Oxidation of microbial iron-sulfur centers by the myeloperoxidase-H2O2-halide antimicrobial system

Abstract

Myeloperoxidase, H2O2, and a halide (chloride, bromide, or iodide) form a potent microbicidal system that contributes to the antimicrobial activity of neutrophils. The mechanism of toxicity is not completely understood. Powerful oxidants are formed that presumably attack the microbe at a variety of sites. Among the consequences of this attack is the release of a large proportion of 59Fe of prelabeled organisms. We report here that the myeloperoxidase-H2O2-halide system oxidizes the iron-sulfur centers of model compounds (spinach ferredoxin) and intact microorganisms (Escherichia coli) with the loss of labile sulfide. The oxidation of the iron-sulfur centers of ferredoxin was measured by the fall in absorbance at 420 nm (bleaching) and by the loss of 5,5'-dithiobis-(2-nitrobenzoic acid) reducing activity. The latter compound is a sulfhydryl reagent that is reduced by ferredoxin labile sulfide during denaturation. The oxidation of E. coli iron-sulfur centers by the peroxidase system was determined by the loss of labile sulfide content, as measured by the release of H2S by acid and its reaction with zinc acetate to form ZnS. The halides were effective as components of the peroxidase system in the order I greater than Br greater than Cl. The oxidation of E. coli iron-sulfur centers by the peroxidase system was rapid and preceded the loss of viability. Gentamicin, at a concentration which produced a loss of viability comparable to that of the peroxidase system, did not cause a loss of labile sulfide from E. coli, suggesting that labile sulfide loss is not a nonspecific reflection of the loss of viability, but a direct consequence of the action of the myeloperoxidase system. The oxidation of iron-sulfur centers in microorganisms by the myeloperoxidase-H2O2-halide system may contribute to the death of the organism.