South African Journal of Chemistry
On-line version ISSN 1996-840X
Print version ISSN 0379-4350
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) from one 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 6.8 x 103 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.
Keywords : Cysteine; nitric oxide; nitrosothiol.