Role of the Superoxide Anion in the Myeloperoxidase-mediated Antimicrobial System

An antimicrobial system has been previously described in the polymorphonuclear leukocyte which consists of myeloperoxidase, H2O2, and a halide. In the present study, H2O2 is replaced by xanthine and xanthine oxidase in the isolated myeloperoxidase-mediated antimicrobial system and the contribution of the superoxide anion (O2-) and hydroxyl radicals (·OH) to the antimicrobial effect investigated. Xanthine, xanthine oxidase, chloride, and myeloperoxidase have a marked microbicidal effect on Escherichia coli, Staphylococcus aureus, and Candida tropicalis. The requirement for H2O2 is indicated by the inhibitory effect of catalase. Superoxide dismutase, which catalyzes the conversion of (O2-) to O2 and H2O2, inhibits microbicidal activity at high concentration (g88 µg per ml); however, this is probably due, in large part, to the nonspecific inhibitory effect of protein. Cytochrome c is reduced by (O2-) and thus inhibits the conversion of (O2-) to H2O2. The microbicidal activity of the xanthine-xanthine oxidase-chloride-myeloperoxidase system is inhibited by cytochrome c and superoxide dismutase, at low concentration (4.4 µg per ml), largely reverses this inhibition. Microbicidal activity in the presence of cytochrome c and superoxide dismutase is inhibited by catalase. The hydroxyl radical scavengers, benzoate and ethanol, also have an inhibitory effect on the microbicidal system which is partially reversed by superoxide dismutase. Again, catalase abolishes the microbicidal activity of the reconstituted system. These studies suggest that superoxide dismutase and scavengers of the (O2-) and ·OH radicals influence microbicidal activity through their effect on H2O2 formation. Their interaction offers a possible means of control of the myeloperoxidase-mediated antimicrobial system in the intact cell.