Leukocytic oxygen activation and microbicidal oxidative toxins

Molecular chlorine generated by the myeloperoxidase-hydrogen peroxide-chloride system of phagocytes produces 5-chlorocytosine in bacterial RNA

Myeloperoxidase, a heme enzyme secreted by activated phagocytes, uses H(2)O(2) and Cl(-) to generate the chlorinating intermediate hypochlorous acid (HOCl). This potent cytotoxic oxidant plays a critical role in host defenses against invading pathogens. In this study, we explore the possibility that myeloperoxidase-derived HOCl might oxidize nucleic acids. When we exposed 2'-deoxycytidine to the myeloperoxidase-H(2)O(2)-Cl(-) system, we obtained a single major product that was identified as 5-chloro-2'-deoxycytidine using mass spectrometry, high performance liquid chromatography, UV-visible spectroscopy, and NMR spectroscopy. 5-Chloro-2'-deoxycytidine production by myeloperoxidase required H(2)O(2) and Cl(-), suggesting that HOCl is an intermediate in the reaction. However, reagent HOCl failed to generate 5-chloro-2'-deoxycytidine in the absence of Cl(-). Moreover, chlorination of 2'-deoxycytidine was optimal under acidic conditions in the presence of Cl(-). These results implicate molecular chlorine (Cl(2)), which is in equilibrium with HOCl through a reaction requiring Cl(-) and H(+), in the generation of 5-chloro-2'-deoxycytidine. Activated human neutrophils were able to generate 5-chloro-2'-deoxycytidine. Cellular chlorination was blocked by catalase and heme poisons, consistent with a myeloperoxidase-catalyzed reaction. The myeloperoxidase-H(2)O(2)-Cl(-) system generated similar levels of 5-chlorocytosine in RNA and DNA in vitro. In striking contrast, only cell-associated RNA acquired detectable levels of 5-chlorocytosine when intact Escherichia coli was exposed to the myeloperoxidase system. This observation suggests that oxidizing intermediates generated by myeloperoxidase selectively target intracellular RNA for chlorination. Collectively, these results indicate that Cl(2) derived from HOCl generates 5-chloro-2'-deoxycytidine during the myeloperoxidase-catalyzed oxidation of 2'-deoxycytidine. Phagocytic generation of Cl(2) therefore may constitute one mechanism for oxidizing nucleic acids at sites of inflammation.

Kinetics of oxidation of serotonin by myeloperoxidase compounds I and II

The oxidation of serotonin (5-hydroxytryptamine) by the myeloperoxidase intermediates compounds I and II was investigated by using transient-state spectral and kinetic measurements at 25.0 ± 0.1°C. Rapid scan spectra demonstrated that both compound I and compound II oxidize serotonin via one-electron processes. Rate constants for these reactions were determined using both sequential-mixing and single-mixing stopped-flow techniques. The second order rate constant obtained for the one-electron reduction of compound I to compound II by serotonin is (1.7 ± 0.1) × 107 M-1·s-1, and that for compound II reduction to native enzyme is (1.4 ± 0.1) × 106 M-1·s-1 at pH 7.0. The maximum pH of the compound I reaction with serotonin occurs in the pH range 7.0-7.5. At neutral pH, the rate constant for myeloperoxidase compound I reacting with serotonin is an order of magnitude larger than for its reaction with chloride, (2.2 ± 0.2) × 106 M-1·s-1. A direct competition of serotonin with chloride for myeloperoxidase compound I oxidation was observed. Our results suggest that serotonin may have a role to protect lipoproteins from oxidation and to prevent enzymes from inactivation caused by the potent oxidants HOCl and active oxygen species.Key words: serotonin oxidation, myeloperoxidase, chloride, competition of serotonin, blood platelets, neutrophils.

Oxidation of clozapine and ascorbate by myeloperoxidase

The kinetics and spectra of the reactions of clozapine with compounds I and II of myeloperoxidase were investigated using both single- and sequential-mixing stopped-flow techniques, steady-state kinetics, and spectrophotometric measurements. The results show conclusively that both compounds I and II are reduced in one-electron reactions with clozapine. At pH 7.0 the rate constant for compound I reacting with clozapine is (1.5 +/- 0.1) x 10(6) M(-1) s(-1) and for compound II (4.8 +/- 0.1) x 10(4) M(-1) s(-1). The physiological pH of 7.4 was found to be optimal for the oxidation of clozapine by compound I. The rate constant for compound I reacting with ascorbate is (1.1 +/- 0.1) x 10(6) M(-1) s(-1) and for compound II (1.1 +/- 0.2) x 10(4) M(-1) s(-1), both obtained at pH 7.0. Experiments with both clozapine and ascorbate present showed that ascorbate acts both as a competitive inhibitor and free radical scavenger.

Mass spectrometric quantification of amino acid oxidation products in proteins: insights into pathways that promote LDL oxidation in the human artery wall

Oxidatively damaged low density lipoprotein (LDL) may play an important role in atherogenesis, but the physiologically relevant pathways have proved difficult to identify. Mass spectrometric quantification of stable compounds that result from specific oxidation reactions represents a powerful approach for investigating such mechanisms. Analysis of protein oxidation products isolated from atherosclerotic lesions implicates tyrosyl radical, reactive nitrogen species, and hypochlorous acid in LDL oxidation in the human artery wall. These observations provide chemical evidence for the reaction pathways that promote LDL oxidation and lesion formation in vivo.--Heinecke, J. W. Mass spectrometric quantification of amino acid oxidation products in proteins: insights into pathways that promote LDL oxidation in the human artery wall.

Analysis of localization and function of the COOH-terminal corresponding site of cytochrome b558 in fish neutrophils

Using an antibody against the synthetic peptide corresponding to the COOH-terminal region of human cytochrome b558 large subunit, a broad band was specifically detected in neutrophil lysates from 6 marine fish and 2 freshwater fish by western blotting. Immunofluorescence assay showed that the antibody recognized the epitopes in eel and tilapia neutrophils permeabilized with detergent. These results suggest that the cytochrome b large subunit universally exists in fish neutrophils and that the epitopes are exposed to the cytoplasmic side of fish neutrophils as well as human neutrophils. Furthermore, a synthetic peptide corresponding to the COOH-terminus of the large subunit apparently blocked superoxide production in a specific and dose-dependent fashion in eel and tilapia neutrophils, indicating that the region equivalent to the COOH-terminus of cytochrome b large subunit is responsible for superoxide generation in fish neutrophils.

