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

Abstract

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 × 10¹⁰ 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.