The mechanism of myeloperoxidase-dependent chlorination of monochlorodimedon

A real-time-based in vitro assessment of the oxidative antimicrobial mechanisms of the myeloperoxidase-hydrogen peroxide-halide system

Mammals have evolved a special cellular mechanism for killing invading microbes, which is called the phagocytosis. Neutrophils are the first phagocytosing cells that migrate into the site of infection. In these cells, hypochlorite (HOCl) and other hypohalites, generated in the myeloperoxidase (MPO)-hydrogen peroxide (H₂O₂)-halide system is primarily responsible for oxidative killing. Here, we present a method for assessing these oxidative mechanisms in an in vitro cell-free system in a kinetical real-time-based manner by utilizing a bioluminescent bacterial probe called Escherichia coli-lux. The E. coli-lux method provides a practical tool for assessing the effects of various elementary factors in the MPO-H₂O₂-halide system. Due to the reported versatile intracellular pH and halide concentration during the formation of the phagolysosome and respiratory burst, the antimicrobial activity of the MPO-H₂O₂-halide system undergoes extensive alterations. Here, we show that at a physiological pH or lower, the antimicrobial activity of MPO is high, and the system effectively enhances the H₂O₂-dependent oxidative killing of E. coli by chlorination. The HOCl formed in this reaction is a prominent microbe killer. During the respiratory burst, there is a shift to a more alkaline environment. At pH 7.8, the chlorinating activity of MPO was shown to be absent, and the activity of the HOCl decreased. At this higher pH, the activity of H₂O₂ is enhanced and high enough to kill E. coli without the participation of MPO, and the lowered chloride concentration seemed still to enhance the H₂O₂-dependent killing capacity.

Pharmacophore-based discovery of 2-(phenylamino)aceto-hydrazides as potent eosinophil peroxidase (EPO) inhibitors

There is an increasing interest in developing novel eosinophil peroxidase (EPO) inhibitors, in order to provide new treatment strategies against chronic inflammatory and neurodegenerative diseases caused by eosinophilic disorder. Within this study, a ligand-based pharmacophore model for EPO inhibitors was generated and used for in silico screening of large 3 D molecular structure databases, containing more than 4 million compounds. Hits obtained were clustered and a total of 277 compounds were selected for biological assessment. A class of 2-(phenyl)amino-aceto-hydrazides with different substitution pattern on the aromatic ring was found to contain the most potent EPO inhibitors, exhibiting IC₅₀ values down to 10 nM. The generated pharmacophore model therefore, represents a valuable tool for the selection of compounds for biological testing. The compounds identified as potent EPO inhibitors will serve to initiate a hit to lead and lead optimisation program for the development of new therapeutics against eosinophilic disorders.

Antioxidant Activity/Capacity Measurement. 3. Reactive Oxygen and Nitrogen Species (ROS/RNS) Scavenging Assays, Oxidative Stress Biomarkers, and Chromatographic/Chemometric Assays

There are many studies in which the antioxidant potential of different foods have been analyzed. However, there are still conflicting results and lack of information as a result of unstandardized assay techniques and differences between the principles of the methods applied. The measurement of antioxidant activity, especially in the case of mixtures, multifunctional or complex multiphase systems, cannot be evaluated satisfactorily using a simple antioxidant test due to the many variables influencing the results. In the literature, there are many antioxidant assays that are used to measure the total antioxidant activity/capacity of food materials. In this review, reactive oxygen and nitrogen species (ROS/RNS) scavenging assays are evaluated with respect to their mechanism, advantages, disadvantages, and potential use in food systems. On the other hand, in vivo antioxidant activity (AOA) assays including oxidative stress biomarkers and cellular-based assays are covered within the scope of this review. Finally, chromatographic and chemometric assays are reviewed, focusing on their benefits especially with respect to their time saving, cost-effective, and sensitive nature.

Alkalinity of neutrophil phagocytic vacuoles is modulated by HVCN1 and has consequences for myeloperoxidase activity

The NADPH oxidase of neutrophils, essential for innate immunity, passes electrons across the phagocytic membrane to form superoxide in the phagocytic vacuole. Activity of the oxidase requires that charge movements across the vacuolar membrane are balanced. Using the pH indicator SNARF, we measured changes in pH in the phagocytic vacuole and cytosol of neutrophils. In human cells, the vacuolar pH rose to ~9, and the cytosol acidified slightly. By contrast, in Hvcn1 knock out mouse neutrophils, the vacuolar pH rose above 11, vacuoles swelled, and the cytosol acidified excessively, demonstrating that ordinarily this channel plays an important role in charge compensation. Proton extrusion was not diminished in Hvcn1^-/- mouse neutrophils arguing against its role in maintaining pH homeostasis across the plasma membrane. Conditions in the vacuole are optimal for bacterial killing by the neutral proteases, cathepsin G and elastase, and not by myeloperoxidase, activity of which was unphysiologically low at alkaline pH.

Transiently produced hypochlorite is responsible for the irreversible inhibition of chlorite dismutase

Chlorite dismutases (Clds) are heme b-containing prokaryotic oxidoreductases that catalyze the reduction of chlorite to chloride with the concomitant release of molecular oxygen. Over time, they are irreversibly inactivated. To elucidate the mechanism of inactivation and investigate the role of the postulated intermediate hypochlorite, the pentameric chlorite dismutase of "Candidatus Nitrospira defluvii" (NdCld) and two variants (having the conserved distal arginine 173 exchanged with alanine and lysine) were recombinantly produced in Escherichia coli. Exchange of the distal arginine boosts the extent of irreversible inactivation. In the presence of the hypochlorite traps methionine, monochlorodimedone, and 2-[6-(4-aminophenoxy)-3-oxo-3H-xanthen-9-yl]benzoic acid, the extent of chlorite degradation and release of molecular oxygen is significantly increased, whereas heme bleaching and oxidative modifications of the protein are suppressed. Among other modifications, hypochlorite-mediated formation of chlorinated tyrosines is demonstrated by mass spectrometry. The data obtained were analyzed with respect to the proposed reaction mechanism for chlorite degradation and its dependence on pH. We discuss the role of distal Arg173 by keeping hypochlorite in the reaction sphere for O-O bond formation.

