ATP/PCr levels were the same in groups 3 and 4 denoting the same degree of cerebral hypoxia in all hypoxic animals. Conjugated dienes increased significantly during hypoxia as compared to normoxia (0.142+/-0.017 vs. 0.0+/-0.0) and were decreased insignificantly with indomethacin treatment. Fluorescent compounds were not significantly different among the four groups. Na+,K+-ATPase activity decreased during hypoxia but was not preserved in hypoxic animals pretreated with indomethacin. These data provide direct evidence of the presence of free radicals during hypoxia and the contribution of cyclooxygenase metabolism to their formation.

The effect of Ginkgo biloba in isolated ischemic/reperfused rat heart: a link between vitamin E preservation and prostaglandin biosynthesis

The effect of Ginkgo biloba extract (EGb 761) was studied in rat hearts submitted to ischemia/reperfusion. Isolated hearts perfused in Langendorff mode were subjected to 60 minutes of global ischemia and 15 minutes of reperfusion. EGb 761 was administered by chronic or acute treatment: intra-peritoneal injections of 5 mg/Kg extract for 5 days, or 100 mg /L extract addition to the perfusion buffer, respectively. In hearts not treated with EGb 761, ischemia induced a 20% decrease in the concentration of membrane alpha-tocopherol. This effect was not worsened by reperfusion. alpha-tocopherol consumption was accompanied by about 650% increase in 6-ketoPGF1alpha release within 3 minutes of reperfusion. Moreover, ischemia induced activation of transcription factor NF-kappaB, as compared with the untreated group. In both chronic and acute treatment with EGb 761, heart concentration of alpha-tocopherol was completely spared during ischemia as much as after reperfusion, and a significant decrease of 6-ketoPGF1alpha release was observed at 3 minutes of reperfusion. Nuclear translocation of NF-kappaB was lowered during ischemia. EGb 761 might act as direct free radical scavenger or as tocopheryl radical recycler; in both cases sparing membrane vitamin E should affect phospholipase A2 activity. Finally, EGb 761, by lowering ROS produced during ischemia, challenges nuclear translocation of NF-kappaB.

Identification of Tyr504 as an alternative tyrosyl radical site in human prostaglandin H synthase-2

Hydroperoxides induce formation of a tyrosyl radical on Tyr385 in prostaglandin H synthase (PGHS). The Tyr385 radical initiates hydrogen abstraction from arachidonic acid, thereby mechanistically connecting the peroxidase and cyclooxygenase activities. In both PGHS isoforms the tyrosyl radical undergoes a time-dependent transition from a wide doublet to a wide singlet species; pretreatment with cyclooxygenase inhibitors results in a third type of signal, a narrow singlet [Tsai, A.-L.; Kulmacz, R. J. (2000) Prost. Lipid Med. 62, 231-254]. These transitions have been interpreted as resulting from Tyr385 ring rotation, but could also be due to radical migration from Tyr385 to another tyrosine residue. PATHWAYS analysis of PGHS crystal structures identified four tyrosine residues with favorable predicted electronic coupling: residues 148, 348, 404, and 504 (ovine PGHS-1 numbering). We expressed recombinant PGHS-2 proteins containing single Tyr --> Phe mutations at the target residues, a quadruple mutant with all four tyrosines mutated, and a quintuple mutant, which also contains a Y385F mutation. All mutants bind heme and display appreciable peroxidase activity, and with the exception of the quintuple mutant, all retain cyclooxygenase activity, indicating that neither of the active sites is significantly perturbed. Reaction of the Y148F, Y348F, and Y404F mutants with EtOOH generates a wide singlet EPR signal similar to that of native PGHS-2. However, reaction of the Y504F and the quadruple mutants with peroxide yields persistent wide doublets, and the quintuple mutant is EPR silent. Nimesulide pretreatment of Y504F and the quadruple mutant results in an abnormally small amount of wide doublet signal, with no narrow singlet being formed. Therefore, the formation of an alternative tyrosine radical on Tyr504 probably accounts for the transition from a wide doublet to a wide singlet in native PGHS-2 and for formation of a narrow singlet in complexes of PGHS-2 with cyclooxygenase inhibitors.

