Nitroxyl oxidizes NADPH in a superoxide dismutase inhibitable manner

Nitric oxide synthases (NOS) convert image-arginine and Nω-hydroxy-image-arginine to nitric oxide (√NO) and/or nitroxyl (NO−) in a NADPH-dependent fashion. Subsequently, √NO/superoxide (O2−-derived peroxynitrite (ONOO−) consumes one additional mol NADPH. The related stoichiometry of NO− and NADPH is unclear. We here describe that NO− also oxidizes NADPH in a concentration-dependent manner. In the presence of superoxide dismutase (SOD), which also converts NO− to √NO, nitrite accumulation was almost doubled and no oxidation of NADPH was observed. Nitrate yield from NO− was low, arguing against intermediate ONOO− formation. Thus, biologically formed NO− may function as an effective pro-oxidant unless scavenged by SOD and affect the apparent NADPH stoichiometry of the NOS reaction.