8-Nitro-2'-deoxyguanosine, a specific marker of oxidation by reactive nitrogen species, is generated by the myeloperoxidase-hydrogen peroxide-nitrite system of activated human phagocytes

Reactive intermediates generated by phagocytes damage DNA and may contribute to the link between chronic inflammation and cancer. Myeloperoxidase, a heme protein secreted by activated phagocytes, is a potential catalyst for such reactions. Recent studies demonstrate that this enzyme uses hydrogen peroxide (H2O2) and nitrite (NO2-) to generate reactive nitrogen species which convert tyrosine to 3-nitrotyrosine. We now report that activated human neutrophils use myeloperoxidase, H2O2, and NO2- to nitrate 2'-deoxyguanosine, one of the nucleosides of DNA. Through HPLC, UV/vis spectroscopy, and mass spectrometry, the two major products of this reaction were identified as 8-nitroguanine and 8-nitro-2'-deoxyguanosine. Nitration required each component of the complete enzymatic system and was inhibited by catalase and heme poisons. However, it was independent of chloride ion and little affected by scavengers of hypochlorous acid, suggesting that the reactive agent is a nitrogen dioxide-like species that results from the one-electron oxidation of NO2- by myeloperoxidase. Alternatively, 2'-deoxyguanosine might be oxidized directly by the enzyme to yield a radical species which subsequently reacts with NO2- or NO2* to generate the observed products. Human neutrophils stimulated with phorbol ester also generated 8-nitroguanine and 8-nitro-2'-deoxyguanosine. The reaction required NO2- and was inhibited by catalase and heme poisons, implicating myeloperoxidase in the cell-mediated pathway. These results indicate that human neutrophils use the myeloperoxidase-H2O2-NO2- system to generate reactive species that can nitrate the C-8 position of 2'-deoxyguanosine. Our observations raise the possibility that reactive nitrogen species generated by myeloperoxidase and other peroxidases contribute to nucleobase oxidation and tissue injury at sites of inflammation.

Reaction of myeloperoxidase compound I with chloride, bromide, iodide, and thiocyanate

Myeloperoxidase plays a fundamental role in oxidant production by neutrophils. The enzyme uses hydrogen peroxide to oxidize chloride (Cl-), bromide (Br-), iodide (I-), and the pseudohalide thiocyanate (SCN-) to their respective hypohalous acids. This study for the first time presents transient kinetic measurements of the oxidation of these halides and thiocyanate by the myeloperoxidase intermediate compound I, using the sequential mixing stopped-flow technique. At pH 7 and 15 degrees C, the two-electron reduction of compound I to the native enzyme by Cl- has a second-order rate constant of (2.5 +/- 0.3) x 10(4) M(-1) s(-1), whereas reduction of compound I by SCN- has a second-order rate constant of (9.6 +/- 0.5) x 10(6) M(-1) s(-1). Iodide [(7.2 +/- 0.7) x 10(6) M(-1) s(-1)] is shown to be a better electron donor for compound I than Br- [(1.1 +/- 0.1) x 10(6) M(-1) s(-1)]. The pH dependence studies suggest that compound I reduction by (pseudo-)halides is controlled by a residue with a pKa of about 4.6. The protonation of this group is necessary for optimum (pseudo-)halide anion oxidation. These transient kinetic results are underlined by steady-state spectral and kinetic investigations. SCN- is shown to be most effective in shifting the system myeloperoxidase/hydrogen peroxide from the peroxidatic cycle to the halogenation cycle, whereas iodide is shown to be more effective than bromide which in turn is much more effective than chloride. Decreasing pH increases the rate of this transition. Our results show that thiocyanate is an important substrate of myeloperoxidase in most environments and that hypothiocyanate is likely to contribute to leukocyte antimicrobial activity.

Electron paramagnetic resonance detection of free tyrosyl radical generated by myeloperoxidase, lactoperoxidase, and horseradish peroxidase

Phagocytes secrete the heme protein myeloperoxidase, which is present and active in human atherosclerotic tissue. These cells also generate hydrogen peroxide (H2O2), thereby allowing myeloperoxidase to generate a range of oxidizing intermediates and stable end products. When this system acts on L-tyrosine in vitro, it forms o, o'-dityrosine, which is enriched in atherosclerotic lesions. Myeloperoxidase, therefore, may oxidize artery wall proteins in vivo, cross-linking their L-tyrosine residues. In these studies, we used electron paramagnetic resonance (EPR) spectroscopy to identify an oxidizing intermediate in this reaction pathway and in parallel reactions catalyzed by horseradish peroxidase and lactoperoxidase. Using an EPR flow system to rapidly mix and examine solutions containing horseradish peroxidase, H2O2, and L-tyrosine, we detected free tyrosyl radical (a2,6H = 6.3 G, a3,5H = 1.6 G, and abetaH = 15. 0 G). We then used spin trapping techniques with 2-methyl-2-nitrosopropane (MNP) to further identify this intermediate. The resulting three-line spectrum (aN = 15.6 G) was consistent with an MNP/tyrosyl radical spin adduct. Additional MNP spin trapping studies with ring-labeled L-[13C6]tyrosine yielded a characteristic eight-line EPR spectrum (aN = 15.6 G, a13C (2) = 8.0 G, a13C (1) = 7.1 G, a13C (1) = 1.3 G), indicating that the MNP adduct resulted from trapping a carbon-centered radical located on the aromatic ring of L-tyrosine. This same eight-line spectrum was observed when human myeloperoxidase or bovine lactoperoxidase was substituted for horseradish peroxidase. Furthermore, a partially immobilized MNP/tyrosyl radical spin adduct was detected when we exposed a synthetic polypeptide composed of glutamate and L-tyrosine residues to the myeloperoxidase-H2O2-L-tyrosine system. The broadened EPR signal resulting from this MNP/polypeptide adduct was greatly narrowed by proteolytic digestion with Pronase, confirming that the initial spin-trapped radical was protein-bound. Collectively, these results indicate that peroxidases use H2O2 to convert L-tyrosine to free tyrosyl radical. They also support the idea that free tyrosyl radical initiates cross-linking of L-tyrosine residues in proteins. We suggest that this pathway may play an important role in protein and lipid oxidation at sites of inflammation and in atherosclerotic lesions.

Oxidants and antioxidants in the pathogenesis of atherosclerosis: implications for the oxidized low density lipoprotein hypothesis

The oxidation hypothesis proposes that low density lipoprotein must be oxidatively modified to trigger the pathological events of atherosclerosis. In this article, we evaluate recent studies addressing the pathways that promote low density lipoprotein oxidation in vivo and the impact of antioxidants on atherogenesis in animals, paying particular attention to the clinical implications of these studies for the oxidation hypothesis.

Differential reactivities of hypochlorous and hypobromous acids with purified Escherichia coli phospholipid: formation of haloamines and halohydrins

Hypochlorous (HOCl) and hypobromous (HOBr) acids are strong oxidants derived from myeloperoxidase and eosinophil peroxidase, the major antimicrobial enzymes of neutrophils and eosinophils, respectively. These oxidants are highly reactive with a wide range of biomolecules. At physiological pH, both HOCl and HOBr react readily with amines to form haloamines and with the unsaturated bonds of fatty acids to form halohydrins. We have investigated which of these reactions occur with phosphatidylethanolamine (PE), the predominant phospholipid of Escherichia coli. The formation of haloamines was determined by TLC and colorimetrically and the formation of halohydrins was determined by TLC and GC-MS. With HOCl, chloramines were much the preferred product and chlorohydrins were formed in substantial amounts only when HOCl was in excess of the amount required to convert the amine to the dichloramine. With HOBr at all concentrations, bromamines and bromohydrins were formed concurrently, indicating a greater relative reactivity with unsaturated fatty acids than with HOCl. The bromamine derivatives of PE, and other primary amines, were found to be more reactive than the equivalent chloramines, and were able to brominate the unsaturated bonds of fatty acids. Bromohydrins (formed directly or through the action of bromamines) may, therefore, be suitable biomarkers for the production of HOBr in vivo.

