Kinetics of the reactions of hypochlorous acid and amino acid chloramines with thiols, methionine, and ascorbate

Thiol oxidation by hypochlorous acid and chloramines is a favorable reaction and may be responsible for alterations in regulatory or signaling pathways in cells exposed to neutrophil oxidants. In order to establish the mechanism for such changes, it is necessary to appreciate whether these oxidants are selective for different thiols as compared with other scavengers. We have measured rate constants for reactions of amino acid chloramines with a range of thiols, methionine, and ascorbate, using a combination of stopped-flow and competitive kinetics. For HOCl, rate constants are too fast to measure directly by our system and values relative to reduced glutathione were determined by competition with methionine. For taurine chloramine, the rate constants for reaction with 5-thio-2-nitrobenzoic acid, GSH, methionine, and ascorbate at pH 7.4 were 970, 115, 39, and 13 M(-1) s(-1), respectively. Values for 10 thiols varied by a factor of 20 and showed an inverse relationship to the pK(a) of the thiol group. Rate constants for chloramines of glycine and N-alpha-acetyl-lysine also showed these relationships. Rates increased with decreasing pH, suggesting a mechanism involving acid catalysis. For hypochlorous acid, rates of reaction with 5-thio-2-nitrobenzoic acid, GSH, cysteine, and most of the other thiols were very similar. Relative reactivities varied by less than 5 and there was no dependence on thiol pK(a). Chloramines have the potential to be selective for different cellular thiols depending on their pK(a). For HOCl to be selective, other factors must be important, or its reactions could be secondary to chloramine formation.