Differential effects of oxidizing agents on human plasma .alpha.1-proteinase inhibitor and human neutrophil myeloperoxidase

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Abstract
Human .alpha.1-proteinase inhibitor is easily susceptible to inactivation because of the presence of a methionyl residue at its reactive site. Thus, oxidizing species derived from the myeloperoxidase system (enzyme, H2O2 and Cl-), as well as hypochlorous acid, can inactivate this inhibitor, although H2O2 alone has no effect. Butylated hydroxytoluene, a radical scavenger, partially protects .alpha.-proteinase inhibitor from the myeloperoxidase system and completely protects it from hypochlorous acid. Each oxidant also reacts differently with the inhibitor, in that the myeloperoxidase system and hypochlorous acid can each oxidize as many as 6 methionyl residues, but hypochlorous acid can also oxidize a single tyrosine residue. Myeloperoxidase can be inactivated by hypochlorous acid, by autoxidation in the presence of H2O2 and Cl-, as well as by H2O2 alone. Butylated hydroxytoluene completely protects this enzyme from hypochlorous acid inactivation, does not affect the action of H2O2 and enhances autoinactivation. As many as 6 methionyl residues and 2 tyrosine residues could be oxidized during autoxidation and 6 methionine residues by H2O2 alone. Eight methionine residues and 1 tyrosine residue could be oxidized by hypochlorous acid. The tyrosine residue in myeloperoxidase was oxidized only at a relatively high concentration (600 .mu.M) of hypochlorous acid at which point the enzyme simultaneously and completely lost its enzymatic activity. Loss of activity of myeloperoxidase could also be correlated with the loss of the heme groups present in the enzyme when a relatively high concentration of hypochlorous acid (600 .mu.M) was used and also during autoxidation. Once there is sufficient oxidant to modify 1 of the tyrosine residues, the heme group itself becomes susceptible.