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Oxidants derived from myeloperoxidase (MPO) contribute to inflammatory diseases. In vivo MPO activity is commonly assessed by the accumulation of 3-chlorotyrosine (3-Cl-Tyr), although 3-Cl-Tyr is formed at low yield and is subject to metabolism. Here we show that MPO activity can be assessed using hydroethidine (HE), a probe commonly employed for the detection of superoxide. Using LC/MS/MS, (1)H NMR, and two-dimensional NOESY, we identified 2-chloroethidium (2-Cl-E(+)) as a specific product when HE was exposed to hypochlorous acid (HOCl), chloramines, MPO/H2O2/chloride, and activated human neutrophils. The rate constant for HOCl-mediated conversion of HE to 2-Cl-E(+) was estimated to be 1.5 × 10(5) M(-1)s(-1). To investigate the utility of 2-Cl-E(+) to assess MPO activity in vivo, HE was injected into wild-type and MPO-deficient (Mpo(-/-)) mice with established peritonitis or localized arterial inflammation, and tissue levels of 2-Cl-E(+) and 3-Cl-Tyr were then determined by LC/MS/MS. In wild-type mice, 2-Cl-E(+) and 3-Cl-Tyr were detected readily in the peritonitis model, whereas in the arterial inflammation model 2-Cl-E(+) was present at comparatively lower concentrations (17 versus 0.3 pmol/mg of protein), and 3-Cl-Tyr could not be detected. Similar to the situation with 3-Cl-Tyr, tissue levels of 2-Cl-E(+) were decreased substantially in Mpo(-/-) mice, indicative of the specificity of the assay. In the arterial inflammation model, 2-Cl-E(+) was absent from non-inflamed arteries and blood, suggesting that HE oxidation occurred locally in the inflamed artery. Our data suggest that the conversion of exogenous HE to 2-Cl-E(+) may be a useful selective and sensitive marker for MPO activity in addition to 3-Cl-Tyr.

Peroxiredoxin 2 is a member of the mammalian peroxiredoxin family of thiol proteins that is important in antioxidant defense and redox signaling. We have examined its reactivity with various biological oxidants, in order to assess its ability to act as a direct physiological target for these species. Human erythrocyte peroxiredoxin 2 was oxidized stoichiometrically to its disulfide-bonded homodimer by hydrogen peroxide, as monitored electrophoretically under nonreducing conditions. The protein was highly susceptible to oxidation by adventitious peroxide, which could be prevented by treating buffers with low concentrations of catalase. However, this did not protect peroxiredoxin 2 against oxidation by added H(2)O(2). Experiments measuring inhibition of dimerization indicated that at pH 7.4 catalase and peroxiredoxin 2 react with hydrogen peroxide at comparable rates. A rate constant of 1.3 x 10(7) M(-1) s(-1) for the peroxiredoxin reaction was obtained from competition kinetic studies with horseradish peroxidase. This is 100-fold faster than is generally assumed. It is sufficiently high for peroxiredoxin to be a favored cellular target for hydrogen peroxide, even in competition with catalase or glutathione peroxidase. Reactions of t-butyl and cumene hydroperoxides with peroxiredoxin were also fast, but amino acid chloramines reacted much more slowly. This contrasts with other thiol compounds that react many times faster with chloramines than with hydrogen peroxide. The alkylating agent iodoacetamide also reacted extremely slowly with peroxiredoxin 2. These results demonstrate that peroxiredoxin 2 has a tertiary structure that facilitates reaction of the active site thiol with hydrogen peroxide while restricting its reactivity with other thiol reagents.

Singlet molecular oxygen (.sup.1O.sub.2) has well-established roles in photosynthetic plants, bacteria and fungi.sup.1-3, but not in mammals. Chemically generated .sup.1O.sub.2 oxidizes the amino acid tryptophan to precursors of a key metabolite called N-formylkynurenine.sup.4, whereas enzymatic oxidation of tryptophan to N-formylkynurenine is catalysed by a family of dioxygenases, including indoleamine 2,3-dioxygenase 1.sup.5. Under inflammatory conditions, this haem-containing enzyme is expressed in arterial endothelial cells, where it contributes to the regulation of blood pressure.sup.6. However, whether indoleamine 2,3-dioxygenase 1 forms .sup.1O.sub.2 and whether this contributes to blood pressure control have remained unknown. Here we show that arterial indoleamine 2,3-dioxygenase 1 regulates blood pressure via formation of .sup.1O.sub.2. We observed that in the presence of hydrogen peroxide, the enzyme generates .sup.1O.sub.2 and that this is associated with the stereoselective oxidation of l-tryptophan to a tricyclic hydroperoxide via a previously unrecognized oxidative activation of the dioxygenase activity. The tryptophan-derived hydroperoxide acts in vivo as a signalling molecule, inducing arterial relaxation and decreasing blood pressure; this activity is dependent on Cys42 of protein kinase G1[alpha]. Our findings demonstrate a pathophysiological role for .sup.1O.sub.2 in mammals through formation of an amino acid-derived hydroperoxide that regulates vascular tone and blood pressure under inflammatory conditions. Singlet molecular oxygen, produced by indoleamine 2,3-dioxygenase 1 activity, gives rise to a signalling molecule that regulates arterial relaxation under inflammatory conditions., Author(s): Christopher P. Stanley [sup.1] , Ghassan J. Maghzal [sup.1] [sup.2] , Anita Ayer [sup.1] [sup.2] , Jihan Talib [sup.1] [sup.2] , Andrew M. Giltrap [sup.3] , Sudhir Shengule [sup.1] [...]

