Comparison of Fecal Calprotectin and Myeloperoxidase in Predicting Outcomes in Inflammatory Bowel Disease

BACKGROUND

Biomarkers have been proposed as surrogate treatment targets for the management of inflammatory bowel disease (IBD); however, their relationship with IBD-related complications remains unclear. This study investigated the utility of neutrophil biomarkers fecal calprotectin (fCal) and fecal myeloperoxidase (fMPO) in predicting a complicated IBD course.

METHODS

Participants with IBD were followed for 24 months to assess for a complicated IBD course (incident corticosteroid use, medication escalation for clinical disease relapse, IBD-related hospitalizations/surgeries). Clinically active IBD was defined as Harvey-Bradshaw index >4 for Crohn's disease (CD) and simple clinical colitis activity index >5 for ulcerative colitis (UC). Area under the receiver-operating-characteristics curves (AUROC) and multivariable logistic regression assessed the performance of baseline symptom indices, fCal, and fMPO in predicting a complicated disease IBD course at 24 months.

RESULTS

One hundred and seventy-one participants were included (CD, n = 99; female, n = 90; median disease duration 13 years [interquartile range, 5-22]). Baseline fCal (250 μg/g; AUROC = 0.77; 95% confidence interval [CI], 0.69-0.84) and fMPO (12 μg/g; AUROC = 0.77; 95% CI, 0.70-0.84) predicted a complicated IBD course. Fecal calprotectin (adjusted OR = 7.85; 95% CI, 3.38-18.26) and fMPO (adjusted OR = 4.43; 95% CI, 2.03-9.64) were associated with this end point after adjustment for other baseline variables including clinical disease activity. C-reactive protein (CRP) was inferior to fecal biomarkers and clinical symptoms (pdifference < .05) at predicting a complicated IBD course. A combination of baseline CRP, fCal/fMPO, and clinical symptoms provided the greatest precision at identifying a complicated IBD course.

CONCLUSIONS

Fecal biomarkers are independent predictors of IBD-related outcomes and are useful adjuncts to routine clinical care.

The Fdb3 transcription factor of the Fusarium Detoxification of Benzoxazolinone gene cluster is required for MBOA but not BOA degradation in Fusarium pseudograminearum

A number of cereals produce the benzoxazolinone class of phytoalexins. Fusarium species pathogenic towards these hosts can typically degrade these compounds via an aminophenol intermediate, and the ability to do so is encoded by a group of genes found in the Fusarium Detoxification of Benzoxazolinone (FDB) cluster. A zinc finger transcription factor encoded by one of the FDB cluster genes (FDB3) has been proposed to regulate the expression of other genes in the cluster and hence is potentially involved in benzoxazolinone degradation. Herein we show that Fdb3 is essential for the ability of Fusarium pseudograminearum to efficiently detoxify the predominant wheat benzoxazolinone, 6-methoxy-benzoxazolin-2-one (MBOA), but not benzoxazoline-2-one (BOA). Furthermore, additional genes thought to be part of the FDB gene cluster, based upon transcriptional response to benzoxazolinones, are regulated by Fdb3. However, deletion mutants for these latter genes remain capable of benzoxazolinone degradation, suggesting that they are not essential for this process.

Degradation of the benzoxazolinone class of phytoalexins is important for virulence of Fusarium pseudograminearum towards wheat

Wheat, maize, rye and certain other agriculturally important species in the Poaceae family produce the benzoxazolinone class of phytoalexins on pest and pathogen attack. Benzoxazolinones can inhibit the growth of pathogens. However, certain fungi can actively detoxify these compounds. Despite this, a clear link between the ability to detoxify benzoxazolinones and pathogen virulence has not been shown. Here, through comparative genome analysis of several Fusarium species, we have identified a conserved genomic region around the FDB2 gene encoding an N-malonyltransferase enzyme known to be involved in benzoxazolinone degradation in the maize pathogen Fusarium verticillioides. Expression analyses demonstrated that a cluster of nine genes was responsive to exogenous benzoxazolinone in the important wheat pathogen Fusarium pseudograminearum. The analysis of independent F. pseudograminearum FDB2 knockouts and complementation of the knockout with FDB2 homologues from F. graminearum and F. verticillioides confirmed that the N-malonyltransferase enzyme encoded by this gene is central to the detoxification of benzoxazolinones, and that Fdb2 contributes quantitatively to virulence towards wheat in head blight inoculation assays. This contrasts with previous observations in F. verticillioides, where no effect of FDB2 mutations on pathogen virulence towards maize was observed. Overall, our results demonstrate that the detoxification of benzoxazolinones is a strategy adopted by wheat-infecting F. pseudograminearum to overcome host-derived chemical defences.

A kinetic analysis of the catalase activity of myeloperoxidase

The predominant physiological activity of myeloperoxidase is to convert hydrogen peroxide and chloride to hypochlorous acid. However, this neutrophil enzyme also degrades hydrogen peroxide to oxygen and water. We have undertaken a kinetic analysis of this reaction to clarify its mechanism. When myeloperoxidase was added to hydrogen peroxide in the absence of reducing substrates, there was an initial burst phase of hydrogen peroxide consumption followed by a slow steady state loss. The kinetics of hydrogen peroxide loss were precisely mirrored by the kinetics of oxygen production. Two mols of hydrogen peroxide gave rise to 1 mol of oxygen. With 100 microM hydrogen peroxide and 6 mM chloride, half of the hydrogen peroxide was converted to hypochlorous acid and the remainder to oxygen. Superoxide and tyrosine enhanced the steady-state loss of hydrogen peroxide in the absence of chloride. We propose that hydrogen peroxide reacts with the ferric enzyme to form compound I, which in turn reacts with another molecule of hydrogen peroxide to regenerate the native enzyme and liberate oxygen. The rate constant for the two-electron reduction of compound I by hydrogen peroxide was determined to be 2 x 10(6) M(-1) s(-1). The burst phase occurs because hydrogen peroxide and endogenous donors are able to slowly reduce compound I to compound II, which accumulates and retards the loss of hydrogen peroxide. Superoxide and tyrosine drive the catalase activity because they reduce compound II back to the native enzyme. The two-electron oxidation of hydrogen peroxide by compound I should be considered when interpreting mechanistic studies of myeloperoxidase and may influence the physiological activity of the enzyme.