Impact of myeloperoxidase-LDL interactions on enzyme activity and subsequent posttranslational oxidative modifications of apoB-100

Oxidation of LDL by the myeloperoxidase (MPO)-H2O2-chloride system is a key event in the development of atherosclerosis. The present study aimed at investigating the interaction of MPO with native and modified LDL and at revealing posttranslational modifications on apoB-100 (the unique apolipoprotein of LDL) in vitro and in vivo. Using amperometry, we demonstrate that MPO activity increases up to 90% when it is adsorbed at the surface of LDL. This phenomenon is apparently reflected by local structural changes in MPO observed by circular dichroism. Using MS, we further analyzed in vitro modifications of apoB-100 by hypochlorous acid (HOCl) generated by the MPO-H2O2-chloride system or added as a reagent. A total of 97 peptides containing modified residues could be identified. Furthermore, differences were observed between LDL oxidized by reagent HOCl or HOCl generated by the MPO-H2O2-chloride system. Finally, LDL was isolated from patients with high cardiovascular risk to confirm that our in vitro findings are also relevant in vivo. We show that several HOCl-mediated modifications of apoB-100 identified in vitro were also present on LDL isolated from patients who have increased levels of plasma MPO and MPO-modified LDL. In conclusion, these data emphasize the specificity of MPO to oxidize LDL.

A stable bacterial peroxidase with novel halogenating activity and an autocatalytically linked heme prosthetic group

Reconstructing the phylogenetic relationships of the main evolutionary lines of the mammalian peroxidases lactoperoxidase and myeloperoxidase revealed the presence of novel bacterial heme peroxidase subfamilies. Here, for the first time, an ancestral bacterial heme peroxidase is shown to possess a very high bromide oxidation activity (besides conventional peroxidase activity). The recombinant protein allowed monitoring of the autocatalytic peroxide-driven formation of covalent heme to protein bonds. Thereby, the high spin ferric rhombic heme spectrum became similar to lactoperoxidase, the standard reduction potential of the Fe(III)/Fe(II) couple shifted to more positive values (-145 ± 10 mV at pH 7), and the conformational and thermal stability of the protein increased significantly. We discuss structure-function relationships of this new peroxidase in relation to its mammalian counterparts and ask for its putative physiological role.

Redox properties of myeloperoxidase

The heme-containing enzyme myeloperoxidase (MPO) is secreted from polymorphonuclear leukocytes and monocytes. It is involved in host defence and inflammation by oxidation of numerous small molecules. This review summarises our current results on the determination of redox properties of all intermediates involved in the halogenation and peroxidase cycle of MPO. The standard reduction potentials of the redox couples compound I/native MPO, compound I/compound II of MPO, and compound II/native MPO have been determined to be 1.16 V, 1.35 V, and 0.97 V, respectively, at pH 7 and 25 degrees C. Thus, for the first time, a full description of these important thermodynamic parameters of myeloperoxidase has been performed, allowing a better understanding of its extraordinary reactivity.

Measuring chlorine bleach in biology and medicine

BACKGROUND

Chlorine bleach, or hypochlorous acid, is the most reactive two-electron oxidant produced in appreciable amounts in our bodies. Neutrophils are the main source of hypochlorous acid. These champions of the innate immune system use it to fight infection but also direct it against host tissue in inflammatory diseases. Neutrophils contain a rich supply of the enzyme myeloperoxidase. It uses hydrogen peroxide to convert chloride to hypochlorous acid.

SCOPE OF REVIEW

We give a critical appraisal of the best methods to measure production of hypochlorous acid by purified peroxidases and isolated neutrophils. Robust ways of detecting it inside neutrophil phagosomes where bacteria are killed are also discussed. Special attention is focused on reaction-based fluorescent probes but their visual charm is tempered by stressing their current limitations. Finally, the strengths and weaknesses of biomarker assays that capture the footprints of chlorine in various pathologies are evaluated.

MAJOR CONCLUSIONS

Detection of hypochlorous acid by purified peroxidases and isolated neutrophils is best achieved by measuring accumulation of taurine chloramine. Formation of hypochlorous acid inside neutrophil phagosomes can be tracked using mass spectrometric analysis of 3-chlorotyrosine and methionine sulfoxide in bacterial proteins, or detection of chlorinated fluorescein on ingestible particles. Reaction-based fluorescent probes can also be used to monitor hypochlorous acid during phagocytosis. Specific biomarkers of its formation during inflammation include 3-chlorotyrosine, chlorinated products of plasmalogens, and glutathione sulfonamide.

GENERAL SIGNIFICANCE

These methods should bring new insights into how chlorine bleach is produced by peroxidases, reacts within phagosomes to kill bacteria, and contributes to inflammation. This article is part of a Special Issue entitled Current methods to study reactive oxygen species - pros and cons and biophysics of membrane proteins. Guest Editor: Christine Winterbourn.

Acetaminophen (paracetamol) inhibits myeloperoxidase-catalyzed oxidant production and biological damage at therapeutically achievable concentrations

The heme peroxidase enzyme myeloperoxidase (MPO) is released by activated neutrophils and monocytes, where it uses hydrogen peroxide (H(2)O(2)) to catalyze the production of the potent oxidants hypochlorous acid (HOCl), hypobromous acid (HOBr) and hypothiocyanous acid (HOSCN) from halide and pseudohalide (SCN(-)) ions. These oxidants have been implicated as key mediators of tissue damage in many human inflammatory diseases including atherosclerosis, asthma, rheumatoid arthritis, cystic fibrosis and some cancers. It is shown here that acetaminophen (paracetamol), a phenol-based drug with analgesic and antipyretic actions, is an efficient inhibitor of HOCl and HOBr generation by isolated MPO-H(2)O(2)-halide systems. With physiological halide concentrations, acetaminophen concentrations required for 50% inhibition of oxidant formation (IC(50)) were 77+/-6microM (100mMCl(-)) and 92+/-2microM (100mMCl(-) plus 100microMBr(-)), as measured by trapping of oxidants with taurine. The IC(50) for inhibition of HOCl generation by human neutrophils was ca. 100microM. These values are lower than the maximal therapeutic plasma concentrations of acetaminophen (< or =150microM) resulting from typical dosing regimes. Acetaminophen did not diminish superoxide generation by neutrophils, as measured by lucigenin-dependent chemiluminescence. Inhibition of HOCl production was associated with the generation of fluorescent acetaminophen oxidation products, consistent with acetaminophen acting as a competitive substrate of MPO. Inhibition by acetaminophen was maintained in the presence of heparan sulfate and extracellular matrix, materials implicated in the sequestration of MPO at sites of inflammation in vivo. Overall, these data indicate that acetaminophen may be an important modulator of MPO activity in vivo.

Essential role of proximal histidine-asparagine interaction in mammalian peroxidases

In heme enzymes belonging to the peroxidase-cyclooxygenase superfamily the proximal histidine is in close interaction with a fully conserved asparagine. The crystal structure of a mixture of glycoforms of myeloperoxidase (MPO) purified from granules of human leukocytes prompted us to revise the orientation of this asparagine and the protonation status of the proximal histidine. The data we present contrast with previous MPO structures, but are strongly supported by molecular dynamics simulations. Moreover, comprehensive analysis of published lactoperoxidase structures suggest that the described proximal heme architecture is a general structural feature of animal heme peroxidases. Its importance is underlined by the fact that the MPO variant N421D, recombinantly expressed in mammalian cell lines, exhibited modified spectral properties and diminished catalytic activity compared with wild-type recombinant MPO. It completely lost its ability to oxidize chloride to hypochlorous acid, which is a characteristic feature of MPO and essential for its role in host defense. The presented crystal structure of MPO revealed further important differences compared with the published structures including the extent of glycosylation, interaction between light and heavy polypeptides, as well as heme to protein covalent bonds. These data are discussed with respect to biosynthesis and post-translational maturation of MPO as well as to its peculiar biochemical and biophysical properties.

