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

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