Substrates and products of eosinophil peroxidase

Eosinophil peroxidase has been implicated in promoting oxidative tissue damage in a variety of inflammatory conditions, including asthma. It uses H(2)O(2) to oxidize chloride, bromide and thiocyanate to their respective hypohalous acids. The aim of this study was to establish which oxidants eosinophil peroxidase produces under physiological conditions. By measuring rates of H(2)O(2) utilization by the enzyme at neutral pH, we determined the catalytic rate constants for bromide and thiocyanate as 248 and 223 s(-1) and the Michaelis constants as 0.5 and 0.15 mM respectively. On the basis of these values thiocyanate is preferred 2.8-fold over bromide as a substrate for eosinophil peroxidase. Eosinophil peroxidase catalysed substantive oxidation of chloride only below pH 6.5. We found that when eosinophil peroxidase or myeloperoxidase oxidized thiocyanate, another product besides hypothiocyanite was formed; it also converted methionine into methionine sulphoxide. During the oxidation of thiocyanate, the peroxidases were present as their compound II forms. Compound II did not form when GSH was included to scavenge hypothiocyanite. We propose that the unidentified oxidant was derived from a radical species produced by the one-electron oxidation of hypothiocyanite. We conclude that at plasma concentrations of bromide (20-120 microM) and thiocyanate (20-100 microM), hypobromous acid and oxidation products of thiocyanate are produced by eosinophil peroxidase. Hypochlorous acid is likely to be produced only when substrates preferred over chloride are depleted. Thiocyanate should be considered to augment peroxidase-mediated toxicity because these enzymes can convert relatively benign hypothiocyanite into a stronger oxidant.

Oxidation of tryptophan by redox intermediates of myeloperoxidase and inhibition of hypochlorous acid production

The neutrophil enzyme myeloperoxidase catalyzes the oxidation of tyrosine to tyrosyl radicals, which cross-link to proteins and initiate lipid peroxidation. Tryptophan is present in plasma at about the same concentration as tyrosine and has a similar one-electron reduction potential. In this investigation, we have determined the ability of myeloperoxidase to catalyze the oxidation of tryptophan to assess whether or not this reaction may contribute to oxidative stress at sites of inflammation. We show that tryptophan is a poor substrate for myeloperoxidase because, even though it reacts rapidly with compound I (kI 2.1 x 10(6) M(-1)s(-1)), it reacts sluggishly with compound II (kII 7 M(-1)s(-1)). Tryptophan reversibly inhibited production of hypochlorous acid by purified myeloperoxidase by converting the enzyme to a mixture of compound II and compound III. It gave 50% inhibition (I50) at a concentration of 2 microM. In contrast, it was an ineffective inhibitor of hypochlorous acid production by human neutrophils (I50 80 microM) unless superoxide dismutase was present (I50 5 microM). We propose that compound I of myeloperoxidase will oxidize tryptophan at sites of inflammation. Enzyme turnover will result from the reaction of superoxide or tyrosine with compound II. Thus, tryptophan radicals are potential candidates for exacerbating oxidative stress during inflammation.

Biomarkers of myeloperoxidase-derived hypochlorous acid

Hypochlorous acid is the major strong oxidant generated by neutrophils. The heme enzyme myeloperoxidase catalyzes the production of hypochlorous acid from hydrogen peroxide and chloride. Although myeloperoxidase has been implicated in the tissue damage that occurs in numerous diseases that involve inflammatory cells, it has proven difficult to categorically demonstrate that it plays a crucial role in any pathology. This situation should soon be rectified with the advent of sensitive biomarkers for hypochlorous acid. In this review, we outline the advantages and limitations of chlorinated tyrosines, chlorohydrins, 5-chlorocytosine, protein carbonyls, antibodies that recognize HOCl-treated proteins, and glutathione sulfonamide as potential biomarkers of hypochlorous acid. Levels of 3-chlorotyrosine and 3,5-dichlorotyrosine are increased in proteins after exposure to low concentrations of hypochlorous acid and we conclude that their analysis by gas chromatography and mass spectrometry is currently the best method available for probing the involvement of oxidation by myeloperoxidase in the pathology of particular diseases. The appropriate use of other biomarkers should provide complementary information.Keywords-Free radicals, Myeloperoxidase, Neutrophil oxidant, Hypochlorous acid, Chlorotyrosine, Chlorohydrin, Oxidant biomarker

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.

Comparison of mono- and dichlorinated tyrosines with carbonyls for detection of hypochlorous acid modified proteins

Hypochlorous acid is a potent oxidant capable of oxidizing and chlorinating proteins. Based on its indiscriminant reactivity, it is proposed to play a major role in tissue damage associated with a range of inflammatory diseases. We have determined the relative tendencies for formation of protein carbonyls, chlorinated tyrosine residues, and epitopes recognized by an antibody raised against hypochlorous acid oxidized protein (HOP-1) when albumin is treated with hypochlorous acid. We have also tested the specificity of the HOP-1 antibody by measuring how effectively it recognizes proteins oxidized by hypobromous acid. 3-Chlorotyrosine, along with a new marker of hypochlorous acid dependent protein modification, 3, 5-dichlorotyrosine, was formed at the lowest doses of hypochlorous acid that were capable of generating protein carbonyls. Comparatively high doses of hypochlorous acid were needed to generate epitopes recognized by HOP-1, which were also produced by hypobromous acid. Our study demonstrates that it is advantageous to measure protein carbonyls and HOP-1 epitopes in conjunction with chlorinated tyrosines when attempting to identify the oxidants responsible for inflammatory tissue damage.

