Reaction of hydroxyl radicals with S-nitrosothiols: determination of rate constants and end product analysis

The reaction of the hydroxyl radical (.OH) with S-nitroso derivatives of cysteine, acetylcysteine and glutathione was studied at neutral and acidic pH. The second-order rate constants were determined by a competition kinetic method using a deoxyribose-thiobarbituric acid assay. The rate constants were diffusion controlled and were 2.27, 1.94 and 1.46 x 10(10) dm3 mol-1 s-1, for S-nitrosocysteine, S-nitrosoacetylcysteine and S-nitrosoglutathione respectively, at neutral pH. The major products of the degradation induced by .OH were found to be the corresponding disulfide (-S-S-) and nitrite (NO2-) at neutral pH as well as at pH 3. Simultaneous proton formation has also been observed. A plausible mechanism based on the formation of an intermediate thiol radical (RS.), as a result of electron transfer from the S-nitrosothiols (RSNOs) to .OH, is proposed for the formation of disulfide and nitrite at both pHs. The high rate constant values and the degradation of these compounds demonstrate the potential role of .OH in RSNO metabolism under physiological conditions.