Effects of MCI-186 upon neutrophil-derived active oxygens

Reactions of 3-methyl-1-phenyl-2-pyrazoline-5-one (MCI-186) with hypochlorous acid and superoxide were analysed by spectrophotometry and mass spectrometry. The results were applied to the neutrophil system to evaluate the scavenging activity of neutrophil-derived active oxygen species by MCI-186. MCI-186 reacted rapidly with hypochlorous acid (1 x 10(6) M(-1)s(-1)) to form a chlorinated intermediate, followed by a slow conversion to a new spectrum. MCI-186 consumed 3 moles of hypochlorous acid and did not react with superoxide. The newly synthesized fluorescence probes, 2-[6-(4'-amino)-phenoxy-3H-xanthen-3-on-9-yl]benzoic acid (APF) and 2-[6-(4'-hydroxy)phenoxy-3H-anthen-3-on-9-yl]benzoic acid (HPF) successfully detected neutrophil-derived active oxygens (Setsukinai K, Urano Y, Kakinuma K, Majima HJ, Nagano T. Development of novel fluorescence probes that can reliably detect reactive oxygen species and distinguish specific species. J Biol Chem 2003; 278: 3170-3175). The rate constants for the reaction of hypochlorous acid with MCI-186 and fluorescence probes was in the order of MCI-186 > APF > HPF. Fluorescence due to the oxidation of APF and HPF was observed with the stimulated neutrophils. The result that the intensity from APF oxidation was higher than that from HPF oxidation is compatible with reports that APF selectively reacts with hypochlorous acid. Fluorescence due to oxidation of both APF and HPF decreased when the reactions were carried out in the presence of a fluorescence probe and MCI-186 in a dose-dependent manner. These results indicate that MCI-186 effectively scavenges neutrophil-derived hypochlorous acid and other active oxygens.

Myeloperoxidase: a target for new drug development?

Myeloperoxidase (MPO), a member of the haem peroxidase-cyclooxygenase superfamily, is abundantly expressed in neutrophils and to a lesser extent in monocytes and certain type of macrophages. MPO participates in innate immune defence mechanism through formation of microbicidal reactive oxidants and diffusible radical species. A unique activity of MPO is its ability to use chloride as a cosubstrate with hydrogen peroxide to generate chlorinating oxidants such as hypochlorous acid, a potent antimicrobial agent. However, evidence has emerged that MPO-derived oxidants contribute to tissue damage and the initiation and propagation of acute and chronic vascular inflammatory disease. The fact that circulating levels of MPO have been shown to predict risks for major adverse cardiac events and that levels of MPO-derived chlorinated compounds are specific biomarkers for disease progression, has attracted considerable interest in the development of therapeutically useful MPO inhibitors. Today, detailed information on the structure of ferric MPO and its complexes with low- and high-spin ligands is available. This, together with a thorough understanding of reaction mechanisms including redox properties of intermediates, enables a rationale attempt in developing specific MPO inhibitors that still maintain MPO activity during host defence and bacterial killing but interfere with pathophysiologically persistent activation of MPO. The various approaches to inhibit enzyme activity of MPO and to ameliorate adverse effects of MPO-derived oxidants will be discussed. Emphasis will be put on mechanism-based inhibitors and high-throughput screening of compounds as well as the discussion of physiologically useful HOCl scavengers.

The vinyl-sulfonium bond in human myeloperoxidase: Impact on compound I formation and reduction by halides and thiocyanate

In human myeloperoxidase (MPO) the heme is covalently attached to the protein via two ester linkages and a unique sulfonium ion linkage between the sulfur atom of Met243 and the beta-carbon of the vinyl ring on pyrrole ring A. Here, we have investigated the variant Met243Val produced in Chinese hamster ovary cells in order to elucidate the role of the electron withdrawing sulfonium bond in compound I formation and reduction. Disruption of this MPO-typical bond causes a blue-shifted UV-vis spectrum and an increase in the heme flexibility. This had no impact on compound I formation mediated by hydrogen peroxide (2.2x10(7) M(-1)s(-1) at pH 7.0 and 25 degrees C). Compared with wild-type recombinant MPO the cyanide association rate with ferric Met243Val was significantly enhanced as were also the calculated apparent bimolecular compound I reduction rates by iodide (>10(8) M(-1)s(-1)) and thiocyanate (>10(8) M(-1)s(-1)). By contrast, the overall chlorination and bromination activities were decreased by 98.1% and 87.4%, respectively, compared with the wild-type protein. Compound I reduction by chloride was slower than compound I decay to a compound II-like species (0.4 s(-1)), whereas compound I reduction by bromide was about 10-times slower (1.3x10(4) M(-1)s(-1)) than the wild-type rate. These findings are discussed with respect to the known crystal structure of MPO and its bromide complex as well as the known redox chemistry of its intermediates and substrates.

Disruption of the aspartate to heme ester linkage in human myeloperoxidase: impact on ligand binding, redox chemistry, and interconversion of redox intermediates

In human heme peroxidases the prosthetic group is covalently attached to the protein via two ester linkages between conserved glutamate and aspartate residues and modified methyl groups on pyrrole rings A and C. Here, monomeric recombinant myeloperoxidase (MPO) and the variants D94V and D94N were produced in Chinese hamster ovary cell lines. Disruption of the Asp(94) to heme ester bond decreased the one-electron reduction potential E'(0) [Fe(III)/Fe(II)] from 1 to -55 mV at pH 7.0 and 25 degrees C, whereas the kinetics of binding of low spin ligands and of compound I formation was unaffected. By contrast, in both variants rates of compound I reduction by chloride and bromide (but not iodide and thiocyanate) were substantially decreased compared with the wild-type protein. Bimolecular rates of compound II (but not compound I) reduction by ascorbate and tyrosine were slightly diminished in D94V and D94N. The presented biochemical and biophysical data suggest that the Asp(94) to heme linkage is no precondition for the autocatalytic formation of the other two covalent links found in MPO. The findings are discussed with respect to the known active site structure of MPO and its complexes with ligands.

