oa South African Journal of Chemistry - Kinetics and mechanism of formation of S-nitrosocysteine : research article



The kinetics and mechanism of the nitrosation of cysteine by nitrous acid has been studied in acidic medium. The stoichiometry of the reaction is strictly 1:1, with the formation of one mole of S-nitrosocysteine (CySNO) fromone mole of nitrous acid. Only two nitrosating agents were detected: nitrous acid itself and protonated nitrous acid which is the hydrated form of the nitrosonium cation, NO+. Nitric oxide itself was not detected as a nitrosant. The bimolecular rate constant for the direct nitrosation of cysteine by nitrous acid was determined to be 6.4±1.1 L mol-1 s-1, while nitrosation by the nitrosonium cation has a rate constant of 6.8x103 L mol-1 s-1. CySNO is short-lived, and decomposes completely to cystine and nitric oxide within 100 s in the presence of micromolar quantities of Cu(II) ions. Since the physiological environment contains many metal ions and metalloenzymes, it is unlikely that CySNO will be an effective carrier of NO.


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