REACTION-KINETICS FOR NITROSATION OF CYSTEINE AND GLUTATHIONE IN AEROBIC NITRIC-OXIDE SOLUTIONS AT NEUTRAL PH - INSIGHTS INTO THE FATE AND PHYSIOLOGICAL-EFFECTS OF INTERMEDIATES GENERATED IN THE NO/O-2 REACTION

The critical regulatory function of nitric oxide (NO) in many physiologic processes is well established. However, in an aerobic aqueous environment NO is known to generate one or more reactive and potentially toxic nitrogen oxide (NOx) metabolites. This has led to the speculation that mechanisms must exist in vive by which these reactive intermediates are detoxified, although the nature of these mechanisms has yet to be elucidated. This report demonstrates that among the primary bioorganic products of the reaction of cellular constituents with the intermediates of the NO/O-2 reaction are S-nitrosothiol (S-NO) adducts. Anaerobic solutions of NO are not capable of nitrosating cysteine or glutathione, while S-NO adducts of these amino acids are readily formed in the presence of O-2 and NO. Investigation of the kinetics for the formation of these S-NO adducts has revealed a rate equation of d[RSNO]/dt = k(SNO)[NO](2)[O-2], where k(SNO) = (6 +/- 2) X 10(6) M(-2) s(-1), a value identical to that for the formation of reactive intermediates in the autoxidation of NO. Competition studies performed with a variety of amino acids, glutathione, and azide have shown that cysteine residues have an affinity for the NOx species that is 3 orders of magnitude greater than that of the nonsulfhydryl amino acids, and > 10(6) times greater than that of the exocyclic amino groups of DNA bases. The dipeptide alanyltyrosine reacts with the intermediates of the NO/O-2 reaction with an affinity 150 times less than that of the sulfhydryl-containing compounds. Furthermore, Chinese hamster V79 lung fibroblasts depleted of glutathione display enhanced cytotoxicity on exposure to NO. Together, these results suggest that the S-NO adduct of glutathione may represent a physiological scavenger of NOx species and that enzymes containing cysteine residues critical to their function may be subject to inhibition by reactive intermediates generated in the NO/O-2 reaction.