The reaction of superoxide radicals with S-nitrosoglutathione and the products of its reductive heterolysis.

ABSTRACT

Generation of superoxide radicals (0.01-0.1 microm s(-1)) by radiolysis of aqueous solutions containing S-nitrosoglutathione (45-160 microm, pH 3.8-7.3) resulted in loss of this solute at rates varying with solute concentration, radical generation rate, and pH. The results were quantitatively consistent with the loss being attributed to competition between reaction of superoxide with S-nitrosoglutathione (rate constant 300 +/- 100 m(-1) s(-1)) and the pH-dependent disproportionation of superoxide/hydroperoxyl. This rate constant is much lower than previous estimates and seven orders of magnitude lower than the rate constants between superoxide and superoxide dismutase or superoxide and nitric oxide. This indicates that interaction between superoxide and S-nitrosoglutathione is unlikely to be biologically important, contrary to previous suggestions that reaction could serve to prevent the rapid reaction between superoxide and nitric oxide. Reductive homolysis of S-nitrosoglutathione by the carbon dioxide radical anion, a model for biological reductants such as disulfide radical anions, occurred with a rate constant of 7.4 x 10(8) m(-1) s(-1) and produced nitric oxide stoichiometrically. Thiyl radicals were not produced, indicating the alternative homolysis route to generate nitroxyl did not occur.