Reactions of Superoxide with Myeloperoxidase

When neutrophils ingest bacteria, they discharge superoxide and myeloperoxidase into phagosomes. Both are essential for killing of the phagocytosed micro-organisms. It is generally accepted that superoxide is a precursor of hydrogen peroxide which myeloperoxidase uses to oxidize chloride to hypochlorous acid. Previously, we demonstrated that superoxide modulates the chlorination activity of myeloperoxidase by reacting with its ferric and compound II redox states. In this investigation we used pulse radiolysis to determine kinetic parameters of superoxide reacting with redox forms of myeloperoxidase and used these data in a steady-state kinetic analysis. We provide evidence that superoxide reacts with compound I and compound III. Our estimates of the rate constants for the reaction of superoxide with compound I, compound II, and compound III are 5 x 10(6) M-1 s-1, 5.5 +/- 0.4 x 10(6) M-1 s-1, and 1.3 +/- 0.2 x 10(5) M-1 s-1, respectively. These reactions define new activities for myeloperoxidase. It will act as a superoxide dismutase when superoxide reacts consecutively with ferric myeloperoxidase and compound III. It will also act as a superoxidase by using hydrogen peroxide to oxidize superoxide via compound I and compound II. The favorable kinetics of these reactions indicate that, within the confines of a phagosome, superoxide will react with myeloperoxidase and affect the reactions it will catalyze. These interactions of superoxide and myeloperoxidase will have a major influence on the way neutrophils use oxygen to kill bacteria. Consequently, superoxide should be viewed as a cosubstrate that myeloperoxidase uses to elicit bacterial killing.

Modeling the reactions of superoxide and myeloperoxidase in the neutrophil phagosome: implications for microbial killing

Neutrophils kill bacteria by ingesting them into phagosomes where superoxide and cytoplasmic granule constituents, including myeloperoxidase, are released. Myeloperoxidase converts chloride and hydrogen peroxide to hypochlorous acid (HOCl), which is strongly microbicidal. However, the role of oxidants in killing and the species responsible are poorly understood and the subject of current debate. To assess what oxidative mechanisms are likely to operate in the narrow confines of the phagosome, we have used a kinetic model to examine the fate of superoxide and its interactions with myeloperoxidase. Known rate constants for reactions of myeloperoxidase have been used and substrate concentrations estimated from neutrophil morphology. In the model, superoxide is generated at several mm/s. Most react with myeloperoxidase, which is present at millimolar concentrations, and rapidly convert the enzyme to compound III. Compound III turnover by superoxide is essential to maintain enzyme activity. Superoxide stabilizes at approximately 25 microM and hydrogen peroxide in the low micromolar range. HOCl production is efficient if there is adequate chloride supply, but further knowledge on chloride concentrations and transport mechanisms is needed to assess whether this is the case. Low myeloperoxidase concentrations also limit HOCl production by allowing more hydrogen peroxide to escape from the phagosome. In the absence of myeloperoxidase, superoxide increases to >100 microM but hydrogen peroxide to only approximately 30 microM. Most of the HOCl reacts with released granule proteins before reaching the bacterium, and chloramine products may be effectors of its antimicrobial activity. Hydroxyl radicals should form only after all susceptible protein targets are consumed.

pH-dependent regulation of myeloperoxidase activity

The balance between peroxidase and chlorinating activities of myeloperoxidase (MPO) is very important for the enhancement of antimicrobial action and prevention of damage caused by hypochlorite. In the present paper, the peroxidase and chlorinating activities have been studied at various pH values. The possibility of using neutrophil protein solution for the evaluation of MPO activity has been demonstrated. It is shown that at neutral pH MPO had higher affinity to peroxidase substrate guaiacol: at pH 7.4, chloride ions did not compete with guaiacol up to the concentration of 150 mM. At acidic pH, chlorinating activity of MPO dominates: only hypochlorite production can be detected at equal chloride and guaiacol concentrations of 15 mM. However, horseradish peroxidase does not exhibit any difference in activity in the presence of chloride ions even at acidic pH values. It was demonstrated by MALDI-TOF mass-spectrometry that the amount of hypochlorite produced is sufficient to modify phospholipids (with formation of Cl- and Br-hydrins and lyso-derivatives) only at acidic pH (5.0). Thus, in the presence of phenolic peroxidase substrate, MPO chlorinating activity can be displayed at acidic pH only. It can lead to elimination of hypochlorite production in normal tissues at neutral pH (7.4) and its enhancement in phagosomes where the pH range is 4.7-6.0.

Role of the covalent glutamic acid 242-heme linkage in the formation and reactivity of redox intermediates of human myeloperoxidase

In human myeloperoxidase the heme is covalently attached to the protein via two ester linkages between the carboxyl groups of Glu242 and Asp94 and modified methyl groups on pyrrole rings A and C of the heme as well as a sulfonium ion linkage between the sulfur atom of Met243 and the beta-carbon of the vinyl group on pyrrole ring A. In the present study, wild-type recombinant myeloperoxidase (recMPO) and the variant Glu242Gln were produced in Chinese hamster ovary cells and investigated in a comparative sequential-mixing stopped-flow study in order to elucidate the role of the Glu242-heme ester linkage in the individual reaction steps of both the halogenation and peroxidase cycle. Disruption of the ester bond increased heme flexibility, blue shifted the UV-vis spectrum, and, compared with recMPO, decelerated cyanide binding (1.25 x 10(4) versus 1.6 x 10(6) M(-)(1) s(-)(1) at pH 7 and 25 degrees C) as well as compound I formation mediated by either hydrogen peroxide (7.8 x 10(5) versus 1.9 x 10(7) M(-)(1) s(-)(1)) or hypochlorous acid (7.5 x 10(5) versus 2.3 x 10(7) M(-)(1) s(-)(1)). The overall chlorination and bromination activity of Glu242Gln was 2.0% and 24% of recMPO. The apparent bimolecular rate constants of compound I reduction by chloride (65 M(-)(1) s(-)(1)), bromide (5.4 x 10(4) M(-)(1) s(-)(1)), iodide (6.4 x 10(5) M(-)(1) s(-)(1)), and thiocyanate (2.2 x10(5) M(-)(1) s(-)(1)) were 500, 25, 21, and 63 times decreased compared with recMPO. By contrast, Glu242Gln compound I reduction by tyrosine was only 5.4 times decreased, whereas tyrosine-mediated compound II reduction was 60 times slower compared with recMPO. The effects of exchange of Glu242 on electron transfer reactions are discussed.

Interaction of hypohalous acids and heme peroxidases with unsaturated phosphatidylcholines

The formation of chlorohydrins, bromohydrins, and iodohydrins from 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) by the myeloperoxidase-hydrogen peroxide-halide system was evaluated by means of matrix-assisted laser desorption/ionisation time-of-flight (MALDI-TOF) mass spectrometry. This approach allows to detect different kinds of the halogenation reaction even in one mass spectrum. Using a mixture of Cl-, Br-, I-, and SCN- at physiological concentrations, a bromination of POPC dominates by the MPO-hydrogen peroxide-halide system. Hypothiocyanite does apparently not react with the double bond of POPC, but increasing amounts of SCN- cause a decrease of the bromohydrin peaks. An interconversion between different hypohalous acids produced by the myeloperoxidase-hydrogen peroxide-halide system determines the pattern of halogenohydrins in POPC. Especially, hypochlorous acid is able to oxidise Br- to hypobromous acid.