Initiation of rapid, P53-dependent growth arrest in cultured human skin fibroblasts by reactive chlorine species

Oxidants produced by neutrophils have been implicated in causing cancers associated with chronic inflammation. Hypochlorous acid is the most potent oxidant produced by these cells in appreciable amounts. It reacts with amines to form chloramines, which are weaker oxidants but are mutagenic. Recently, we showed that sublethal doses of hypochlorous acid increased levels of the transcription factor protein 53 (p53) and the wild-type activating fragment-1/cyclin-dependent kinase inhibitory protein-1 (WAF1/CIP1) in cultured human skin fibroblasts. WAF1/CIP1 is an important intermediate in the pathway leading to growth arrest. We now show that low doses of hypochlorous acid and physiological chloramines lead to an inhibition of both DNA synthesis and division of cultured human skin fibroblasts. Inhibition of DNA synthesis occurred within 1 h of hypochlorous acid treatment, was maintained for 24 h, and returned to a normal rate after 48 h. Cell division was inhibited by hypochlorous acid and chloramines for 48 h and returned to normal 72 h after treatment. Growth arrest was dependent on p53 because it was blocked when cells were transfected with a p53-binding oligonucleotide. We propose that reactive chlorine species will initiate WAF1/CIP1-dependent growth arrest that will counteract their mutagenic effects and minimize the possibility of the malignant transformation of cells surrounding sites of inflammation.

Nitrite as a substrate and inhibitor of myeloperoxidase. Implications for nitration and hypochlorous acid production at sites of inflammation

Myeloperoxidase is a heme enzyme of neutrophils that uses hydrogen peroxide to oxidize chloride to hypochlorous acid. Recently, it has been shown to catalyze nitration of tyrosine. In this study we have investigated the mechanism by which it oxidizes nitrite and promotes nitration of tyrosyl residues. Nitrite was found to be a poor substrate for myeloperoxidase but an excellent inhibitor of its chlorination activity. Nitrite slowed chlorination by univalently reducing the enzyme to an inactive form and as a consequence was oxidized to nitrogen dioxide. In the presence of physiological concentrations of nitrite and chloride, myeloperoxidase catalyzed little nitration of tyrosyl residues in a heptapeptide. However, the efficiency of nitration was enhanced at least 4-fold by free tyrosine. Our data are consistent with a mechanism in which myeloperoxidase oxidizes free tyrosine to tyrosyl radicals that exchange with tyrosyl residues in peptides. These peptide radicals then couple with nitrogen dioxide to form 3-nitrotyrosyl residues. With neutrophils, myeloperoxidase-dependent nitration required a high concentration of nitrite (1 mM), was doubled by tyrosine, and increased 4-fold by superoxide dismutase. Superoxide is likely to inhibit nitration by reacting with nitrogen dioxide and/or tyrosyl radicals. We propose that at sites of inflammation myeloperoxidase will nitrate proteins, even though nitrite is a poor substrate, because the co-substrate tyrosine will be available to facilitate the reaction. Also, production of 3-nitrotyrosine will be most favorable when the concentration of superoxide is low.

Myeloperoxidase

This review covers recent advances in the biology of myeloperoxidase. Mechanisms of posttranslational processing and how these fail in some of the common deficiency mutants are discussed. We also review the enzymology that points to myeloperoxidase having a number of physiologic substrates in addition to chloride and the evidence that it produces hypochlorous acid in the neutrophil phagosome in sufficient quantities to be bactericidal. Evidence is accumulating that myeloperoxidase-derived oxidants modify biologic macromolecules and cell-regulatory pathways and that they play a role in atherosclerosis. Investigation of disease incidence in relation to a polymorphism in the promoter region of the gene has produced interesting associations. These links with inflammatory diseases can now be pursued further using specific biomarkers of myeloperoxidase activity.

Current concepts of the actions of paracetamol (acetaminophen) and NSAIDs

There is much uncertainty about the mechanism of action of paracetamol (acetaminophen). It is commonly stated that, unlike the non-steroidal anti-inflammatory drugs (NSAIDs), it is a weak inhibitor of the synthesis of prostaglandins. This conclusion is made largely from studies in which the synthesis of prostaglandins was measured in homogenized tissues. However, in several cellular systems, paracetamol is an inhibitor of the synthesis of prostaglandins with IC(50) values ranging from approximately 4 microM to 200 microM. Paracetamol is not bound significantly to plasma proteins and therefore the concentrations in plasma can be equated directly with those used in in vitro experiments. After oral doses of 1 g, the peak plasma concentrations of paracetamol are approximately 100 microM and the plasma concentrations are therefore in the range where marked inhibition of the synthesis of prostaglandins should occur in some cells. Paracetamol is metabolized by the peroxidase component of prostaglandin H synthase but the relationship of this to inhibition of the cyclooxygenase or peroxidase activities of the enzyme is unclear. Paracetamol is also metabolized by several other peroxidases, including myeloperoxidase, the enzyme in neutrophils which is responsible for the production of hypochlorous acid (HOCl). The metabolism of paracetamol by myeloperoxidase leads to the decreased total production of HOC1 by both intact neutrophils and isolated myeloperoxidase, even though the initial rate of production of HOC1 is increased. The IC(50) value, derived from inhibition of the total production of HOC1 by isolated myeloperoxidase, is 81 microM. Several NSAIDs inhibit functions of neutrophils in media containing low concentrations of protein but their effects, in contrast to that of paracetamol, are generally produced only at concentrations greater than those of the unbound drug in plasma during treatment with the NSAIDs. However, neutrophils isolated during treatment with NSAIDs, such as piroxicam, ibuprofen and indomethacin show decreased function. Paracetamol has little or no anti-inflammatory activity by itself but may potentiate the clinical activity of NSAIDs in the treatment of rheumatoid arthritis.

Transient and steady-state kinetics of the oxidation of substituted benzoic acid hydrazides by myeloperoxidase