Isotope dilution mass spectrometric quantification of 3-nitrotyrosine in proteins and tissues is facilitated by reduction to 3-aminotyrosine

Oxidative damage by reactive nitrogen species has been implicated in the pathogenesis of atherosclerosis and other inflammatory diseases. The mechanisms of tissue damage are poorly understood, however, because the toxic intermediates are short-lived. Previous in vitro studies have suggested that 3-nitrotyrosine represents a specific marker of protein oxidation by reactive nitrogen species. The detection of this nitrated aromatic amino acid may thus serve as an indicator of tissue injury by nitrogen species in vivo. Here we describe a highly sensitive and specific analytical method for quantifying free and protein-bound 3-nitrotyrosine. The assay involves acid hydrolysis of proteins, isolation of 3-nitrotyrosine by ion exchange chromatography, and reduction of 3-nitrotyrosine to 3-aminotyrosine with dithionite. The reduced amino acid is then converted to its n-propyl, per-heptafluorobutyryl derivative and quantified by isotope dilution gas chromatography negative-ion chemical ionization mass spectrometry. Attomole levels of 3-nitrotyrosine can be reproducibly measured in this manner. Quantifying 3-nitrotyrosine levels of tissues by stable isotope dilution gas chromatography/mass spectrometry should provide a powerful tool for exploring the impact of reactive nitrogen species on oxidative reactions in vivo.

Relative chlorinating, nitrating, and oxidizing capabilities of neutrophils determined with phagocytosable probes

The capabilities of stimulated neutrophils to initiate intraphagosomal and extracellular chlorination, nitration, and other oxidative reactions has been evaluated using a fluorescent particle and soluble phenolic compounds as target molecules. Neutrophils activated by the soluble stimulus, phorbol myristate acetate, both chlorinated fluorescein that was covalently attached to polyacrylamide microspheres and initiated tyrosine dimerization. When nitrite ion was present at millimolar concentration levels in the medium, nitration of the phenolic rings also occurred; the relative extent of nitration increased as the nitrite concentration was increased. Myeloperoxidase (MPO) also catalyzed nitration and chlorination of fluorescein and the fluorescein-conjugated particles in cell-free solutions; the relative nitration yields increased with increasing [NO2-]/[Cl-] ratios. Nitration did not involve intermediary formation of nitrating agents derived from reaction between MPO-generated HOCl and NO2- because this reaction also occurred in chloride-free media and direct addition of HOCl to solutions containing NO2- and fluorescein gave only chlorinated products. In marked contrast to these extracellular reactions, intraphagosomal nitration of the fluorescein-conjugated particles could not be detected (even at [NO2-] as high as 0.1 M), whereas chlorination of the probe was extensive. These data indicate that intraphagosomal aromatic nitration in neutrophils is negligible, although extracellular nitration of phenolic compounds by secreted MPO could occur at physiological concentration levels of NO2-.

Intraphagosomal chlorination dynamics and yields determined using unique fluorescent bacterial mimics

Fluorescein was covalently attached through a cystamine linker group to carboxy-derivatized polyacrylamide microspheres to generate phagocytosable particles containing fluorescent reporter groups. A unique feature of these beads is that the dye was recoverable in near-quantitative yield from intracellular environments by thiol reduction of the cystamine disulfide bond. Fluorescence microscopy indicated that individual neutrophils could bind as many as approximately 20 serum-opsonized beads, although no appreciable cellular association was observed for unopsonized beads. By using methyl viologen to quench external fluorescence, it was demonstrated that 70-90% of the neutrophil-associated fluorescein on opsonized beads was inaccessible to the medium. The particle-bound fluorescein underwent near-stoichiometric conversion to chlorinated derivatives when reacted with HOCl or the cell-free myeloperoxidase (MPO)-H2O2-Cl- system; products were identified by HPLC separation and electrospray ionization mass spectrometry of the recovered dye. Fluorescence changes accompanying phagocytosis were consistent with chlorination of the dye; fluorescence spectrometric and chemical trapping measurements indicated that intraphagosomal chlorination was far more extensive than extracellular chlorination. Yields of recovered chlorofluoresceins determined by HPLC indicated that sufficient HOCl had been produced intracellularly to kill entrapped bacteria. Fluorescein chlorination coincided approximately with phagocytosis and stimulated uptake of O2 by the cells. Demonstration that HOCl is produced within phagosomes in sufficient concentrations to kill bacteria on a time scale associated with death constitutes strong evidence in support of a primary role for HOCl in the microbicidal action of neutrophils.

Toxicity of peroxynitrite and related reactive nitrogen species toward Escherichia coli

The toxicity of peroxynitrite toward Escherichia coli (expressed as LD50, the concentration required to kill 50% of the bacteria) was found to be independent of bacterial cell densities over a wide experimental range, spanning 10(6)-10(10) colony-forming units/mL; the magnitude of LD50 was also pH-independent over the range pH 5.9-8.3. This highly unusual behavior can be quantitatively reproduced by a dynamical model in which (i) ONO2H is identified as the toxic form of the oxidant and (ii) the bulk of the added peroxynitrite decays to nitrate ion under these conditions. From the model, one estimates that 10(6)-10(7) ONO2H molecules are required to kill a bacterium, indicating a very high intrinsic toxicity (cf. HOCl, for which LD50 = 10(7)-10(8) molecules/cell of E. coli). Nearly complete protection was observed when bicarbonate ion was added to the buffer, even when concentrations of peroxynitrite exceeded 50 times the LD50 measured in the absence of bicarbonate. Consistent with previous reports, combinations of H2O2 and NO and, in weakly acidic media, H2O2 and NO2- were found to exhibit enhanced toxicities relative to the individual reactants. Protection by bicarbonate was utilized to assess the potential role of intermediary formation of ONO2H in bacterial killing in these systems. Approximately 25% protection by bicarbonate was observed for media containing H2O2 and NO2-, consistent with a minor contribution to killing by ONO2H under the experimental conditions. No protection was observed for media containing H2O2 and *NO in both anaerobic and aerobic environments, excluding extracellularly generated ONO2H as a participant in these bactericidal reactions.

