Electron-paramagnetic resonance spectroscopy using N-methyl-image-glucamine dithiocarbamate iron cannot discriminate between nitric oxide and nitroxyl: implications for the detection of reaction products for nitric oxide synthase

Purified neuronal nitric oxide synthase (NOS) does not produce nitric oxide (NO) unless high concentrations of superoxide dismutase (SOD) are added, suggesting that nitroxyl (NO−) or a related molecule is the principal reaction product of NOS, which is SOD-dependently converted to NO. This hypothesis was questioned by experiments using electron paramagnetic resonance spectroscopy and iron N-methyl-image-glucamine dithiocarbamate (Fe-MGD) as a trap for NO. Although NOS and the NO donor S-nitroso-N-acetyl-penicillamine produced an electron paramagnetic resonance signal, the NO− donor, Angeli’s salt (AS) did not. AS is a labile compound that rapidly hydrolyzes to nitrite, and important positive control experiments showing that AS was intact were lacking. On reinvestigating this crucial experiment, we find identical MGD2-Fe-NO complexes both from S-nitroso-N-acetyl-penicillamine and AS but not from nitrite. Moreover, the yield of MGD2-Fe-NO complex from AS was stoichiometric even in the absence of SOD. Thus, MGD2-Fe directly detects NO−, and any conclusions drawn from MGD2-Fe-NO complexes with respect to the nature of the primary NOS product (NO, NO−, or a related N-oxide) are invalid. Thus, NOS may form NO− or related N-oxides instead of NO.