Myeloperoxidase is the most abundant protein in neutrophils and catalyzes the production of hypochlorous acid. This potent oxidant plays a central role in microbial killing and inflammatory tissue damage. 4-Aminobenzoic acid hydrazide (ABAH) is a mechanism-based inhibitor of myeloperoxidase that is oxidized to radical intermediates that cause enzyme inactivation. We have investigated the mechanism by which benzoic acid hydrazides (BAH) are oxidized by myeloperoxidase, and we have determined the features that enable them to inactivate the enzyme. BAHs readily reduced compound I of myeloperoxidase. The rate constants for these reactions ranged from 1 to 3 x 10(6) M-1 s-1 (15 degrees C, pH 7.0) and were relatively insensitive to the substituents on the aromatic ring. Rate constants for reduction of compound II varied between 6.5 x 10(5) M-1 s-1 for ABAH and 1.3 x 10(3) M-1 s-1 for 4-nitrobenzoic acid hydrazide (15 degrees C, pH 7.0). Reduction of both compound I and compound II by BAHs adhered to the Hammett rule, and there were significant correlations with Brown-Okamoto substituent constants. This indicates that the rates of these reactions were simply determined by the ease of oxidation of the substrates and that the incipient free radical carried a positive charge. ABAH was oxidized by myeloperoxidase without added hydrogen peroxide because it underwent auto-oxidation. Although BAHs generally reacted rapidly with compound II, they should be poor peroxidase substrates because the free radicals formed during peroxidation converted myeloperoxidase to compound III. We found that the reduction of ferric myeloperoxidase by BAH radicals was strongly influenced by Hansch's hydrophobicity constants. BAHs containing more hydrophilic substituents were more effective at converting the enzyme to compound III. This implies that BAH radicals must hydrogen bond to residues in the distal heme pocket before they can reduce the ferric enzyme. Inactivation of myeloperoxidase by BAHs was related to how readily they were oxidized, but there was no correlation with their rate constants for reduction of compounds I or II. We propose that BAHs destroy the heme prosthetic groups of the enzyme by reducing a ferrous myeloperoxidase-hydrogen peroxide complex.

Hypochlorous acid activates the tumor suppressor protein p53 in cultured human skin fibroblasts

The carcinogenicity associated with chronic inflammation has been attributed to neutrophils and the oxidants they produce. Neutrophils accumulate at sites of chronic inflammation, where they are stimulated to produce hydrogen peroxide which is converted to hypochlorous acid by coreleased myeloperoxidase. We report here that levels of the tumor suppressor protein p53 were increased in cultured human skin fibroblasts that had been incubated with stimulated neutrophils. The increase in p53 required the myeloperoxidase-dependent generation of hypochlorous acid and could be mimicked by exposing cells to a flux of hypochlorous acid produced by purified myeloperoxidase and a hydrogen peroxide-generating system. Levels of p53 were very sensitive to hypochlorous acid, with fluxes as low as 0.2 microM per min being effective. Levels of the p53-dependent protein WAF1/CIP1 were also elevated when fibroblasts were treated with hypochlorous acid. This result indicates that the p53 in the cells treated with hypochlorous acid was transcriptionally active. Hydrogen peroxide alone also elevated p53 and WAF1/CIP1, but the fluxes required were nearly 10-fold higher than those that were effective for hypochlorous acid. Our results implicate hypochlorous acid in the neutrophil-dependent initiation of a signal transduction pathway which could minimize the carcinogenicity of chronic inflammation.

The activation of gold complexes by cyanide produced by polymorphonuclear leukocytes. III. The formation of aurocyanide by myeloperoxidase

There is considerable evidence that the anti-rheumatic gold complexes are activated by their conversion to aurocyanide. In order to understand the mechanism of production of aurocyanide, we investigated the involvement of myeloperoxidase in the reaction. This haem enzyme of neutrophils and monocytes uses hydrogen peroxide to oxidise chloride and thiocyanate to hypochlorous acid and hypothiocyanite, respectively. When aurothiomalate (10 microM) was incubated with thiocyanate (200 microM), hydrogen peroxide (100 microM) and myeloperoxidase (20 nM), it was transformed to a product that was spectrally identical to authentic aurocyanide. Aurothiomalate was quantitatively converted to aurocyanide in about 10 min at pH 6.0 and in 40 min at pH 7.4. Aurocyanide formation occurred after myeloperoxidase had used all the hydrogen peroxide available to produce hypothiocyanite. Thus, the cyanide must have formed from the slow decomposition of hypothiocyanite. The rate of aurocyanide production was increased in the presence of 100 mM chloride, which indicates that hypochlorous acid accelerates the formation of cyanide. Hypochlorous acid (100 to 400 microM) reacted non-enzymatically with thiocyanate (200 microM) and aurothiomalate (10 microM) to produce aurocyanide. Thus, aurocyanide is produced by two processes, involving both the formation of hypothiocyanite and hypochlorous acid. Aurocyanide is an effective inhibitor of the respiratory burst of neutrophils and monocytes and the proliferation of lymphocytes. Therefore, aurothiomalate may attenuate inflammation by acting as a pro-drug which is reliant on neutrophils and monocytes to produce hypothiocyanite. When the hypothiocyanite decays to hydrogen cyanide, the pro-drug is converted to aurocyanide which then suppresses further oxidant production by these inflammatory cells.

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.

Neutrophil-mediated activation of mitoxantrone to metabolites which form adducts with DNA

In this study we have shown that phorbol ester-stimulated human neutrophils are able to oxidatively activate mitoxantrone and result in covalent incorporation of the drug into cellular DNA. The use of the myeloperoxidase inhibitor sodium azide confirmed that the activation and covalent binding of mitoxantrone to cellular DNA was due to its metabolism by the haem enzyme myeloperoxidase. Phorbol ester-stimulated neutrophils were also able to oxidatively metabolise mitoxantrone and facilitate extracellular covalent binding of the drug to calf thymus DNA. These results suggest that myeloperoxidase may contribute to the mode of action of mitoxantrone.

Myeloperoxidase-dependent generation of a tyrosine peroxide by neutrophils

It has recently been shown that tyrosyl radicals react with superoxide to form a peroxide adduct of tyrosine. Since myeloperoxidase oxidizes tyrosine to its radical, and neutrophils and monocytes contain myeloperoxidase as well as produce superoxide, we have investigated whether tyrosine peroxide could be a significant product of tyrosine oxidation by these cells. Oxidation of tyrosine by purified myeloperoxidase and a superoxide-generating system, and by stimulated human neutrophils, was found to generate peroxide adducts as detected in the xylenol orange (FOX) assay and by HPLC. Superoxide, hydrogen peroxide, and myeloperoxidase were required for formation of the peroxide. Dityrosine was also formed in each system, and in the presence of superoxide dismutase, suppression of tyrosine peroxide formation gave elevated formation of dityrosine. Quantitative estimates indicate that at physiological tyrosine concentration the peroxide is likely to be formed in preference to dityrosine and to be a significant product of neutrophils. This metastable peroxide therefore has the potential to contribute to neutrophil- or monocyte-mediated tissue injury.

