Nitrite as a substrate and inhibitor of myeloperoxidase - Implications fornitration 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 c atalyze nitration of tyrosine. In this study we have investigated the mecha nism by which it oxidizes nitrite and promotes nitration of tyrosyl residue s. Nitrite was found to be a poor substrate for myeloperoxidase but an exce llent inhibitor of its chlorination activity. Nitrite slowed chlorination b y univalently reducing the enzyme to an inactive form and as a consequence was oxidized to nitrogen dioxide. In the presence of physiological concentr ations 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 wit h a mechanism in which myeloperoxidase oxidizes free tyrosine to tyrosyl ra dicals that exchange with tyrosyl residues in peptides, These peptide radic als then couple with nitrogen dioxide to form 3-nitrotyrosyl residues. With neutrophils, myeloperoxidase-dependent nitration required a high concentra tion of nitrite (1 mM), was doubled by tyrosine, and increased 4-fold by su peroxide dismutase, Superoxide is likely to inhibit nitration by reacting w ith nitrogen dioxide and/or tyrosyl radicals. We propose that at sites of i nflammation myeloperoxidase will nitrate proteins, even though nitrite is a poor substrate, because the co-substrate tyrosine will be available to fac ilitate the reaction. Also, production of 3-nitrotyrosine will be most favo rable when the concentration of superoxide is low.