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.

The structure of mammalian cyclooxygenases

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

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

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

Structural characterization of arachidonyl radicals formed by aspirin-treated prostaglandin H synthase-2

Peroxide-generated tyrosyl radicals in both prostaglandin H synthase (PGHS) isozymes have been demonstrated to couple the peroxidase and cyclooxygenase activities by serving as the immediate oxidant for arachidonic acid (AA) in cyclooxygenase catalysis. Acetylation of Ser-530 of PGHS-1 by aspirin abolishes all oxygenase activity and transforms the peroxide-induced tyrosyl radical from a functional 33-35-gauss (G) wide doublet/wide singlet to a 26-G narrow singlet unable to oxidize AA. In contrast, aspirin-treated PGHS-2 (ASA-PGHS-2) no longer forms prostaglandins but retains oxygenase activity forming 11(R)- and 15(R)-hydroperoxyeicosatetraenoic acid and also retains the EPR line-shape of the native peroxide-induced 29-30-G wide singlet radical. To evaluate the functional role of the wide singlet radical in ASA-PGHS-2, we have examined the ability of this radical to oxidize AA in single-turnover EPR studies. Anaerobic addition of AA to ASA-PGHS-2 immediately after formation of the wide singlet radical generated either a 7-line EPR signal similar to the pentadienyl AA radical obtained in native PGHS-2 or a 26-28-G singlet radical. These EPR signals could be accounted for by a pentadienyl radical and a strained allyl radical, respectively. Experiments using 11d-AA, 13(R)d-AA, 15d-AA, 13,15d(2)-AA, and octadeuterated AA (d(8)-AA) confirmed that the unpaired electron in the pentadienyl radical is delocalized over C11, C13, and C15. A 6-line EPR radical was observed when 16d(2)-AA was used, indicating only one strongly interacting C16 hydrogen. These results support a functional role for peroxide-generated tyrosyl radicals in lipoxygenase catalysis by ASA-PGHS-2 and also indicate that the AA radical in ASA-PGHS-2 is more constrained than the corresponding radical in native PGHS-2.

Mechanism of vitamin E inhibition of cyclooxygenase activity in macrophages from old mice: role of peroxynitrite

Vitamin E inhibits cyclooxygenase activity in macrophages from old mice by reducing peroxynitrite production. PGE(2) is a proinflammatory mediator that has been linked to a variety of age-associated diseases such as cancer, arthritis, and cardiovascular disease. Furthermore in the aged, increased cyclooxygenase (COX)-2-mediated PGE(2) production contributes to decline in T-cell-mediated function. Previously we reported that increased macrophage PGE(2) production in the aged is due to higher COX-2 activity and that supplementation with vitamin E significantly reduced the age-associated increase in macrophage PGE(2) production posttranslationally without changing COX-2 expression. Peroxynitrite, a product of nitric oxide (NO) and superoxide (O(-)(2)), increases the activity of COX without affecting its expression. Thus, we investigated if vitamin E inhibits COX activity through decreasing peroxynitrite formation. Macrophages from old mice had higher PGE(2) levels, COX activity, and NO levels than those from young mice, all of which were significantly reduced by vitamin E. When added individually, inhibitors of NO and O(-)(2) did not significantly reduce COX activity; however, when the inhibitors were combined, COX activity was significantly reduced in macrophages from old mice fed 30 ppm vitamin E. Increasing NO levels alone using SNAP or O(-)(2) levels, using X/XO, had no effect; however, increasing peroxynitrite levels using Sin-1 or X/XO + SNAP significantly increased COX activity in macrophages from old mice fed 500, but not those fed 30 ppm vitamin E. These data strongly suggest that peroxynitrite plays an important role in the vitamin E-induced inhibition of COX activity. These findings have important implications for designing interventions to reverse and/or delay age-associated dysregulation of immune and inflammatory responses and diseases associated with them.