Kinetics of chlorination of monochlorodimedone by myeloperoxidase

The phagocyte-derived enzyme myeloperoxidase has been recently implicated in the pathogenesis of atherosclerosis, because it catalyzes the reaction of hydrogen peroxide with chloride ions to give the highly toxic oxidant hypochlorous acid. The aim of this study was to determine the dependence of this reaction on the concentration of hydrogen peroxide and of the enzyme by means of the photometric monochlorodimedone assay. The initial rate of hypochlorous acid formation increased less than proportionally with increasing myeloperoxidase concentrations. Variation of the concentration of hydrogen peroxide had a biphasic effect, with an optimal concentration of hydrogen peroxide. Above this concentration enzyme destruction is apparently predominant. The progress curves of hypochlorous acid formation showed two distinct maxima. It was concluded that hypochlorous acid not only reacts with monochlorodimedone but also with the amino groups of myeloperoxidase to form intermediary chloramines that may further chlorinate monochlorodimedone. This was supported by the kinetics in the presence of the amino compound glycine, a competitive substrate for chlorination by hypochlorous acid. In the presence of high concentrations of glycine the progress curve rises continuously, yielding a greatly increased concentration of chlorinating species, either hypochlorous acid or chloramines. We concluded that glycine protects myeloperoxidase against hypochlorous acid-induced self-destruction.

Dissociation of DNA double strand by hypohalous acids

Calf thymus DNA was treated with authentic HOCl, and hypohalous acid-generating systems. This caused a decrease in fluorescence of ethidium-DNA complexes when ethidium bromide was subsequently added to the DNA. The fluorescence continued to decrease up to 30 min after adding HOCl. Loss in fluorescence was proportional to the concentration of HOCl and was complete when a 3-fold excess of HOCl was added to the DNA. No significant decrease in the fluorescence was observed when the chlorination was carried out in the presence of a concentration of monochlorodimedone (MCD) equivalent to that of HOCl. MCD is known to react stoichiometrically with HOCl. The decrease in fluorescence was completely inhibited by H2O2, ascorbate and glutathione (GSH). We have estimated the rate constant for the reaction of HOCl with H2O, to be 1-2 x 10(5) M(-1)s(-1). When compared with authentic HOCl, HOCl-generating systems (Cl + H2O2 + MPO or chloroperoxidase) were found to be inefficient in damaging DNA. This result most likely arises because the rate constant for reaction of HOCl with H2O2 is about 1000-fold faster than that for the reaction with DNA. HOBr and HOI generating systems also had a limited ability to damage DNA. We conclude that good chlorine acceptors and antioxidants protect DNA from hypohalous acid-induced oxidative damage.

Comparison of HOCl traps with myeloperoxidase inhibitors in prevention of low density lipoprotein oxidation

In this study, the production of the highly toxic oxidant hypochlorous acid (HOCl) by the phagocytic enzyme myeloperoxidase (MPO) was quantitated and the concomitant alterations of low density lipoprotein (LDL) were analyzed in view of the potential role of LDL in atherosclerosis. Using the monochlorodimedone assay, it was found that HOCl is produced in micromolar concentrations. The kinetics of the decrease of tryptophan fluorescence appeared to be a sensitive method to monitor LDL alterations under near in vivo conditions. Therefore, this method was used to subsequently compare the effectiveness of MPO inhibitors that block production of HOCl with compounds that act as HOCl traps. The efficiency of MPO inhibitors to prevent LDL damage increased in the series benzohydroxamic acid < salicylhydroxamic acid < 3-amino-1,2,4-triazole < sodium azide < potassium cyanide < p-hydroxy-benzoic acid hydrazide, while for the HOCl traps the protective efficiency increased in the series glycine < taurine < methionine. We conclude that HOCl traps may have high potential therapeutic impact in vivo due to their low toxicity, although high concentrations of them would have to reach sites of inflammation. In contrast, only low concentrations of a specific MPO inhibitor would be required to irreversibly inhibit the enzyme.

Mechanism of reaction of myeloperoxidase with nitrite

Myeloperoxidase (MPO) is a major neutrophil protein and may be involved in the nitration of tyrosine residues observed in a wide range of inflammatory diseases that involve neutrophils and macrophage activation. In order to clarify if nitrite could be a physiological substrate of myeloperoxidase, we investigated the reactions of the ferric enzyme and its redox intermediates, compound I and compound II, with nitrite under pre-steady state conditions by using sequential mixing stopped-flow analysis in the pH range 4-8. At 15 degrees C the rate of formation of the low spin MPO-nitrite complex is (2.5 +/- 0.2) x 10(4) m(-1) s(-1) at pH 7 and (2.2 +/- 0.7) x 10(6) m(-1) s(-1) at pH 5. The dissociation constant of nitrite bound to the native enzyme is 2.3 +/- 0.1 mm at pH 7 and 31.3 +/- 0.5 micrometer at pH 5. Nitrite is oxidized by two one-electron steps in the MPO peroxidase cycle. The second-order rate constant of reduction of compound I to compound II at 15 degrees C is (2.0 +/- 0.2) x 10(6) m(-1) s(-1) at pH 7 and (1.1 +/- 0.2) x 10(7) m(-1) s(-1) at pH 5. The rate constant of reduction of compound II to the ferric native enzyme at 15 degrees C is (5.5 +/- 0.1) x 10(2) m(-1) s(-1) at pH 7 and (8.9 +/- 1.6) x 10(4) m(-1) s(-1) at pH 5. pH dependence studies suggest that both complex formation between the ferric enzyme and nitrite and nitrite oxidation by compounds I and II are controlled by a residue with a pK(a) of (4.3 +/- 0.3). Protonation of this group (which is most likely the distal histidine) is necessary for optimum nitrite binding and oxidation.

The sulfonium ion linkage in myeloperoxidase. Direct spectroscopic detection by isotopic labeling and effect of mutation

The heme group of myeloperoxidase is covalently linked via two ester bonds to the protein and a unique sulfonium ion linkage involving Met(243). Mutation of Met(243) into Thr, Gln, and Val, which are the corresponding residues of eosinophil peroxidase, lactoperoxidase, and thyroid peroxidase, respectively, and into Cys was performed. The Soret band in the optical absorbance spectrum in the oxidized mutants is now found at approximately 411 nm. Both the pyridine hemochrome spectra and resonance Raman spectra of the mutants are affected by the mutation. In the Met(243) mutants the affinity for chloride has decreased 100-fold. All mutants have lost their chlorination activity, except for the M243T mutant, which still has 15% activity left. By Fourier transform infared difference spectroscopy it was possible to specifically detect the (13)CD(3)-labeled methionyl sulfonium ion linkage. We conclude that the sulfonium ion linkage serves two roles. First, it serves as an electron-withdrawing substituent via its positive charge, and, second, together with its neighboring residue Glu(242), it appears to be responsible for the lower symmetry of the heme group and distortion from the planar conformation normally seen in heme-containing proteins.

