Formation of peroxynitrite during thiol-mediated reduction of sodium nitroprusside.

Aerobic incubations of equimolar concentrations (5-500 microM) of sodium nitroprusside (SNP) and dithiothreitol (DTT) carried out at pH 7.4 in the absence of light caused a concentration-dependent increase in the rates of oxidation of dihydrorhodamine-123. The enhancement of the rates of oxidation under such conditions was only partially sensitive to the inhibition by 100 mM dimethyl sulfoxide implying the involvement of both peroxynitrite and hydroxyl radicals in the observed effects. The oxidation of dihydrorhodamine-123 in the presence of SNP and DTT was nearly completely abolished by superoxide dismutase (20 U/ml). It was found that such an effect of the enzyme was related primarily to the stabilization of an intermediate of SNP reduction formed upstream to the liberation of nitrosonium ligand. Increased rates of oxidation of dihydrorhodamine-123 were also observed during the reduction of SNP with either L-cysteine or glutathione. It is concluded that thiol-mediated reduction of SNP under aerobic conditions is accompanied by the formation of oxygen-derived free radicals. Nitrosonium ligand liberated from the product(s) of SNP reduction is, under such conditions, converted to peroxynitrite.