The importance of proteins in defense against oxidation

Free radicals are a normal feature of cellular oxygen metabolism. However, free radical-associated damage is an important factor in many pathological and toxicological processes. For a long time, lipid peroxidation, mediated by oxygen-derived free radicals, was probably the most extensively investigated process. From more recent studies, it has become evident that proteins are also the targets of free radicals, and this has important implication for their activity, unfolding, and degradation, as well as in cell functioning. After giving a brief overview of the key role of proteins in the overall antioxidant defense, this review examines their role as targets of oxidation reactions, taking into account the reactivity of amino acid residues and some of their oxidation products. In light of recent data, we then consider the specific role of sulfur-containing amino acids in protein degradation and their possible interplay with the reversal of limited oxidative lesions. The participation of proteins in the overall antioxidant defense is also discussed, specifically the role of metallothionein as an intracellular antioxidant and that of albumin as a circulating antioxidant.

Biotransformation of xenobiotics by amine oxidases

Although the cytochrome P450 (CYP) system ranks first in terms of catalytic versatility and the wide range of xenobiotics it detoxifies or activates to reactive intermediates, the contribution of amine oxidases and in particular of monoamine oxidases (MAOs) to the metabolism of xenobiotics is far from negligible but has been largely neglected. In this review on the involvement of amine oxidases in the metabolism of xenobiotics, the major characteristics reported for the CYP system (protein, reaction, tissue distribution, subcellular localisation, substrates, inhibitors, inducers, genetic polymorphism, impact of different physiopathological conditions on the activity, turnover) will be compared, whenever possible, with the corresponding characteristics of amine oxidases (MAOs in particular). The knowledge of the involvement of MAO-A, -B or both in the metabolism of a drug allows us to predict interactions with selective or non-selective MAO inhibitors (e.g. the metabolism of a drug deaminated by both forms of MAO is not necessarily inhibited in vivo by a selective MAO-A or -B inhibitor). If a drug is metabolized by MAOs, competitive interactions can occur with other drugs that are MAO substrates, e.g. with beta-adrenoceptor agonists and antagonists, prodrugs of dopamine, serotonin 5-HT1-receptor agonists as well as with primaquine, flurazepam and citalopram. Moreover, the knowledge of the involvement of MAOs in the metabolism of a drug may suggest possible, although not obligatory, interactions with tyramine-containing food or drink, with over the counter medicines sold to relieve the symptoms of coughs and colds (generally containing the indirectly-acting sympathomimetic amine phenylpropanolamine) or with phenylephrine-containing preparations. Finally, biotransformation by amine oxidases, as by CYP, does not always lead to detoxication but can produce toxic compounds.

Vitamin E and macrophage cyclooxygenase regulation in the aged

Aging is associated with increased evidence of cardiovascular disease (CVD). Atherosclerosis, a major cause of CVD, is an inflammatory process whose development is influenced by several proinflammatory mediators. Products of arachidonic acid metabolism, in particular, prostaglandin (PG) E(2) and thromboxane (TX) A(2), play an important role in the development of atherosclerosis. We showed previously that the aged have higher PGE(2) production compared with their young counterparts. This age-associated increase in PGE(2) production is mainly a consequence of increased cyclooxygenase (COX) activity. We demonstrated further that increased COX activity in old mice is due to the increased expression of mRNA and protein for the inducible form of COX, COX-2. Vitamin E has been shown to reduce PGE(2) production and risk of CVD. In aged mice, we showed that a vitamin E-induced decrease in PGE(2) production is due to decreased COX activity. However, vitamin E had no effect on COX mRNA and protein levels, indicating a post-translational regulation of COX by vitamin E. Further experiments indicated that vitamin E decreases COX activity through reducing formation of peroxynitrite, a hydroperoxide shown to be involved in the activation of COX-2. Other homologues of tocopherols were also effective in inhibiting COX activity, but their degree of inhibition varied. The varied potency to inhibit COX activity was not explained totally by differences in their antioxidant capacity. Vitamin E-induced inhibition of COX activity might contribute to its effect of reducing CVD risk.