Characterization of the Asp94 and Glu242 mutants in myeloperoxidase, the residues linking the heme group via ester bonds

The heme group of all mammalian peroxidases is covalently linked to the protein matrix via two esterbonds, as we have recently shown by Fourier transform infrared (FTIR) difference spectroscopy [Kooter, I. M., Pierik, A.J., Merkx, M., Averill, B.A., Moguilevsky, N., Bollen, A. & Wever, R. (1997) J. Am. Chem. Soc. 119, 11542-11543]. We have examined the effects of mutation of Asp94 and Glu242, responsible for those ester bonds in myeloperoxidase, on the spectroscopic properties and catalytic activity of this enzyme. Mutation of Asp94 in myeloperoxidase results in two species. The first species has spectroscopic characteristics similar to that of wild-type myeloperoxidase. The second species has spectroscopic characteristics similar to that of Met243-->Gln mutant, and it is therefore concluded that, besides loss of the ester bond involving Asp94, this species also has lost the sulfonium ion linkage that is also characteristic of myeloperoxidase. The Asp94-->Asn mutant still has about 30% residual peroxidase activity while for the Asp94-->Val mutant only a few percentage activity is left. When Glu242 is mutated the sulfonium ion linkage is not affected, but this residue together with its neighbouring residue Met243, according to resonance Raman spectra, is responsible for the low symmetry of the heme group. Mutation of either of these residues results in loss of the bowed distortion from the planar conformation, and in a heme group with higher symmetry. For the Glu242-->Gln mutant 8% residual peroxidase activity is found.

Thiocyanate and chloride as competing substrates for myeloperoxidase

The neutrophil enzyme myeloperoxidase uses H2O2 to oxidize chloride, bromide, iodide and thiocyanate to their respective hypohalous acids. Chloride is considered to be the physiological substrate. However, a detailed kinetic study of its substrate preference has not been undertaken. Our aim was to establish whether myeloperoxidase oxidizes thiocyanate in the presence of chloride at physiological concentrations of these substrates. We determined this by measuring the rate of H2O2 loss in reactions catalysed by the enzyme at various concentrations of each substrate. The relative specificity constants for chloride, bromide and thiocyanate were 1:60:730 respectively, indicating that thiocyanate is by far the most favoured substrate for myeloperoxidase. In the presence of 100 mM chloride, myeloperoxidase catalysed the production of hypothiocyanite at concentrations of thiocyanate as low as 25 microM. With 100 microM thiocyanate, about 50% of the H2O2 present was converted into hypothiocyanite, and the rate of hypohalous acid production equalled the sum of the individual rates obtained when each of these anions was present alone. The rate of H2O2 loss catalysed by myeloperoxidase in the presence of 100 mM chloride doubled when 100 microM thiocyanate was added, and was maximal with 1mM thiocyanate. This indicates that at plasma concentrations of thiocyanate and chloride, myeloperoxidase is far from saturated. We conclude that thiocyanate is a major physiological substrate of myeloperoxidase, regardless of where the enzyme acts. As a consequence, more consideration should be given to the oxidation products of thiocyanate and to the role they play in host defence and inflammation.

Neutrophils convert tyrosyl residues in albumin to chlorotyrosine

Hypochlorous acid chlorinates tyrosyl residues in small peptides to produce chlorotyrosine. Detection of chlorotyrosine has the potential to unequivocally identify the contribution hypochlorous acid makes to inflammation. I have developed a selective and sensitive HPLC assay for measuring chlorotyrosine. When albumin was exposed to reagent hypochlorous acid, or that produced by myeloperoxidase and stimulated neutrophils, tyrosyl residues in the protein were converted to chlorotyrosine. About 2% of the hypochlorous acid generated by neutrophils was accounted for by the formation of chlorotyrosine. These results demonstrate that chlorotyrosine will be a useful marker for establishing a role for hypochlorous acid in host defence and inflammation.

Chlorination of tyrosyl residues in peptides by myeloperoxidase and human neutrophils

Hypochlorous acid is the major strong oxidant generated by human neutrophils, and it has the potential to cause much of the tissue damage that these inflammatory cells promote. It is produced from hydrogen peroxide and chloride by the heme enzyme myeloperoxidase. To unequivocally establish that hypochlorous acid contributes to inflammation, a stable and unique marker for its reaction with biomolecules needs to be identified. In this investigation we have found that reagent hypochlorous acid reacts with tyrosyl residues in small peptides and converts them to chlorotyrosine. Purified myeloperoxidase in combination with hydrogen peroxide and chloride, as well as stimulated human neutrophils, chlorinated tyrosine in the peptide Gly-Gly-Tyr-Arg. Rather than reacting directly with the aromatic ring of tyrosine, hypochlorous acid initially reacted with an amine group of the peptide to form a chloramine. The chloramine then underwent an intramolecular reaction with the tyrosyl residue to convert it to chlorotyrosine. This indicates that tyrosyl residues in proteins that are close to amine groups will be susceptible to chlorination. Peroxidases are the only enzymes capable of chlorinating an aromatic ring. Furthermore, myeloperoxidase is the only human enzyme that produces hypochlorous acid under physiological conditions. Therefore, chlorotyrosine will be a specific marker for the production of hypochlorous acid in vivo and for the involvement of myeloperoxidase in inflammatory tissue damage.

The characterization of antioxidants

The role of antioxidants in nutrition is an area of increasing interest. Antioxidants are used (1) to prolong the shelf life and maintain the nutritional quality of lipid-containing foods, and (2) to modulate the consequences of oxidative damage in the human body. This review discusses what an antioxidant is and how the properties of antioxidants may be characterized.

Myeloperoxidase-mediated activation of xenobiotics by human leukocytes

Peripheral blood leukocytes contain a variety of enzymes that are capable of metabolising xenobiotics. The enzyme myeloperoxidase (MPO) appears to be the most important for drug metabolism. MPO is a peroxidase/oxidase and generates the powerful oxidant hypochlorous acid. MPO- or MPO-generated oxidants are capable of oxidizing a wide variety of compounds and a broad range of functional groups, especially those that contain nitrogen and sulfur. Leukocytes have a role in immune response; therefore, reactive intermediates generated by leukocyte metabolism of xenobiotics may have a role in idiosyncratic drug reactions, particularly those that are immune-mediated such as drug-induced lupus or agranulocytosis.