Extensive peroxynitrite activity during progressive stages of central nervous system inflammation

Nitric oxide (NO) production has been associated with disease activity in multiple sclerosis and experimental autoimmune encephalomyelitis (EAE). This free radical can be transformed by superoxide to peroxynitrite, an extremely toxic oxidant which causes lipid peroxidation. In addition, peroxynitrite nitrates tyrosine residues, resulting in nitrotyrosine, which can be identified immunohistochemically. The results of this study indicate that peroxynitrite is formed very early during EAE development, correlating with clinical disease activity. Nitrotyrosine-positive cells display a widespread distribution in brain and spinal cord during severe disease and are associated with both perivascular infiltrates and parenchymal sites. Double-staining procedures demonstrated that a subpopulation of CD11b-positive cells (macrophages/microglia) reacted with nitrotyrosine antibodies. Immunostaining for inducible NO synthase demonstrated a similar distribution as nitrotyrosine staining. These experiments indicate that peroxynitrite is formed during progressive stages of disease activity.

Formation of reactive nitrogen species during peroxidase-catalyzed oxidation of nitrite. A potential additional mechanism of nitric oxide-dependent toxicity

Involvement of peroxynitrite (ONOO-) in inflammatory diseases has been implicated by detection of 3-nitrotyrosine, an allegedly characteristic protein oxidation product, in various inflamed tissues. We show here that nitrite (NO2-), the primary metabolic end product of nitric oxide (NO.), can be oxidized by the heme peroxidases horseradish peroxidase, myeloperoxidase (MPO), and lactoperoxidase (LPO), in the presence of hydrogen peroxide (H2O2), to most likely form NO.2, which can also contribute to tyrosine nitration during inflammatory processes. Phenolic nitration by MPO-catalyzed NO2- oxidation is only partially inhibited by chloride (Cl-), the presumed major physiological substrate for MPO. In fact, low concentrations of NO2- (2-10 microM) catalyze MPO-mediated oxidation of Cl-, indicated by increased chlorination of monochlorodimedon or 4-hydroxyphenylacetic acid, most likely via reduction of MPO compound II. Peroxidase-catalyzed oxidation of NO2-, as indicated by phenolic nitration, was also observed in the presence of thiocyanate (SCN-), an alternative physiological substrate for mammalian peroxidases. Collectively, our results suggest that NO2-, at physiological or pathological levels, is a substrate for the mammalian peroxidases MPO and lactoperoxidase and that formation of NO2. via peroxidase-catalyzed oxidation of NO2- may provide an additional pathway contributing to cytotoxicity or host defense associated with increased NO. production.

Evidence for the activation of myeloperoxidase by f-Meth-Leu-Phe prior to its release from neutrophil granulocytes

Activity and release of myeloperoxidase (MPO) was measured in heparinized whole blood samples after activation of neutrophil granulocytes by the chemoattractant N-formyl-methionyl-leucyl-phenylalanine (fMLP) using two different methods: (i) by determination of the amount of MPO released into the blood plasma using a MPO enzyme-immunoassay, and (ii) simultaneously, by measuring the remaining activity within the neutrophils by flow cytometry using the Bayer Technicon H3. Although a part of MPO was released immediately after addition of fMLP, remaining MPO activity within the neutrophils surprisingly increased during the first minutes after incubation. Subsequently, MPO activity dropped due to a continuous release of MPO. In addition to fMLP, granulocyte-macrophage colony stimulating factor (GM-CSF) enhanced MPO activity in neutrophils. These results indicate that MPO is present in resting granulocytes in an inactive or only partially active form and is activated by fMLP and GM-CSF.

Mass spectrometric quantification of markers for protein oxidation by tyrosyl radical, copper, and hydroxyl radical in low density lipoprotein isolated from human atherosclerotic plaques

Lipoprotein oxidation has been implicated in the pathogenesis of atherosclerosis. However, the physiologically relevant pathways mediating oxidative damage have not yet been identified. Three potential mechanisms are tyrosyl radical, hydroxyl radical, and redox active metal ions. Tyrosyl radical forms o,o'-dityrosine cross-links in proteins. The highly reactive hydroxyl radical oxidizes phenylalanine residues to o-tyrosine and m-tyrosine. Metal ions oxidize low density lipoprotein (LDL) by poorly understood pathways. To explore the involvement of tyrosyl radical, hydroxyl radical, and metal ions in atherosclerosis, we developed a highly sensitive and quantitative method for measuring levels of o, o'-dityrosine, o-tyrosine, and m-tyrosine in proteins, lipoproteins, and tissue, using stable isotope dilution gas chromatography-mass spectrometry. We showed that o,o'-dityrosine was selectively produced in LDL oxidized with tyrosyl radical. Both o-tyrosine and o, o'-dityrosine were major products when LDL was oxidized with hydroxyl radical. Only o-tyrosine was formed in LDL oxidized with copper. Similar profiles of oxidation products were observed in bovine serum albumin oxidized with the three different systems. Applying these findings to LDL isolated from human atherosclerotic lesions, we detected a 100-fold increase in o,o'-dityrosine levels compared to those in circulating LDL. In striking contrast, levels of o-tyrosine and m-tyrosine were not elevated in LDL isolated from atherosclerotic tissue. Analysis of fatty streaks revealed a similar pattern of oxidation products; compared with normal aortic tissue, there was a selective increase in o,o'-dityrosine with no change in o-tyrosine. The detection of a selective increase of o,o'-dityrosine in LDL isolated from vascular lesions is consistent with the hypothesis that oxidative damage in human atherosclerosis is mediated in part by tyrosyl radical. In contrast, these observations do not support a role for free metal ions as catalysts of LDL oxidation in the artery wall.

Human neutrophils employ the myeloperoxidase-hydrogen peroxide-chloride system to convert hydroxy-amino acids into glycolaldehyde, 2-hydroxypropanal, and acrolein. A mechanism for the generation of highly reactive alpha-hydroxy and alpha,beta-unsaturated aldehydes by phagocytes at sites of inflammation

Reactive aldehydes derived from reducing sugars and lipid peroxidation play a critical role in the formation of advanced glycation end (AGE) products and oxidative tissue damage. We have recently proposed another mechanism for aldehyde generation at sites of inflammation that involves myeloperoxidase, a heme enzyme secreted by activated phagocytes. We now demonstrate that human neutrophils employ the myeloperoxidase-H202-chloride system to produce alpha-hydroxy and alpha,beta-unsaturated aldehydes from hydroxy-amino acids in high yield. Identities of the aldehydes were established using mass spectrometry and high performance liquid chromatography. Activated neutrophils converted L-serine to glycolaldehyde, an alpha-hydroxyaldehyde which mediates protein cross-linking and formation of Nepsilon-(carboxymethyl)lysine, an AGE product. L-Threonine was similarly oxidized to 2-hydroxypropanal and its dehydration product, acrolein, an extremely reactive alpha,beta-unsaturated aldehyde which alkylates proteins and nucleic acids. Aldehyde generation required neutrophil activation and a free hydroxy-amino acid; it was inhibited by catalase and heme poisons, implicating H202 and myeloperoxidase in the cellular reaction. Aldehyde production by purified myeloperoxidase required H202 and chloride, and was mimicked by reagent hypochlorous acid (HOCl) in the absence of enzyme, suggesting that the reaction pathway involves a chlorinated intermediate. Collectively, these results indicate that the myeloperoxidase-H202-chloride system of phagocytes converts free hydroxy-amino acids into highly reactive alpha-hydroxy and alpha,beta-unsaturated aldehydes. The generation of glycolaldehyde, 2-hydroxypropanal, and acrolein by activated phagocytes may thus play a role in AGE product formation and tissue damage at sites of inflammation.