Peroxidases

The family of human peroxidases described includes myeloperoxidase, eosinophil peroxidase, uterine peroxidase, lactoperoxidase, salivary peroxidase, thyroid peroxidase and prostaglandin H1/2 synthases. The chemical identity of the peroxidase compound I and II oxidation states for the different peroxidases are compared. The identities of the distal and proximal amino acids of the catalytic site of each peroxidase are also compared. The gene characteristics and chromosomal location of the human peroxidase family have been tabulated and their molecular evolution discussed. Myeloperoxidase polymorphism and the mutations identified so far that affect myeloperoxidase activity and modulate their susceptibility to disease is described. The mechanisms for hypohalous and hypothiocyanate formation by the various peroxidases have been compared. The cellular function of the peroxidases and their hypohalites have been described as well as their inflammatory effects. The peroxidase catalysed cooxidation of drugs and xenobiotics that results in oxygen activation by redox cycling has been included. Low-density lipoprotein oxidation (initiation of atherosclerosis), chemical carcinogenesis, idiosyncratic drug reactions (e.g. agranulocytosis), liver necrosis or teratogenicity initiated by the cooxidation of endogenous substrates, plasma amino acids, drugs and xenobiotics catalysed by peroxidases or peroxidase containing cells have also been compared. Finally, peroxidase inhibitors currently in use for treating various diseases are described.

Cyclooxygenase mechanisms

Several advances have occurred in the past year in our understanding of cyclooxygenase catalysis. The role of specific heme oxidation states in the formation of catalytically competent tyrosyl radicals has been defined; the identity of physiological hydroperoxide activators has been established; and the participation of individual amino acids in substrate binding and oxygenation has been elucidated.

The formation of stable fatty acid substrate complexes in prostaglandin H(2) synthase-1

We have developed a protocol to purify apo-ovine (o) prostaglandin endoperoxide H(2) synthase-1 (PGHS-1) to homogeneity from ram seminal vesicles. The resulting apo enzyme can then be reconstituted with Co(3+)-protoporphyrin IX instead of Fe(3+)-protoporphyrin IX to produce a native-like, but functionally inert, enzyme suitable for the production of enzyme:fatty acid substrate complexes for biophysical characterization. Co(3+)-protoporphyrin IX reconstituted oPGHS-1 (Co(3+)-oPGHS-1) displays a Soret band at 426 nm that shifts to 406 nm upon reduction. This behavior is similar to that of cobalt-reconstituted horseradish peroxidase and myoglobin and suggests, along with resonance Raman spectroscopy, that the Co(3+)-protoporphyrin IX group is one in a six-coordinate, cobalt(III) state. However, Co(3+)-oPGHS-1 does not display cyclooxygenase or peroxidase activity, nor does the enzyme produce prostaglandin products when incubated with [1-(14)C]arachidonic acid. The cocrystallization of Co(3+)-oPGHS-1 and the substrate arachidonic acid (AA) has been achieved using sodium citrate as the precipitant in the presence of the nonionic detergent N-octyl-beta-d-glucopyranoside. Crystals are hexagonal, belonging to the space group P6(5)22, with cell dimensions of a = b = 181.69 A and c = 103.74 A, and a monomer in the asymmetric unit. GC-MS analysis of dissolved crystals indicates that unoxidized AA is bound within the crystals.

Dietary vitamin E and T cell-mediated function in the elderly: effectiveness and mechanism of action

One of the most dramatic and consequence-bearing age-related phenomena is the decline of the immune function with old age. Age-related T cell-mediated immunity dysfunction has been implicated in the etiology of many of the chronic degenerative diseases of the elderly, including arthritis, cancer, autoimmune diseases and increased susceptibility to infectious diseases. T cells from aged individuals are impaired in their response to mitogens and in their cytokine production. In recent years, several studies have emphasized the importance of intracellular anti-oxidant levels for preserving the immune function. Recent progress in understanding the mechanisms of action of anti-oxidants on cellular metabolism, have shown that anti-oxidants may modulate signal transduction and gene expression in immune cells. Vitamin E is widely recognized as a major lipid-soluble chain-breaking anti-oxidant in the biological membrane, where it scavenges free radicals, inhibiting the initiation and chain propagation of lipid peroxidation and protecting cellular structures against oxidative stress damage. Experimental studies have provided evidences for a role of vitamin E in protecting the immune system of elderly subjects. This article reviews the studies concerning the effect of both vitamin E deficiency and supplementation on T cell-mediated immune function in aging. Following a chronological pathway, the present article will also discuss the knowledge regarding the underlying mechanism of action of vitamin E.