Oxidation of desferrioxamine to nitroxide free radical by activated human neutrophils

Human neutrophils activated by PMA were found to induce the formation of a nitroxide radical from DFO. The presence of SOD was necessary to permit the formation of the DFO radical. The inactive phorbol ester did not induce DFO radical, and DL-sphinganine suppressed the radical produced by the active phorbol ester. Other cell stimuli (Zymocel and the chemotactic peptide) also induced the formation of the DFO radical, although radical concentration was very much lower than with PMA. Participation of NO, OH or 1O2 was ruled out by the inability of NG-methyl-L-arginine, NG-nitro-L-arginine, DMSO, mannitol, histidine, and methionine to inhibit the formation of DFO radical produced by PMA-activated cells. Furthermore, PMA-activated cells did not produce detectable levels of NO2-, a stable oxidation product of NO, and D2O, which enhances the lifetime of singlet oxygen, did not modify the intensity or the lifetime of DFO radical. The involvement of cell MPO was suggested by the inhibition of the DFO radical observed after treatment with catalase or with antihuman MPO antibodies. Also, HOCl was found to induce the DFO radical in cell-free reactions, but our data indicate that the reaction leading to DFO radical formation by neutrophils involves the reduction of MPO compound II back to the active enzyme (ferric-MPO). Anti-inflammatory drugs strongly increased the DFO radical produced by activated neutrophils. On the contrary, none of these drugs was able to increase the DFO radical produced by HOCl. Histidine and methionine that inhibited the DFO radical intensity in cell-free reactions, were shown to act directly on HOCl. Experiments with MPO-H2O2 in SOD- and Cl(-)-free conditions showed the formation of DFO radical and confirmed the hypothesis of the involvement of compound II. The conversion of compound II to ferric MPO by DFO optimized the enzymatic activity of neutrophils, and in the presence of monochlorodimedon (compound II promoting agent) we measured an increased HOCl production. When DFO was modified by conjugation with hydroxyethyl starch, it lost the ability to produce the radical either by neutrophils or by MPO-H2O2 and did not increase HOCl production. The inability of these DFO derivatives to produce potentially toxic species might explain their reported lower toxicity in vivo.

Effect of NPC 15669, an inhibitor of neutrophil recruitment and neutrophil-mediated inflammation, on neutrophil function in vitro

The new anti-inflammatory agent N-[9H-(2,7-dimethylfluorenyl-9-methoxy)carbonyl]-L-leucine (NPC 15669) inhibits inflammation in several animal models dependent upon neutrophil activation and recruitment into the inflammatory lesion. NPC 15669 appears to elicit its pharmacological action by inhibiting the cell surface expression of CD11b/CD18 (Mac-1) on the neutrophil and subsequent adhesion of the neutrophil to the vascular endothelium. The current study sought to further characterize the action of NPC 15669 on neutrophil function. In the range of 1-100 microM, this fluorene enhanced superoxide production in a concentration-dependent fashion. Using spin trapping/ESR spectroscopy, NPC 15669 was found to inhibit myeloperoxidase (MPO)-dependent hydroxyl radical primarily by scavenging hypochlorous acid, and secondarily by inhibiting agonist-stimulated degranulation as assessed by MPO and elastase release. These studies demonstrated that NPC 15669, in addition to inhibiting adhesion, alters other neutrophil functions. Whether the pharmacological activities described for NPC 15669 resulted directly from changes in Mac-1 expression or through some other mechanism is currently under investigation.

Oxidant-scavenging activities of ampicillin and sulbactam and their effects on neutrophil functions

Luminol-enhanced luminescence is a method used to measure formation of reactive oxygen intermediates important in the ability of neutrophils to kill microbes. Several studies have demonstrated that under some conditions of incubation, ampicillin can inhibit neutrophil-derived luminol-enhanced luminescence. We evaluated the mechanism(s) by which ampicillin inhibited the luminescent response of stimulated neutrophils. We also investigated sulbactam, a beta-lactamase inhibitor which has been given in combination with ampicillin and other beta-lactam antibiotics to increase their spectra, for possible similar effects. Both ampicillin and sulbactam attenuated luminol-enhanced luminescence by approximately 40%. Superoxide production was not prevented by added ampicillin, nor was superoxide scavenged by it. Myeloperoxidase reacts with H2O2 and Cl- to generate OCl-, which is believed to be the oxidizer of luminol that is primarily responsible for enhancement of neutrophil-derived luminescence. Hydroxyl radicals (HO.), which may also oxidize luminol, resulting in luminescence, can be formed from O2- and H2O2 via either myeloperoxidase-dependent (involving intermediate OCl-) or myeloperoxidase-independent (through a metal ion catalyst) reactions. Ampicillin scavenged H2O2 and OCl- and prevented 95% of Fenton reaction-generated HO. from reacting with 5,5-dimethyl-1-pyrroline-N-oxide. Sulbactam was found to scavenge OCl- and HO., but less avidly than ampicillin did. Neither ampicillin nor sulbactam inhibited myeloperoxidase activity. Sublethal concentrations of sulbactam had no significant effect on neutrophil killing of Staphylococcus aureus and Escherichia coli. Our results demonstrate a mechanism(s) by which ampicillin inhibits luminol-enhanced luminescence from stimulated neutrophils, namely, through scavenging of the oxidant(s) primarily responsible for the generation of luminescence.

Myeloperoxidase-catalyzed iodination and coupling

Myeloperoxidase (MPO), which displays considerable amino acid sequence homology with thyroid peroxidase (TPO) and lactoperoxidase (LPO), was tested for its ability to catalyze iodination of thyroglobulin and coupling of two diiodotyrosyl residues within thyroglobulin to form thyroxine. After 1 min of incubation in a system containing goiter thyroglobulin, I-, and H2O2, the pH optimum of MPO-catalyzed iodination was markedly acidic (approximately 4.0), compared to LPO (approximately 5.4) and TPO (approximately 6.6). The presence of 0.1 N Cl- or Br- shifted the pH optimum for MPO to about 5.4 but had little or no effect on TPO- or LPO-catalyzed iodination. At pH 5.4, 0.1 N Cl- and 0.1 N Br- had a marked stimulatory effect on MPO-catalyzed iodination. At pH 4.0, however, iodinating activity of MPO was almost completely inhibited by 0.1 N Cl- or Br-. Inhibition of chlorinating activity of MPO by Cl- at pH 4.0 has been previously described. When iodination of goiter thyroglobulin was performed with MPO plus the H2O2 generating system, glucose-glucose oxidase, at pH 7.0, the iodinating activity was markedly increased by 0.1 N Cl-. Under these conditions iodination and thyroxine formation were comparable to values observed with TPO. MPO and TPO were also compared for coupling activity in a system that measures coupling of diiodotyrosyl residues in thyroglobulin in the absence of iodination. MPO displayed very significant coupling activity, and, like TPO, this activity was stimulated by a low concentration of free diiodotyrosine (1 microM). The thioureylene drugs, propylthiouracil and methimazole, inhibited MPO-catalyzed iodination both reversibly and irreversibly, in a manner similar to that previously described for TPO-catalyzed iodination.

