Nitroxyl (NO−): a substrate for superoxide dismutase

The interactions of Cu, Zn superoxide dismutase (SOD) with nitroxyl (NO−) and nitric oxide (NO), both of which are thought to be biologically significant, have been studied but remain undefined. Having previously noted that NO− can reduce Cu (II), Zn SOD aerobically, we now report that it also can do so anaerobically and that Cu, Zn SOD can catalyze the elimination of NO− in the absence of O2. NO− acts as a reductant of ferricytochrome c anaerobically, but in the presence of O2 causes the oxidation of ferrocytochrome c and NADPH. Equivalent fluxes of NO−, and NO+O2−, were able to comparably oxidize NADPH, but the oxidation by NO+O2− was more than fivefold more sensitive to inhibition by Cu, Zn SOD than was the oxidation by NO−. Thus Cu, Zn SOD inhibited NADPH oxidation by NO− by a route independent of catalyzing the dismutation of O2−. Plausible mechanisms for those observations are offered and rate constants are estimated.