Acid adaptation sensitizes Salmonella typhimurium to hypochlorous acid

Acid adaptation of Salmonella typhimurium at a pH of 5.0 to 5.8 for one to two cell doublings resulted in marked sensitization of the pathogen to halogen-based sanitizers including chlorine (hypochlorous acid) and iodine. Acid-adapted S. typhimurium was more resistant to an anionic acid sanitizer than was its nonadapted counterpart. A nonselective plating medium of tryptose phosphate agar plus 1% pyruvate was used throughout the study to help recover chemically stressed cells. Mechanisms of HOCl-mediated inactivation of acid-adapted and nonadapted salmonellae were investigated. Hypochlorous acid oxidized a higher percentage of cell surface sulfhydryl groups in acid-adapted cells than in nonadapted cells, and sulfhydryl oxidation was correlated with cell inactivation. HOCl caused severe metabolic disruptions in acid-adapted and nonadapted S. typhimurium, such as respiratory loss and inability to restore the adenylate energy charge from a nutrient-starved state. Sensitization of S. typhimurium to hypochlorous acid by acid adaptation also involved increased permeability of the cell surface because nonadapted cells treated with EDTA became sensitized. The results of this study establish that acid-adapted S. typhimurium cells are highly sensitized to HOCl oxidation and that inactivation by HOCl involves changes in membrane permeability, inability to maintain or restore energy charge, and probably oxidation of essential cellular components. This study provides a basis for improved practical technologies to inactivate Salmonella and implies that acid pretreatment of food plant environments may increase the efficacy of halogen sanitizers.

Pathways for oxidation of low density lipoprotein by myeloperoxidase: tyrosyl radical, reactive aldehydes, hypochlorous acid and molecular chlorine

Many lines of evidence implicate oxidation of low density lipoprotein (LDL) in the pathogenesis of atherosclerosis, a chronic inflammatory disease. The physiologically relevant mechanisms have not been identified, but phagocytic white cells may play an important role because macrophage-rich lesions characterize the disorder. Recent studies have shown that myeloperoxidase, a heme enzyme secreted only by phagocytes, is present in human atherosclerotic tissue. The enzyme is a potent catalyst of LDL oxidation in vitro, it co-localizes with macrophages in lesions, and it generates products that are detectable in atherosclerotic plaque. These findings suggest that myeloperoxidase may promote LDL oxidation in the artery wall. This article reviews the enzyme's ability to generate a range of oxidants, including tyrosyl radical, reactive aldehydes, hypochlorous acid and molecular chlorine. These products have the potential to damage host molecules as well as microbes, suggesting a mechanism that may contribute to atherosclerotic vascular disease.

Role of oxidants in microbial pathophysiology

Reactive oxidant species (superoxide, hydrogen peroxide, hydroxyl radical, hypohalous acid, and nitric oxide) are involved in many of the complex interactions between the invading microorganism and its host. Regardless of the source of these compounds or whether they are produced under normal conditions or those of oxidative stress, these oxidants exhibit a broad range of toxic effects to biomolecules that are essential for cell survival. Production of these oxidants by microorganisms enables them to have a survival advantage in their environment. Host oxidant production, especially by phagocytes, is a counteractive mechanism aimed at microbial killing. However, this mechanism may be contribute to a deleterious consequence of oxidant exposure, i.e., inflammatory tissue injury. Both the host and the microorganism have evolved complex adaptive mechanisms to deflect oxidant-mediated damage, including enzymatic and nonenzymatic oxidant-scavenging systems. This review discusses the formation of reactive oxidant species in vivo and how they mediate many of the processes involved in the complex interplay between microbial invasion and host defense.

Phagocytic activity and oxidative burst of granulocytes in persons with myeloperoxidase deficiency

In the present study, phagocytosis and the oxidative metabolism of neutrophil granulocytes from five clinically healthy persons with different degrees of myeloperoxidase deficiency were investigated and compared to those of normal persons. The identification of individuals with myeloperoxidase deficiency was performed with the Bayer/Technicon H3 blood cell counter, which differentiates the leukocytes by measuring the peroxidase activity. Neutrophils of three out of five investigated myeloperoxidase deficient persons showed extremely low peroxidase indices (-53 and lower), but only the neutrophils of one person totally lacked myeloperoxidase. This was demonstrated by comparing myeloperoxidase mass concentration measured with an enzyme immunoassay, lack of HOCl production, and was further confirmed by measuring luminol- and lucigenin-enhanced chemiluminescence. Characteristically, myeloperoxidase deficient granulocytes showed a strikingly decreased luminol-enhanced chemiluminescence while the lucigenin-enhanced chemiluminescence was significantly increased compared to normal granulocytes. Although there is a DNA sequence homology of about 70%, the activity of peroxidase in eosinophils was not affected in any myeloperoxidase deficient person investigated. Moreover, a person with a very rare defect of eosinophil peroxidase had completely normal myeloperoxidase activity. The lack of myeloperoxidase activity is compensated for by an increased phagocytic activity, an increased production of superoxide anion (lucigenin-chemiluminescence) and probably by an alternative metabolism of H2O2; since persons lacking myeloperoxidase activity do not normally suffer from severe infections, H2O2 is obviously metabolized to other reactive oxygen substrates than HOCl, e.g. to OH-radicals.

Manganese-based superoxide dismutase mimetics inhibit neutrophil infiltration in vivo

In a previous study (Hardy et al. (1994) J. Biol. Chem. 269, 18535-18540), we observed that the manganese-based superoxide dismutase mimetic Mn(II)-dichloro(1,4,7,10,13-pentaazacyclopentadecane) (MnPAM) inhibited neutrophil-mediated cell injury in vitro. We have extended these studies with the low molecular weight superoxide dismutase mimic to evaluate the role of superoxide in neutrophil-mediated tissue injury in vivo. In a dose-dependent manner, MnPAM inhibited colonic tissue injury and neutrophil accumulation into the colonic tissue induced by the intracolonic instillation of dilute aqueous acetic acid in mice. Tissue injury was assessed by visual and histological analysis. Neutrophil infiltration was determined by tissue myeloperoxidase activity and confirmed by histological analysis. Two novel Mn(II) dichloro complexes of the carbon-substituted macrocycles 2-methyl-1,4,7,10,13-pentaazacyclopentadecane (MnMAM) and 2-(2-methylpropyl)-1,4,7,10,13-pentaazacyclopentadecane (MnBAM) effectively catalyzed the dismutation of superoxide with catalytic rate constants (kcat) of 3. 31 x 10(7) M-1 s-1 and 1.91 x 10(7) M-1 s-1, respectively, as determined by stopped-flow kinetic analysis at pH 8.1 and 21 degrees C. The superoxide dismutase mimetics MnMAM and MnBAM also attenuated dilute aqueous acetic acid-induced tissue injury and neutrophil infiltration into colonic tissue; however, two Mn(II) complexes that had little or no detectable SOD activity (kcat </= 0.1 x 10(7) M-1 s-1), specifically the Mn(II) dichloro complexes of 1,4,7,10,13-pentaazacyclohexadecane and 1,4,7,11,14-pentaazacycloheptadecane, failed to inhibit the colonic tissue injury or infiltration of neutrophils in mice treated intracolonically with dilute aqueous acetic acid. These results are consistent with a proinflammatory role for superoxide in the mediation of neutrophil infiltration in vivo.