Structural insights into the stereochemistry of the cyclooxygenase reaction

Cyclooxygenases are bifunctional enzymes that catalyse the first committed step in the synthesis of prostaglandins, thromboxanes and other eicosanoids. The two known cyclooxygenases isoforms share a high degree of amino-acid sequence similarity, structural topology and an identical catalytic mechanism. Cyclooxygenase enzymes catalyse two sequential reactions in spatially distinct, but mechanistically coupled active sites. The initial cyclooxygenase reaction converts arachidonic acid (which is achiral) to prostaglandin G2 (which has five chiral centres). The subsequent peroxidase reaction reduces prostaglandin G2 to prostaglandin H2. Here we report the co-crystal structures of murine apo-cyclooxygenase-2 in complex with arachidonic acid and prostaglandin. These structures suggest the molecular basis for the stereospecificity of prostaglandin G2 synthesis.

Lack of cyclooxygenase-2 activity in HT-29 human colorectal carcinoma cells

Induction of cyclooxygenase-2 (COX-2) is an early event in the sequence of polyp formation to colon carcinogenesis. COX-2 is at elevated levels in human colorectal cancers and in tumors and polyps of mouse models of colorectal cancer. Mutation of the adenomatous polyposis coli (APC) gene is the initial event leading to colorectal cancer. Colorectal cells in culture which express mutant APC are often used to examine the association of COX-2 expression and apoptosis. The expression of full-length APC in HT-29 cells, a human colorectal carcinoma cell line which normally expresses truncated APC and highly expresses COX-2, inhibits cell growth through increased apoptosis and results in a down-regulation of COX-2 protein. In this report, we examine whether down-regulation of COX-2 is directly linked to the increase in apoptosis observed in these HT-29-APC cells. We present evidence that COX-2 and apoptosis are not linked since COX-2, although expressed, is catalytically inactive. Interestingly, the COX-2 cloned from HT-29 cells is catalytically active when transfected into HCT-116 cells, a colorectal cell line which normally does not express COX-2, but is not active in the HT-29 cell line itself.

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.

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.

An electron spin resonance spin-trapping investigation of the free radicals formed by the reaction of mitochondrial cytochrome c oxidase with H2O2

The reaction of purified bovine mitochondrial cytochrome c oxidase (CcO) and hydrogen peroxide was studied using the ESR spin-trapping technique. A protein-centered radical adduct was trapped by 5, 5-dimethyl-1-pyrroline N-oxide and was assigned to a thiyl radical adduct based on its hyperfine coupling constants of aN = 14.7 G and abetaH = 15.7 G. The ESR spectra obtained using the nitroso spin traps 3,5-dibromo-4-nitrosobenzenesulfonic acid (DBNBS) and 2-methyl-2-nitrosopropane (MNP) indicated that both DBNBS/.CcO and MNP/.CcO radical adducts are immobilized nitroxides formed by the trapping of protein-derived radicals. Alkylation of the free thiols on the enzyme with N-ethylmaleimide (NEM) prevented 5, 5-dimethyl-1-pyrroline N-oxide adduct formation and changed the spectra of the MNP and DBNBS radical adducts. Nonspecific protease treatment of MNP-d9/.NEM-CcO converted its spectrum from that of an immobilized nitroxide to an isotropic three-line spectrum characteristic of rapid molecular motion. Super-hyperfine couplings were detected in this spectrum and assigned to the MNP/.tyrosyl adduct(s). The inhibition of either CcO or NEM-CcO with potassium cyanide prevented detectable MNP adduct formation, indicating heme involvement in the reaction. The results indicate that one or more cysteine residues are the preferred reductant of the presumed ferryl porphyrin cation radical residue intermediate. When the cysteine residues are blocked with NEM, one or more tyrosine residues become the preferred reductant, forming the tyrosyl radical.