Peptide methionine sulfoxide reductase from Escherichia coli and Mycobacterium tuberculosis protects bacteria against oxidative damage from reactive nitrogen intermediates

Inducible nitric oxide synthase (iNOS) plays an important role in host defe nse. Macrophages expressing MOS release the reactive nitrogen intermediates (RNI) nitrite and S-nitrosoglutathlone (GSNO), which are bactericidal in v itro at a pH characteristic of the phagosome of activated macrophages. We s ought to characterize the active intrabacterial forms of these RNI and thei r molecular targets. Peptide methionine sulfoxide reductase (MsrA; EC 1.8.4 .6) catalyzes the reduction of methionine sulfoxide (Met-O) in proteins to methionine (Met). E. coli lacking MsrA were hypersensitive to killing not o nly by hydrogen peroxide, but also by nitrite and GSNO. The wild-type pheno type was restored by transformation with plasmids encoding msrA from E. col i or M. tuberculosis, but not by an enzymatically inactive mutant msrA, ind icating that Met oxidation was involved in the death of these cells. It see med paradoxical that nitrite and GSNO kill bacteria by oxidizing Met residu es when these RNI cannot themselves oxidize Met. However, under anaerobic c onditions, neither nitrite nor GSNO was bactericidal. Nitrite and GSNO can both give rise to NO, which may react with superoxide produced by bacteria during aerobic metabolism, forming peroxynitrite, a known oxidant of Met to Met-O. Thus, the findings are consistent with the hypotheses that nitrite and GSNO kill E. coli by intracellular conversion to peroxynitrite, that in tracellular Met residues in proteins constitute a critical target for perox ynitrite, and that MsrA can be essential for the repair of peroxynitrite-me diated intracellular damage.