Molecular chlorine generated by the myeloperoxidase-hydrogen peroxide-chloride system of phagocytes converts low density lipoprotein cholesterol into a family of chlorinated sterols

Oxidation of low density lipoprotein (LDL) may be of critical importance in triggering the pathological events of atherosclerosis. Myeloperoxidase, a heme protein secreted by phagocytes, is a potent catalyst for LDL oxidation in vitro, and active enzyme is present in human atherosclerotic lesions. We have explored the possibility that reactive intermediates generated by myeloperoxidase target LDL cholesterol for oxidation. LDL exposed to the myeloperoxidase-H2O2-Cl- system at acidic pH yielded a family of chlorinated sterols. The products were identified by mass spectrometry as a novel dichlorinated sterol, cholesterol alpha-chlorohydrin (6beta-chlorocholestane-(3beta,5alpha)-diol), cholesterol beta-chlorohydrin (5alpha-chlorocholestane-(3beta, 6beta)-diol), and a structurally related cholesterol chlorohydrin. Oxidation of LDL cholesterol by myeloperoxidase required H2O2 and Cl-, suggesting that hypochlorous acid (HOCl) was an intermediate in the reaction. However, HOCl failed to generate chlorinated sterols under chloride-free conditions. Since HOCl is in equilibrium with molecular chlorine (Cl2) through a reaction which requires Cl- and H+, this raised the possibility that Cl2 was the actual chlorinating intermediate. Consonant with this hypothesis, HOCl oxidized LDL cholesterol in the presence of Cl- and at acidic pH. Moreover, in the absence of Cl- and at neutral pH, Cl2 generated the same family of chlorinated sterols as the myeloperoxidase-H2O2-Cl- system. Finally, direct addition of Cl2 to the double bond of cholesterol accounts for dichlorinated sterol formation by myeloperoxidase. Collectively, these results indicate that Cl2 derived from HOCl is the chlorinating intermediate in the oxidation of cholesterol by myeloperoxidase. Our observations suggest that Cl2 generation in acidic compartments may constitute one pathway for oxidation of LDL cholesterol in the artery wall.

Human neutrophils employ chlorine gas as an oxidant during phagocytosis

Reactive oxidants generated by phagocytes are of central importance in host defenses, tumor surveillance, and inflammation. One important pathway involves the generation of potent halogenating agents by the myeloperoxidase-hydrogen peroxide-chloride system. The chlorinating intermediate in these reactions is generally believed to be HOCl or its conjugate base, ClO-. However, HOCl is also in equilibrium with Cl2, raising the possibility that Cl2 executes oxidation/ halogenation reactions that have previously been attributed to HOCl/ClO-. In this study gas chromatography-mass spectrometric analysis of head space gas revealed that the complete myeloperoxidase-hydrogen peroxide-chloride system generated Cl2. In vitro studies demonstrated that chlorination of the aromatic ring of free L-tyrosine was mediated by Cl2 and not by HOCl/ClO-. Thus, 3-chlorotyrosine serves as a specific marker for Cl2-dependent oxidation of free L-tyrosine. Phagocytosis of L-tyrosine encapsulated in immunoglobulin- and complement-coated sheep red blood cells resulted in the generation of 3-chlorotyrosine. Moreover, activation of human neutrophils adherent to a L-tyrosine coated glass surface also stimulated 3-chlorotyrosine formation. Thus, in two independent models of phagocytosis human neutrophils convert L-tyrosine to 3-chlorotyrosine, indicating that a Cl2-like oxidant is generated in the phagolysosome. In both models, synthesis of 3-chlorotyrosine was inhibited by heme poisons and the peroxide scavenger catalase, implicating the myeloperoxidase-hydrogen peroxide system in the reaction. Collectively, these results demonstrate that myeloperoxidase generates Cl2 and that human neutrophils use an oxidant with characteristics identical to those of Cl2 during phagocytosis. Moreover, our observations suggest that phagocytes exploit the chlorinating properties of Cl2 to execute oxidative and cytotoxic reactions at sites of inflammation and vascular disease.

Human phagocytes employ the myeloperoxidase-hydrogen peroxide system to synthesize dityrosine, trityrosine, pulcherosine, and isodityrosine by a tyrosyl radical-dependent pathway

Myeloperoxidase, a heme protein secreted by activated phagocytes, may be a catalyst for lipoprotein oxidation in vivo. Active myeloperoxidase is a component of human atherosclerotic lesions, and atherosclerotic tissue exhibits selective enrichment of protein dityrosine cross-links, a well characterized product of myeloperoxidase. Tyrosylation of lipoproteins with peroxidase-generated tyrosyl radical generates multiple protein-bound tyrosine oxidation products in addition to dityrosine. The structural characterization of these products would thus serve as an important step in determining the role of myeloperoxidase in lipoprotein oxidation in the artery wall. We now report the identification and characterization of four distinct tyrosyl radical addition products generated by human phagocytes. Activated neutrophils synthesized three major fluorescent products from -tyrosine; on reverse phase HPLC, each compound coeluted with fluorescent oxidation products formed by myeloperoxidase. We purified the oxidation products to apparent homogeneity by cation and anion exchange chromatographies and identified the compounds as dityrosine (3,3'-dityrosine), trityrosine (3,3',5',3"-trityrosine) and pulcherosine (5-[4"-(2-carboxy-2-aminoethyl)phenoxy]3, 3'-dityrosine) by high resolution NMR spectroscopy and mass spectrometry. Additionally, we have found that dityrosine is a precursor to trityrosine, but not pulcherosine. In a search for a precursor to pulcherosine, we identified isodityrosine (3-[4'-(2-carboxy-2-aminoethyl)phenoxy]tyrosine), a non-fluorescent product of L-tyrosine oxidation by human phagocytes. Our results represent the first identification of this family of tyrosyl radical addition products in a mammalian system. Moreover, these compounds may serve as markers specific for tyrosyl radical-mediated oxidative damage in atherosclerosis and other inflammatory conditions.

