Formation of singlet oxygen by the myeloperoxidase-mediated antimicrobial system.

Myeloperoxidase. H,O,, and a halide form a potent antimicrobial system which is operative in the polymorphonuclear leukocyte (PMN). The formation of singlet oxygen (‘0,) by this system is suggested by (a) the conversion of 2,5diphenylfuran to its specific singlet oxygen product cisdibenzoylethylene (cis-DBE); (b) the inhibition of this reaction by the ‘0, quenchers p-carotene, bilirubin, histidine, and 1,4-diazabicyclo[2,2,2Ioctane (DABCO); and (c) the stimulation of conversion by D,O, which prolongs the lifetime of IO2 in solution. Diphenylfuran conversion by the myeloperoxidase system has a pH optimum of 4.5 and an optimum H,O, concentration of 30 pm and the halides vary in effectiveness in the order Br > Cl> I-. Reagent H,O, could be replaced by the H,O,-generating system, glucose + glucose oxidase. Hypochlorous acid, which is formed by the myeloperoxidase/HaOz/chloride system, also converts diphenylfuran to cis-DBE. As with the myeloperoxidase system, this conversion has an acid pH optimum, is inhibited by ‘0, quenchers, and is stimulated by D,O. Diphenylfuran conversion by HOC1 is increased by chloride, but not by H202. in the concentrations employed in the myeloperoxidase system. Our studies suggest that ‘0, is formed by the myeloperoxidase/H,O,/chloride system and that the mechanism involves the initial oxidation of chloride to HOC1 and the subsequent decomposition of HOCl, particularly in the presence of excess chloride, to form ‘0,. The singlet oxygen so formed may participate in the microbicidal activity of the isolated myeloperoxidase system and of the intact PMN.