Synergistic activity of acetohydroxamic acid on prokaryotes under oxidative stress: The role of reactive nitrogen species

One-electron oxidation of acetohydroxamic acid (aceto-HX) initially gives rise to nitroxyl (HNO), which can be further oxidized to nitric oxide (NO) or react with potential biological targets such as thiols and metallo-proteins. The distinction between the effects of NO and HNO in vivo is masked by the reversible redox exchange between the two congeners and by the Janus-faced behavior of NO and HNO. The present study examines the ability of aceto-HX to serve as an HNO donor or an NO donor when added to Escherichia coli and Bacillus subtilis subjected to oxidative stress by comparing its effects to those of NO and commonly used NO and HNO donors. The results demonstrate that: (i) the effects of NO and HNO on the viability of prokaryotes exposed to H2O2 depend on the type of the bacterial cell; (ii) NO synergistically enhances H2O2-induced killing of E. coli, but protects B. subtilis depending on the extent of cell killing by H2O2; (iii) the HNO donor Angeli׳s salt alone has no effect on the viability of the cells; (iv) Angeli׳s salt synergistically enhances H2O2-induced killing of B. subtilis, but not of E. coli; (v) aceto-HX alone (1-4 mM) has no effect on the viability of the cells; (vi) aceto-HX enhances the killing of both cells induced by H2O2 and metmyoglobin, which may be attributed in the case of B. subtilis to the formation of HNO and to further oxidation of HNO to NO in the case of E. coli; (vii) the synergistic activity of aceto-HX on the killing of both cells induced by H2O2 alone does not involve reactive nitrogen species. The effect of aceto-HX on prokaryotes under oxidative stress is opposite to that of other hydroxamic acids on mammalian cells.