Kinetics of oxidation of tyrosine and dityrosine by myeloperoxidase compounds I and II. Implications for lipoprotein peroxidation studies

The oxidation of lipoproteins is considered to play a key role in atherogenesis, and tyrosyl radicals have been implicated in the oxidation reaction. Tyrosyl radicals are generated in a system containing myeloperoxidase, H2O2, and tyrosine, but details of this enzyme-catalyzed reaction have not been explored. We have performed transient spectral and kinetic measurements to study the oxidation of tyrosine by the myeloperoxidase intermediates, compounds I and II, using both sequential mixing and single-mixing stopped-flow techniques. The one-electron reduction of compound I to compound II by tyrosine has a second order rate constant of (7.7 +/- 0.1) x 10(5) M-1 s-1. Compound II is then reduced by tyrosine to native enzyme with a second order rate constant of (1.57 +/- 0.06) x 10(4) M-1 s-1. Our study further revealed that, compared with horseradish peroxidase, thyroid peroxidase, and lactoperoxidase, myeloperoxidase is the most efficient catalyst of tyrosine oxidation at physiological pH. The second order rate constant for the myeloperoxidase compound I reaction with tyrosine is comparable with that of its compound I reaction with chloride: (4.7 +/- 0.1) x 10(6) M-1 s-1. Thus, although chloride is considered the major myeloperoxidase substrate, tyrosine is able to compete effectively for compound I. Steady state inhibition studies demonstrate that chloride binds very weakly to the tyrosine binding site of the enzyme. Coupling of tyrosyl radicals yields dityrosine, a highly fluorescent stable compound that had been identified as a possible marker for lipoprotein oxidation. We present spectral and kinetic data showing that dityrosine is further oxidized by both myeloperoxidase compounds I and II. The second order rate constants we determined for dityrosine oxidation are (1.12 +/- 0.01) x 10(5) M-1 s-1 for compound I and (7.5 +/- 0.3) x 10(2) M-1 s-1 for compound II. Therefore, caution must be exercised when using dityrosine as a quantitative index of lipoprotein oxidation, particularly in the presence of myeloperoxidase and H2O2.

Cloning and characterization of the katB gene of Pseudomonas aeruginosa encoding a hydrogen peroxide-inducible catalase: purification of KatB, cellular localization, and demonstration that it is essential for optimal resistance to hydrogen peroxide

Pseudomonas aeruginosa is an obligate aerobe that is virtually ubiquitous in the environment. During aerobic respiration, the metabolism of dioxygen can lead to the production of reactive oxygen intermediates, one of which includes hydrogen peroxide. To counteract the potentially toxic effects of this compound, P. aeruginosa possesses two heme-containing catalases which detoxify hydrogen peroxide. In this study, we have cloned katB, encoding one catalase gene of P. aeruginosa. The gene was cloned on a 5.4-kb EcoRI fragment and is composed of 1,539 bp, encoding 513 amino acids. The amino acid sequence of the P. aeruginosa katB was approximately 65% identical to that of a catalase from a related species, Pseudomonas syringae. The katB gene was mapped to the 71- to 75-min region of the P. aeruginosa chromosome, the identical region which harbors both sodA and sodB genes encoding both manganese and iron superoxide dismutases. When cloned into a catalase-deficient mutant of Escherichia coli (UM255), the recombinant P. aeruginosa KatB was expressed (229 U/mg) and afforded this strain resistance to hydrogen peroxide nearly equivalent to that of the wild-type E. coli strain (HB101). The KatB protein was purified to homogeneity and determined to be a tetramer of approximately 228 kDa, which was in good agreement with the predicted protein size derived from the translated katB gene. Interestingly, KatB was not produced during the normal P. aeruginosa growth cycle, and catalase activity was greater in nonmucoid than in mucoid, alginate-producing organisms. When exposed to hydrogen peroxide and, to a greater extent, paraquat, total catalase activity was elevated 7- to 16-fold, respectively. In addition, an increase in KatB activity caused a marked increase in resistance to hydrogen peroxide. KatB was localized to the cytoplasm, while KatA, the "housekeeping" enzyme, was detected in both cytoplasmic and periplasmic extracts. A P. aeruginosa katB mutant demonstrated 50% greater sensitivity to hydrogen peroxide than wild-type bacteria, suggesting that KatB is essential for optimal resistance of P. aeroginosa to exogenous hydrogen peroxide.

The stimulus-sensitive H2O2-generating system present in human fat-cell plasma membranes is multireceptor-linked and under antagonistic control by hormones and cytokines

Previous work demonstrated that human fat-cells possess a plasma-membrane-bound H2O2-generating system that is activated by insulin. Here we show that this system is under antagonistic control by various hormones and cytokines that typically act through several distinct receptor families. Similarly to insulin, oxytocin and tumour necrosis factor alpha acted as stimulators of NADPH-dependent H2O2 generation, whereas isoprenaline, a beta-adrenergic agonist, had inhibitory effects. Surprisingly, the acidic and basic isoforms of fibroblast growth factor as well as homodimeric platelet-derived growth factor AA and BB had antagonistic stimulatory and inhibitory effects on NADPH-dependent H2O2 generation. The agents tested acted at discrete ligand-specific receptors and their mechanisms of action were membrane-delimited and occurred in the absence of ATP. These findings implied that established pathways of signal transduction, including receptor kinases or second-messenger-dependent protein kinases A and C, were not involved and placed the stimulus-sensitive H2O2-generating system in a position comparable with adenylate cyclase. It was concluded that the stimulus-sensitive H2O2-generating system of human fat-cells meets all criteria of a universal signal-transducing system for hormones and cytokines that may link ligand binding to cell-surface receptors to changes in the intracellular redox equilibrium.

Bactericidal properties of hydrogen peroxide and copper or iron-containing complex ions in relation to leukocyte function

Various combinations of hydrogen peroxide, reductant (ascorbic acid and superoxide ion), and copper or iron salts and their coordination complexes were examined to determine their cytotoxicity toward several bacteria with diverse metabolic capabilities and cell envelope structures. Four sets of bactericidal conditions were identified, comprising: (1) high concentration levels (5-100 mM) of H2O2 in the absence of exogenous metal ions and reductant; (2) ferrous or ferric coordination complexes plus enzymatically generated O2.- and H2O2 at relatively low steady-state concentration levels; (3) cupric ion plus low concentration levels of H2O2 (1 microM-1 mM) and ascorbate (10 microM-4 mM); (4) cuprous ion (or cupric ion plus ascorbate) in the absence of O2 and H2O2. Rates of losses in viabilities increased proportionately with increases in the concentration of H2O2 in metal-free environments and with each of the components in the Cu2+/ascorbate/H2O2 bactericidal assay system. Oxidant levels required for equivalent killing increased with increasing cell densities of the bacterial suspensions over the range investigated (2 x 10(7)-2 x 10(9) cfu/ml). Other experimental conditions or other combinations of reagents, most notably Fe3+/ascorbate/H2O2 systems, did not generate bactericidal environments. The patterns of response of the three organisms tested, Streptococcus lactis, Escherichia coli, and Pseudomonas aeruginosa, were similar, suggesting common bactericidal mechanisms. However, preliminary evidence suggests that the lethal lesions caused by the various bactericidal conditions are distinct: As discussed, each of the four bactericidal conditions could conceivably be attained within the phagosomes of leukocytes, although none has as yet been identified.