Mechanism of inactivation of myeloperoxidase by 4-aminobenzoic acid hydrazide

Hypochlorous acid is the most powerful oxidant generated by neutrophils and is likely to contribute to the damage mediated by these inflammatory cells. The haem enzyme myeloperoxidase catalyses its production from hydrogen peroxide and chloride. 4-Aminobenzoic acid hydrazide (ABAH) is a potent inhibitor of hypochlorous acid production. In this investigation we show that, in the presence of hydrogen peroxide, ABAH irreversibly inactivates myeloperoxidase. ABAH was oxidized by myeloperoxidase, and kinetic analysis of the inactivation conformed to that for a mechanism-based inhibitor. Inactivation was exacerbated by concentrations of hydrogen peroxide greater than 50 microM and by the absence of oxygen. Hydrogen peroxide alone caused minimal inactivation. Reduced glutathione inhibited the oxidation of ABAH as well as the irreversible inhibition of myeloperoxidase. In the presence of oxygen, ABAH and hydrogen peroxide initially converted myeloperoxidase into compound III, which subsequently lost haem absorbance. In the absence of oxygen, the enzyme was converted into ferrous myeloperoxidase and its haem groups were rapidly destroyed. We propose that myeloperoxidase oxidizes ABAH to a radical that reduces the enzyme to its ferrous intermediate. Ferrous myeloperoxidase reacts either with oxygen to allow enzyme turnover, or with hydrogen peroxide to give irreversible inactivation.

Involvement of superoxide and myeloperoxidase in oxygen-dependent killing of Staphylococcus aureus by neutrophils

We have used a quantitative assay that measures independent rate constants for phagocytosis and killing of Staphylococcus aureus to investigate the involvement of superoxide and myeloperoxidase in bacterial killing by human neutrophils. To inhibit superoxide-dependent processes, superoxide dismutase was cross-linked to immunoglobulin G and the conjugate was attached to the surface of S. aureus via protein A in its cell wall. Myeloperoxidase was inhibited with azide, and myeloperoxidase-deficient neutrophils were used. Adding the NADPH oxidase inhibitor diphenyleneiodonium, to prevent superoxide production, decreased the killing rate to 25%, indicating that oxidative killing mechanisms predominate in this system. The rate constant for killing of S. aureus with superoxide dismutase attached was 70% of that for control bacteria linked to inactivated enzyme. Superoxide dismutase had no effect in the presence of diphenyleneiodonium. The rate of killing was decreased to 33% in the presence of azide and to 40% with myeloperoxidase-deficient neutrophils. Superoxide dismutase had no effect in the presence of azide. On the assumption that the oxidative and nonoxidative components of killing can be considered separately, the oxidative rate was decreased by almost half by superoxide dismutase and was about six times lower when myeloperoxidase was inactive. We conclude that myeloperoxidase-dependent processes are strongly favored by human neutrophils as their prime mechanism of oxidative killing of S. aureus and that superoxide makes a direct contribution to killing. Our results also suggest that superoxide acts in conjunction with a myeloperoxidase-dependent pathway.

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.

Myeloperoxidase oxidizes mitoxantrone to metabolites which bind covalently to DNA and RNA

The anticancer agent mitoxantrone is readily oxidized by the human haem enzyme myeloperoxidase and hydrogen peroxide. Intercalation of mitoxantrone with DNA inhibited oxidation of the drug by myeloperoxidase. However, at a physiological ionic strength, significant oxidation of the drug was evident. At a H2O2:mitoxantrone ratio of 1.0, myeloperoxidase oxidized mitoxantrone to a metabolite (product B) which associated reversibly with DNA. At greater hydrogen peroxide concentrations, two further metabolites were produced (products C and D), neither of which associated reversibly with DNA, as indicated by the absence of any spectral change in the presence of DNA. Long exposure of the products derived from the oxidation of [14C]mitoxantrone by myeloperoxidase resulted in a time-dependent covalent binding of the activated drug to both DNA and RNA. The amount of DNA adduct increased linearly with the extent of oxidation of mitoxantrone (up to a H2O2:mitoxantrone ratio of 5.0). No adducts resulted from exposure of the oxidized product B to DNA, but adducts formed following further oxidation of B by myeloperoxidase. The myeloperoxidase-catalysed oxidation of mitoxantrone to products capable of interacting covalently and non-covalently with nucleic acids may represent an important mode of action of mitoxantrone against acute myeloid leukemias since these cells (including neutrophils, monocytes and their precursors) contain high levels of myeloperoxidase.

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.

Inhibition of myeloperoxidase by benzoic acid hydrazides

Myeloperoxidase is the most abundant protein in neutrophils and catalyses the conversion of H2O2 and chloride into HOCl. To help clarify the role of this enzyme in bacterial killing and inflammation, a specific and potent inhibitor needs to be identified. We have studied a series of benzoic acid hydrazides and found that in general they inhibit the peroxidation activity of myeloperoxidase with an IC50 value of less than 10 microM. The IC50 values of derivatives with substituents containing oxygen or nitrogen were related to their Hammett substituent constants. This indicates that myeloperoxidase oxidizes the hydrazide group of these compounds, and the degree to which they inhibit the enzyme is dependent on the ease of their oxidation. Unsubstituted benzoic acid hydrazide and its 4-chloro derivative were poor inhibitors of peroxidation. Thus it is likely that hydrogen-bonding of the enzyme to substituents containing oxygen or nitrogen increases the binding affinity of the hydrazides and enhances their oxidation by myeloperoxidase. 4-Aminobenzoic acid hydrazide (ABAH) was the most potent inhibitor of peroxidation. It irreversibly inhibited HOCl production by the purified enzyme, having an IC50 value of 0.3 microM. With neutrophils stimulated with opsonized zymosan or phorbol myristate acetate, ABAH inhibited HOCl production by up to 90% and the IC50 values were 16 microM and 2.2 microM respectively. In the presence of superoxide dismutase, these values decreased to 6.4 microM and 0.6 microM respectively. ABAH had no effect on superoxide radical (O2-.) production and degranulation by neutrophils, nor did it inhibit catalase or glutathione peroxidase. Thus ABAH is an effective and selective inhibitor that should be useful for determining the contribution of myeloperoxidase to oxidant-mediated reactions of neutrophils.