Interaction of methyl-xanthines with myeloperoxidase. An anti-inflammatory mechanism

1. Inhibition of myeloperoxidase (MPO)-catalyzed reactions by methyl-substituted xanthines has been investigated. 2. Except for theobromine and caffeine, all xanthines tested were potent inhibitors of the MPO-H2O2-Cl- system. 3. In contrast to methyl substitution in the 1 or 8 position of xanthine, substitution in the 3 or 7 position had a marked effect on the inhibition of MPO catalysis. 4. Two different inhibitory mechanisms were induced; scavenging of hypochlorous acid (HOCl) generated by the MPO system and accumulation of Compound II (ferryl MPO) which is inactive as a catalyst of Cl- oxidation.

Human hemi-myeloperoxidase. Initial chlorinating activity at neutral pH, compound II and III formation, and stability towards hypochlorous acid and high temperature

Human neutrophilic myeloperoxidase (MPO) is involved in the defence mechanism of the body against micro-organisms. The enzyme catalyses the generation of the strong oxidant hypochlorous acid (HOCl) from hydrogen peroxide and chloride ions. In normal neutrophils MPO is present in the dimeric form (140 kDa). The disulphide-linked protomers each consist of a heavy subunit and a light one. Reductive alkylation converts the dimeric enzyme into two promoters, 'hemi-myeloperoxidase'. We studied the initial activities of human dimeric MPO and hemi-MPO at the physiological pH of 7.2 and found no significant differences in chlorinating activity. These results indicate that, at least at neutral pH, the protomers of MPO function independently. The absorption spectra of MPO compounds II and III, both inactive forms concerning HOCl generation, and the rate constants of their formation were the same for dimeric MPO and hemi-MPO, but hemi-MPO required a slightly larger excess of H2O2 for complete conversion. Hemi-MPO was less stable at a high temperature (80 degrees C) as compared to the dimeric enzyme. Furthermore, the resistance of the chlorinating activity of hemi-MPO against its oxidative product hypochlorous acid was somewhat lower (IC50 = 32 microM HOCl) compared to dimeric MPO (IC50 = 50 microM HOCl). The higher stability of dimeric MPO in the presence of its oxidative product compared to that of monomeric MPO might be the reason for the occurrence of MPO as a dimer.

Mechanisms by which clofazimine and dapsone inhibit the myeloperoxidase system. A possible correlation with their anti-inflammatory properties

The mechanisms by which two anti-leprotic drugs (clofazimine and dapsone), both with anti-inflammatory properties, inhibit myeloperoxidase (MPO)-catalysed reactions, were investigated. The disappearance of NADH fluorescence was used as an assay for its oxidation. Chloride stimulated the oxidation of NADH in the MPO-H2O2 system in a concentration-dependent manner (50-fold at 150 mM NaCl). Under these conditions Cl- is oxidized and the oxidant formed, presumably hypochlorous acid (HOCl), oxidizes NADH. Observations demonstrating the effect of the drugs on the MPO system, are: (1) Inhibition of Cl(-)-stimulated oxidation of NADH. (2) Inhibition of polypeptide modification in a model protein, thyroglobulin (TG). (3) Protection of MPO against loss of catalytic activity caused by chlorinating oxidants generated by the system. (4) Inhibition of haemoglobin oxidation. Only dapsone was active here. HPLC analyses suggested that the drugs were not significantly metabolized in the MPO-H2O2 system in the absence of Cl-. Bleaching of clofazimine was stimulated by Cl- in the MPO system, suggesting the involvement of HOCl. Clofazimine was found to be a more potent scavenger of HOCl than dapsone when the inhibition of NADH oxidation by reagent HOCl was used as an assay. This finding is also supported by HPLC analyses which indicated a greater sensitivity of HOCl for clofazimine than for dapsone. Relatively low concentrations of dapsone inhibited the oxidation of oxygenated haemoglobin (HbO2), suggesting that the drug was not metabolized to its N-hydroxylated derivative which is thought to be responsible for methaemoglobin (metHb) formation in vivo. It is proposed that the inhibitory mechanism of action of clofazimine is to scavenge chlorinating oxidants generated by the MPO-Cl(-)-H2O2 system, while dapsone converts MPO into its inactive compound II (ferryl) form. The different inhibitory mechanisms of clofazimine and dapsone towards the MPO system may contribute to the anti-inflammatory actions of the drugs.

Mechanism of inhibition of myeloperoxidase by anti-inflammatory drugs

Hypochlorous acid (HOCl) is the most powerful oxidant produced by human neutrophils, and should therefore be expected to contribute to the damage caused by these inflammatory cells. It is produced from H2O2 and Cl- by the heme enzyme myeloperoxidase (MPO). We used a H2O2-electrode to assess the ability of a variety of anti-inflammatory drugs to inhibit conversion of H2O2 to HOCl. Dapsone, mefenamic acid, sulfapyridine, quinacrine, primaquine and aminopyrine were potent inhibitors, giving 50% inhibition of the initial rate of H2O2 loss at concentrations of about 1 microM or less. Phenylbutazone, piroxicam, salicylate, olsalazine and sulfasalazine were also effective inhibitors. Spectral investigations showed that the inhibitors acted by promoting the formation of compound II, which is an inactive redox intermediate of MPO. Ascorbate reversed inhibition by reducing compound II back to the active enzyme. The characteristic properties that allowed the drugs to inhibit MPO reversibly were ascertained by determining the inhibitory capacity of related phenols and anilines. Inhibition increased as substituents on the aromatic ring became more electron withdrawing, until an optimum reduction potential was reached. Beyond this optimum, their inhibitory capacity declined. The best inhibitor was 4-bromoaniline which had an I50 of 45 nM. An optimum reduction potential enables inhibitors to reduce MPO to compound II, but prevents them from reducing compound II back to the active enzyme. Exploitation of this optimum reduction potential will help in targeting drugs against HOCl-dependent tissue damage.

The influence of superoxide on the production of hypochlorous acid by human neutrophils

Human neutrophils stimulated with opsonized zymosan promoted hypochlorous acid (HOCl)-dependent loss of monochlorodimedon. Formation of HOCl was completely inhibited by catalase, and it was also inhibited up to 70% by SOD. There was no inhibition by desferal, DTPA, mannitol or dimethylsulphoxide, which excluded the involvement of .OH. Our results indicate that generation of O2- by neutrophils enables these cells to enhance their production of HOCl. Furthermore, inhibition of neutrophil processes by SOD and catalase does not necessarily implicate .OH. We propose that O2- may potentiate oxidant damage at inflammatory sites by boosting the myeloperoxidase-dependent production of HOCl.