Characterization of complex formation between Gi2 and octyl glucoside solubilized neutrophil N-formyl peptide chemoattractant receptor by sedimentation velocity

The reversible formation of complexes between N-formyl peptide chemoattractant receptor (FPR) and Gi2 protein was analyzed by velocity sedimentation in linear sucrose density gradients. FPR complexed with heterotrimeric Gi2, sediments at different rate than uncomplexed FPR and the two forms have apparent sedimentation coefficients of 7S and 4S, respectively. The biochemical variables important for the reconstitution of the 7S complex from the 4S receptor and Gi2 were studied. The formation of 7S was saturable with Gi2 and addition of excess Gi did not cause oligomerization. The reconstituted 7S complex was stable under a variety of conditions including octyl glucoside concentrations below and above the critical micellar concentration. The optimum pH for the reconstitution is between 7 and 9, where the 4S and 7S species sedimented reproducibly, at distinct positions in the gradient. Below pH 6 both the 4S and the 7S species appear to undergo denaturation and form precipitates. Magnesium ions have no significant effect on the sedimentation of either forms of FPR. Reconstitution was stable up to a NaCl concentration of 0.2 M. At 1 M NaCl reconstitution was inhibited and at 3 M salt FPR aggregated. Since guanine nucleotides GTP, GTP gamma S, GDP beta S selectively dissociated the 7S complex in a concentration-dependent manner and adenine nucleotides had no effect, we conclude that the FPR-Gi2 system displays a vacant guanyl nucleotide binding site, the hallmark of a functional guanine nucleotide exchange complex. Moreover, our results indicate that the reconstitution of FPR-Gi2 complexes is reproducible at physiologically relevant conditions, shows selectivity, specificity, and biochemically functional properties consistent with a specific and functional interaction between solubilized FPR and G protein.

Transformation of lupus-inducing drugs to cytotoxic products by activated neutrophils

Drug-induced lupus is a serious side effect of certain medications, but the chemical features that confer this property and the underlying pathogenesis are puzzling. Prototypes of all six therapeutic classes of lupus-inducing drugs were highly cytotoxic only in the presence of activated neutrophils. Removal of extracellular hydrogen peroxide before, but not after, exposure of the drug to activated neutrophils prevented cytotoxicity. Neutrophil-dependent cytotoxicity required the enzymatic action of myeloperoxidase, resulting in the chemical transformation of the drug to a reactive product. The capacity of drugs to serve as myeloperoxidase substrates in vitro was associated with the ability to induce lupus in vivo.

Cholesterol chlorohydrin synthesis by the myeloperoxidase-hydrogen peroxide-chloride system: potential markers for lipoproteins oxidatively damaged by phagocytes

Myeloperoxidase, a heme protein secreted by activated phagocytes, uses hydrogen peroxide to produce potent cytotoxins. One important substrate is chloride, which is converted to hypochlorous acid (HOCl). This diffusible oxidant plays a critical role in the destruction of invading pathogens. Under pathological conditions, HOCl may also injure normal tissue. Recent studies have shown that myeloperoxidase is a component of human atherosclerotic lesions. Because oxidized lipoproteins may play a central role in atherogenesis, we have explored the possibility that cholesterol is a target for damage by myeloperoxidase. Three major classes of sterol oxidation products were apparent when cholesterol-phosphatidylcholine multilamellar vesicles which had been exposed to a myeloperoxidase-hydrogen peroxide-chloride system were subsequently analyzed by normal-phase thin layer chromatography. The products were identified by gas chromatography-mass spectrometry as cholesterol alpha- and beta-chlorohydrins (6 beta-chlorocholestane-3 beta,5 alpha-diol and 5 alpha-chlorocholestane-3 beta,6 beta-diol), cholesterol alpha- and beta-epoxides (cholesterol 5 alpha,6 alpha-epoxide and cholesterol 5 beta,6 beta-epoxide), and a novel cholesterol chlorohydrin. Conversion of cholesterol to the oxidation products required active myeloperoxidase, hydrogen peroxide, and halide and could be blocked by catalase or by scavengers of HOCl. Moreover, in the absence of the enzymatic system, reagent HOCl generated the same distribution of products. These results indicate that myeloperoxidase can convert cholesterol to chlorohydrins and epoxides by a reaction involving HOCl. Other oxygenated sterols are cytotoxic and mutagenic and are potent regulators of cholesterol homeostasis in cultured mammalian cells. Cholesterol chlorohydrins might similarly mediate powerful biological effects in the artery wall. Because chlorohydrins are stable under our experimental conditions, chlorinated sterols may prove useful as markers for lipoproteins oxidatively damaged by activated phagocytes.

Variation in hydrogen peroxide sensitivity between different strains of Neisseria gonorrhoeae is dependent on factors in addition to catalase activity

Catalase, which catalyzes the reduction of hydrogen peroxide to oxygen and water, is considered the primary defense of Neisseria gonorrhoeae against exogenous hydrogen peroxide. Recent reports have demonstrated drastically different sensitivities of the organism to hydrogen peroxide ranging from greater than 80% survival after challenge with 30 mM hydrogen peroxide to less than 0.001% survival after challenge with 10 mM hydrogen peroxide. In this study, we have examined the hydrogen peroxide sensitivities of six clinical gonococcal isolates. The study demonstrates that the variations in gonococcal hydrogen peroxide sensitivities previously reported can be attributed to (i) differences in experimental methods employed or (ii) variation among different gonococcal strains. All of the gonococcal isolates examined generated similar concentrations of catalase, implying that the differences in the H2O2 sensitivity observed may depend on factors in addition to catalase.

Oxygen-dependent modulation of release and activity of polymorphonuclear leukocyte granule products

Polymorphonuclear leukocytes are important in the defense against the anaerobic microflora of infected gingival pockets. One part of this defense is release of antibacterial granule products by polymorphonuclear leukocytes into the pockets. The aim of the present study was to compare the efficiency of polymorphonuclear leukocytes in releasing granule products under aerobic and anaerobic conditions. Polymorphonuclear leukocytes were exposed to serum-opsonized zymosan under aerobic and anaerobic conditions. The levels of released granule products were determined by combining measurements of activity with enzyme-linked immunosorbent assays. The level of released elastase was twice as high in anaerobic as in aerobic reaction mixtures. A similar difference was not detected for myeloperoxidase. However, myeloperoxidase was inactivated after its release under aerobic conditions. The release of lactoferrin was an efficient under aerobic as under anaerobic conditions. The effect of aerobic conditions on the release of elastase and the inactivation of myeloperoxidase could be ascribed to oxidants formed in the myeloperoxidase-H2O2-chloride system. Also, the activity of the released cytoplasmic enzyme lactate dehydrogenase was inactivated by oxidants formed in the myeloperoxidase-H2O2-chloride system. These findings suggest that, in the anaerobic environment of the gingival pocket, elastase and possibly also other azurophilic granule products are released in higher amounts than under fully oxygenated conditions. In this environment, the released products may also escape inactivation by the myeloperoxidase-H2O2-chloride system.