Oxidative metabolism of mitoxantrone by the human neutrophil enzyme myeloperoxidase

The anti-cancer drug mitoxantrone is readily oxidized by the human heme enzyme myeloperoxidase (MPO) and H2O2. Direct oxidation yielded up to three products, which depended on the ratio of H2O2 to mitoxantrone. At an H2O2: mitoxantrone ratio of 1.0, one major product was obtained, with a spectrum and HPLC retention time identical to that resulting from oxidation by horseradish peroxidase. This metabolite is a substituted hexahydronaphtho[2,3-f]quinoxaline-7,12-dione and has been discovered in the urine of patients treated with mitoxantrone, hence implicating MPO in the in vivo metabolism of mitoxantrone. At higher concentrations of H2O2, the oxidation of mitoxantrone was more complex, with two further metabolites being identified. When mitoxantrone was incubated with neutrophils that had been stimulated with phorbol myristate acetate, it was oxidized by an MPO-dependent mechanism. Therefore, it appears that MPO may play a significant role in the clinical activity displayed by mitoxantrone against acute myelogenous leukemias, as neutrophils, monocytes and their bone marrow precursors contain high levels of the enzyme.

Assays using horseradish peroxidase and phenolic substrates require superoxide dismutase for accurate determination of hydrogen peroxide production by neutrophils

We used horseradish peroxidase and either scopoletin, homovanillic acid, or phenol red to measure hydrogen peroxide generated by human neutrophils. With these assays, superoxide dismutase significantly increased the amount of hydrogen peroxide detected. In contrast, it had no effect when the accumulation of hydrogen peroxide was measured with a hydrogen peroxide electrode. We propose that superoxide interferes with horseradish peroxidase-dependent assays so that hydrogen peroxide is underestimated. Thus, when using these assays, superoxide dismutase must be added to neutrophils to ensure that all the hydrogen peroxide they produce is detected.

Superoxide-dependent hydroxylation by myeloperoxidase

When stimulated, neutrophils undergo a respiratory burst converting oxygen to superoxide. Although superoxide is critical for microbial killing by phagocytic cells, the precise role it plays has yet to be established. It has been proposed to optimize their production of hypochlorous acid and to be required for the generation of hydroxyl radicals. Superoxide is also involved in the hydroxylation of salicylate by neutrophils. However, the mechanism of this reaction is unknown. We found that neutrophils stimulated with opsonized zymosan hydroxylated salicylate to produce mainly 2,5-dihydroxybenzoate. Its formation was dependent on superoxide and a heme protein but was independent of hydrogen peroxide and hydroxyl radicals. Production of 2,5-dihydroxybenzoate was enhanced by methionine, which scavenges hypochlorous acid. Neutrophils from an individual with myeloperoxidase deficiency hydroxylated salicylate at only 13% of the level of control cells. Purified human myeloperoxidase and xanthine oxidase plus hypoxanthine hydroxylated salicylate to produce 2,5-dihydroxybenzoate. As with neutrophils, the reaction required superoxide but not hydrogen peroxide and was unaffected by hydroxyl radical scavengers. Thus, myeloperoxidase catalyzes superoxide-dependent hydroxylation. This newly recognized reaction may be relevant to the in vivo functions of superoxide and myeloperoxidase.

Superoxide is an antagonist of antiinflammatory drugs that inhibit hypochlorous acid production by myeloperoxidase

Myeloperoxidase, the most abundant enzyme in neutrophils, catalyses the conversion of hydrogen peroxide and chloride to hypochlorous acid. This potent oxidant has the potential to cause considerable tissue damage in many inflammatory diseases. We have investigated the ability of dapsone, diclofenac, primaquine, sulfapyridine and benzocaine to inhibit hypochlorous acid production by stimulated human neutrophils. The drugs were also tested against purified myeloperoxidase using xanthine oxidase to generate hydrogen peroxide and superoxide. The inhibitory effects of the drugs on hypochlorous acid production, either by cells stimulated with phorbol myristate acetate or by myeloperoxidase and xanthine oxidase, were significantly less than those determined with myeloperoxidase and reagent hydrogen peroxide. Comparable potency was observed only when superoxide dismutase was present to remove superoxide. We also observed that with the xanthine oxidase system, inhibition of hypochlorous acid production by dapsone decreased markedly as the concentration of myeloperoxidase increased. Dapsone was a poor inhibitor of hypochlorous acid production by neutrophils stimulated with opsonized zymosan, regardless of the presence of superoxide dismutase. With this phagocytic stimulus, catalase inhibited hypochlorous acid formation by only 60%, which indicates that a substantial amount of the hypochlorous acid detected originated from within phagosomes. Thus, it is apparent that dapsone is unable to affect intraphagosomal conversion of hydrogen peroxide to hypochlorous acid. All the drugs inhibit myeloperoxidase reversibly by trapping it as its inactive redox intermediate, compound II. We propose that superoxide limits the potency of the drugs by reducing compound II back to the active enzyme. Furthermore, under conditions where the activity of myeloperoxidase exceeds that of the hydrogen peroxide-generating system, which is most likely to occur in phagosomes, partial inhibition of myeloperoxidase need not affect hypochlorous acid production. We conclude that drugs that inhibit myeloperoxidase by converting it to compound II are unlikely to be effective against hypochlorous acid-mediating tissue damage.