Abstract: NADPH oxidases (NOX) are superoxide anion radical (O2 −•)-generating enzymes. They form a family of seven members, each with a specific tissue distribution. They function as electron transport chains across membranes, using NADPH as electron donor to reduce molecular oxygen to O2 −•. NOX have multiple biological functions, ranging from host defense to inflammation and cellular signaling. Measuring NOX activity is crucial in understanding the roles of these enzymes in physiology and pathology. Many of the methods used to measure NOX activity are based on the detection of small molecules that react with NOX-generated O2 −• or its direct dismutation product hydrogen peroxide (H2O2) to form fluorescent, luminescent, or colored products. Initial techniques were developed to measure the activity of the phagocyte isoform NOX2 during the oxidative burst of stimulated polymorphonuclear leukocytes, which generate large quantities of O2 −•. However, other members of the NOX family generate much less O2 −• and hence H2O2, and their activity is difficult to distinguish from other sources of these reactive species. In addition, O2 −• and H2O2 are reactive molecules and most probes are prone to artifacts and therefore should be used with appropriate controls and the data carefully interpreted. This review gives an overview of current methods used to measure NOX activity and NOX-derived O2 −• and H2O2 in cells, tissues, isolated systems, and living organisms, describing the advantages and caveats of many established methods with emphasis on more recent technologies and future perspectives. [Copyright &y& Elsevier]

During infection and inflammation, neutrophils and eosinophils produce hypochiorous acid, hypobromous acid, chloramines, and bromamines. These reactive halogen species preferentially oxidize methionine and thiols. It is commonly assumed that they convert methionine to methionine sulfoxide. However, iodine and organic chloramines are known to convert methionine to dehydromethionine, which is a cyclic azasulfonium salt. The potential for this reaction to occur in biologically relevant situations has so far been neglected. Therefore, we investigated the oxidation of methionine and N-terminal methionine residues by biologically relevant reactive halogen species and neutrophils. When hypochlorous acid reacted with methionine, two major products in addition to methionine sulfoxide were formed. They both had molecular masses two mass units lower than that . of methionine and were identified as the diastereomers of dehydromethionine. Hypochlorous acid and chloramines converted methionine to a mixture of approximately 25% dehydromethionine and 75% methionine sulfoxide. Hypobromous acid and bromamines produced upward of 50% dehydromethionine. When methionine was present on the N-termini of peptides, reactive halogen species oxidized them to dehydromethionine with yields as high as 80%. Formylated N-terminal methionines and non-N-terminal methionine residues gave stoichiometric production of the corresponding sulfoxides only. Purified myeloperoxidase used hydrogen peroxide and chloride to catalyze the oxidation of N-terminal methionines to dehydromethionine. Neutrophils oxidized extracellular methiontne to 30% dehydromethionine and 70% methionine sulfoxide. They also oxidized their intracellular methionine to dehydromethionine as well as methionine sulfoxide. We propose that reactive halogen species will produce dehydromethionine and form azasulfonium cations on the N-termini of peptides and proteins during inflammatory events. [ABSTRACT FROM AUTHOR]

Myeloperoxidase uses hydrogen peroxide to oxidize numerous substrates to hypohalous acids or reactive free radicals. Here we show that neutrophils oxidize melatonin to N¹-acetyl-N²-formyl-5-methoxykynuramine (AFMK) in a reaction that is catalyzed by myeloperoxidase. Production of AFMK was highly dependent on superoxide but not hydrogen peroxide. It did not require hypochlorous acid, singlet oxygen, or hydroxyl radical. Purified myeloperoxidase and a superoxide-generating system oxidized melatonin to AFMK and a dimer. The dimer would result from coupling of melatonin radicals. Oxidation of melatonin was partially inhibited by catalase or superoxide dismutase. Formation of AFMK was almost completely eliminated by superoxide dismutase but weakly inhibited by catalase. In contrast, production of melatonin dimer was enhanced by superoxide dismutase and blocked by catalase. We propose that myeloperoxidase uses superoxide to oxidize melatonin by two distinct pathways. One pathway involves the classical peroxidation mechanism in which hydrogen peroxide is used to oxidize melatonin to radicals. Superoxide adds to these radicals to form an unstable peroxide that decays to AFMK. In the other pathway, myeloperoxidase uses superoxide to insert dioxygen into melatonin to form AFMK. This novel activity expands the types of oxidative reactions myeloperoxidase can catalyze. It should be relevant to the way neutrophils use superoxide to kill bacteria and how they metabolize xenobiotics. [ABSTRACT FROM AUTHOR]