Oxidative metabolism of amsacrine by the neutrophil enzyme myeloperoxidase

Oxidative metabolism of the anti-cancer drug amsacrine 4'-(9-acridinylamino) methane-sulphan-m-anisidide has been suggested to account for its cytotoxicity. However, enzymes capable of oxidizing it in non-hepatic tissue have yet to be identified. A potential candidate, that may be relevant to the metabolism of amsacrine in blood and its action in myeloid leukaemias and myelosuppression, is the haem enzyme myeloperoxidase. We have found that the purified human enzyme oxidizes amsacrine to its quinone diimine, either directly or through the production of hypochlorous acid. In comparison, the 4-methyl-5-methylcarboxamide derivative of amsacrine, CI-921 9-[[2-methoxy-4[(methylsulphonyl)-amino]phenyl]amino)-N, 5-dimethyl-4-acridine carboxamide, reacted poorly with myeloperoxidase, although it was oxidized by hypochlorous acid. Detailed studies of the mechanism by which myeloperoxidase oxidizes amsacrine revealed that the semiquinone imine free radical is a likely intermediate in this reaction. Oxidation of amsacrine analogues indicated that factors other than their reduction potential determine how readily they are metabolized by myeloperoxidase. Both amsacrine and CI-921 inhibited production of hypochlorous acid by myeloperoxidase. CI-921 acted by trapping the enzyme as the inactive redox intermediate compound II. Amsacrine inhibited by a different mechanism that may involve conversion of myeloperoxidase to compound III, which is also unable to oxidize Cl-. The susceptibility of amsacrine to oxidation by myeloperoxidase indicates that this reaction may contribute to the cytotoxicity of amsacrine toward neutrophils, monocytes and their precursors.

Oxidation of hydroquinone by myeloperoxidase. Mechanism of stimulation by benzoquinone

Myeloperoxidase (MPO) is a prime candidate for mediating the inflammatory tissue damage of neutrophils because it converts Cl- to the potent oxidant hypochlorous acid. It also oxidizes xenobiotics to reactive free radicals. We have found that the kinetics of oxidation of hydroquinone by myeloperoxidase are inadequately explained by the classical peroxidase mechanism. Peroxidation of hydroquinone displayed a distinct lag phase, which was practically abolished by excluding O2 and was eliminated by adding benzoquinone at the start of the reaction. Superoxide dismutase increased the rate of peroxidation by 40% but did not eliminate the lag phase. Spectral investigations revealed that during the initial phase of the reaction, MPO was converted to oxy-MPO, or compound III, by a mechanism that was not reliant on superoxide. Benzosemiquinone, however, was able to convert ferric-MPO to compound III. Both compound III and ferro-MPO reacted with benzoquinone to regenerate ferric-MPO. We propose that the lag phase occurs because benzosemiquinone reduces ferric-MPO to ferro-MPO, which rapidly binds O2 to form compound III. Since compound III is outside the peroxidation cycle, conversion of hydroquinone to benzoquinone is retarded. However, as benzoquinone accumulates, it oxidizes ferro-MPO and compound III to ferric-MPO, thereby increasing the rate of peroxidation. There is a minimal lag phase under an atmosphere of N2 because ferro-MPO would be rapidly oxidized by benzoquinone, without formation of compound III. We conclude that when substrates produce radicals capable of reducing ferric-MPO, they will be peroxidized efficiently only if oxy-MPO is readily recycled. Furthermore, these radicals will prevent MP3+ from reacting with H2O2, and thereby prevent the enzyme from oxidizing Cl- to hypochlorous acid. Thus, this mechanism could be exploited to prevent hypochlorous acid-mediated inflammatory tissue damage.

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.

Superoxide enhances hypochlorous acid production by stimulated human neutrophils

Stimulated neutrophils undergo a respiratory burst discharging large quantities of superoxide and hydrogen peroxide. They also release myeloperoxidase, which catalyses the conversion of hydrogen peroxide and Cl- to hypochlorous acid. Human neutrophils stimulated with opsonized zymosan promoted the loss of monochlorodimedon. This loss was entirely due to hypochlorous acid, since it did not occur in Cl(-)-free buffer, was inhibited by azide and cyanide, and was enhanced by adding exogenous myeloperoxidase. It was not inhibited by desferal, diethylenetriaminepentaacetic acid, mannitol or dimethylsulfoxide, which excluded involvement of the hydroxyl radical. Approx. 30% of the detectable superoxide generated was converted to hypochlorous acid. As expected, formation of hypochlorous acid was completely inhibited by catalase, but it was also inhibited by up to 70% by superoxide dismutase. Superoxide dismutase also inhibited the production of hypochlorous acid by neutrophils stimulated with phorbol myristate acetate. Our results indicate that generation of superoxide by neutrophils enables these cells to enhance their production of hypochlorous acid. Furthermore, inhibition of neutrophil processes by superoxide dismutase and catalase does not necessarily implicate the hydroxyl radical. It is proposed that superoxide may potentiate oxidant damage at inflammatory sites by optimizing the myeloperoxidase-dependent production of hypochlorous acid.

Influence of superoxide on myeloperoxidase kinetics measured with a hydrogen peroxide electrode

Stimulated neutrophils discharge large quantities of superoxide (O2.-), which dismutates to form H2O2. In combination with Cl-, H2O2 is converted into the potent oxidant hypochlorous acid (HOCl) by the haem enzyme myeloperoxidase. We have used an H2O2 electrode to monitor H2O2 uptake by myeloperoxidase, and have shown that in the presence of Cl- this accurately represents production of HOCl. Monochlorodimedon, which is routinely used to assay production of HOCl, inhibited H2O2 uptake by 95%. This result confirms that monochlorodimedon inhibits myeloperoxidase, and that the monochlorodimedon assay grossly underestimates the activity of myeloperoxidase. With 10 microM-H2O2 and 100 mM-Cl-, myeloperoxidase had a neutral pH optimum. Increasing the H2O2 concentration to 100 microM lowered the pH optimum to pH 6.5. Above the pH optimum there was a burst of H2O2 uptake that rapidly declined due to accumulation of Compound II. High concentrations of H2O2 inhibited myeloperoxidase and promoted the formation of Compound II. These effects of H2O2 were decreased at higher concentrations of Cl-. We propose that H2O2 competes with Cl- for Compound I and reduces it to Compound II, thereby inhibiting myeloperoxidase. Above pH 6.5, O2.- generated by xanthine oxidase and acetaldehyde prevented H2O2 from inhibiting myeloperoxidase, increasing the initial rate of H2O2 uptake. O2.- allowed myeloperoxidase to function optimally with 100 microM-H2O2 at pH 7.0. This occurred because, as previously demonstrated, O2.- prevents Compound II from accumulating by reducing it to ferric myeloperoxidase. In contrast, at pH 6.0, where Compound II did not accumulate, O2.- retarded the uptake of H2O2. We propose that by generating O2.- neutrophils prevent H2O2 and other one-electron donors from inhibiting myeloperoxidase, and ensure that this enzyme functions optimally at neutral pH.

The mechanism of myeloperoxidase-dependent chlorination of monochlorodimedon

Chlorination of monochlorodimedon is routinely used to measure the production of hypochlorous acid catalysed by myeloperoxidase from H2O2 and Cl-. We have found that the myeloperoxidase/H2O2/Cl- system, at pH 7.8, catalysed the loss of monochlorodimedon with a rapid burst phase followed by a much slower steady-state phase. The loss of monochlorodimedon in the absence of Cl- was only 10% of the steady-state rate in the presence of Cl-, which indicates that the major reaction of monochlorodimedon was with hypochlorous acid. During the steady-state reaction, myeloperoxidase was present as 100% compound II, which cannot participate directly in hypochlorous acid formation. Monochlorodimedon was necessary for formation of compound II, since it was not formed in the presence of methionine. Both the amount of hypochlorous acid formed during the burst phase, and the steady-state rate of hypochlorous acid production, increased with increasing concentrations of myeloperoxidase and with decreasing concentrations of monochlorodimedon. Inhibition by monochlorodimedon was competitive with Cl-. From these results, and the ability of myeloperoxidase to slowly peroxidase monochlorodimedon in the absence of Cl-, we propose that the reaction of monochlorodimedon with the myeloperoxidase/H2O2/Cl- system involves a major pathway due to hypochlorous acid-dependent chlorination and a minor peroxidative pathway. Only a small fraction of compound I needs to react with monochlorodimedon instead of Cl- at each enzyme cycle, for compound II to rapidly accumulate. Monochlorodimedon, therefore, cannot be regarded as an inert detector of hypochlorous acid production by myeloperoxidase, but acts to limit the chlorinating activity of the enzyme. In the presence of reducing species that act like monochlorodimedon, the activity of myeloperoxidase would depend on the rate of turnover of compound II. Components of human serum promoted the conversion of ferric-myeloperoxidase to compound II in the presence of H2O2. We suggest, therefore, that in vivo the rate of turnover of compound II may determine the rate of myeloperoxidase-dependent production of hypochlorous acid by stimulated neutrophils.

A pulse radiolysis investigation of the reactions of myeloperoxidase with superoxide and hydrogen peroxide

Using pulse radiolysis, the rate constant for the reaction of ferric myeloperoxidase with O2- to give compound III was measured at pH 7.8, and values of 2.1.10(6) M-1.s-1 for equine ferric myeloperoxidase and 1.1.10(6) M-1.s-1 for human ferric myeloperoxidase were obtained. Under the same conditions, the rate constant for the reaction of human ferric myeloperoxidase with H2O2 to give compound I was 3.1.10(7) M-1.s-1. Our results indicate that although the reaction of ferric myeloperoxidase with O2- is an order of magnitude slower than with H2O2, the former reaction is sufficiently rapid to influence myeloperoxidase-dependent production of hypochlorous acid by stimulated neutrophils.

Superoxide modulates the activity of myeloperoxidase and optimizes the production of hypochlorous acid

Myeloperoxidase catalyses the conversion of H2O2 and Cl- to hypochlorous acid (HOCl). It also reacts with O2- to form the oxy adduct (compound III). To determine how O2- affects the formation of HOCl, chlorination of monochlorodimedon by myeloperoxidase was investigated using xanthine oxidase and hypoxanthine as a source of O2- and H2O2. Myeloperoxidase was mostly converted to compound III, and H2O2 was essential for chlorination. At pH 5.4, superoxide dismutase (SOD) enhanced chlorination and prevented formation of compound III. However, at pH 7.8, SOD inhibited chlorination and promoted formation of the ferrous peroxide adduct (compound II) instead of compound III. We present spectral evidence for a direct reaction between compound III and H2O2 to form compound II, and for the reduction of compound II by O2- to regenerate native myeloperoxidase. These reactions enable compound III and compound II to participate in the chlorination reaction. Myeloperoxidase catalytically inhibited O2- -dependent reduction of Nitro Blue Tetrazolium. This inhibition is explained by myeloperoxidase undergoing a cycle of reactions with O2-, H2O2 and O2-, with compounds III and II as intermediates, i.e., by myeloperoxidase acting as a combined SOD/catalase enzyme. By preventing the accumulation of inactive compound II, O2- enhances the activity of myeloperoxidase. We propose that, under physiological conditions, this optimizes the production of HOCl and may potentiate oxidant damage by stimulated neutrophils.

Mechanism of the inhibition of catalase by ascorbate. Roles of active oxygen species, copper and semidehydroascorbate

Ascorbate reversibly inhibits catalase, and this inhibition is enhanced and rendered irreversible by the prior addition of copper(II)-bishistidine. In the absence of copper, the inhibition was prevented and reversed by ethanol, but not by superoxide dismutase, benzoate, mannitol, thiourea, desferrioxamine, or DETAPAC. In the presence of the copper complex mannitol, benzoate, and superoxide dismutase still had no effect, but thiourea, desferrioxamine, DETAPAC, or additional histidine decreased the extent of inactivation to that seen in the absence of copper. In the presence of copper, ethanol protected at [ascorbate] less than 1 mM, but was ineffective at [ascorbate] greater than 2 mM, even in the absence of oxygen. Although in the absence of copper, complete removal of oxygen provided full protection against inactivation by ascorbate, this protection was not seen if the catalase was briefly preincubated with H2O2 prior to flushing with nitrogen, or if copper was present. In fact, if copper was present, inactivation was enhanced by the removal of oxygen. Increasing the concentration of oxygen from ambient to 100% slowed the inactivation, whether or not copper was present. It is concluded that the initial reversible inactivation involves reaction with H2O2 to form compound I, followed by one electron reduction of compound I to compound II. In the presence of added copper, the initial (reversible) inactivation allows H2O2 to accumulate sufficiently to permit irreversible inactivation. Since in the presence of copper oxygen is not required, and neither the reversible nor the irreversible inactivation was prevented by conventional scavengers of active forms of oxygen, the inactivation is likely mediated by semidehydroascorbate, and/or it may involve site-specific generation of the damaging intermediates.