Mechanistic studies on the conjugation of methyl isocyanate with N-acetyl-cysteine in rats

Abstract

In order to investigate the utility of selected thiols as scavengers of MIC, we first assessed the chemical stability of SMG, AMCC and SMC by measuring their rates of reaction in vitro with thiophan. The inital rates of carbamoylation of thiorphan (0.5 mM) by the above conjugates (0.5 mM) in aqueous buffer at pH 7.4 and 37°C were 2.51, 0.76 and 8.47 μmol L⁻¹ min⁻¹, respectively, indicating that the mercapturate AMCC was the most stable of the three MIC conjugates.

In light of these results, studies were conducted to examine the effect of pretreatment withN-acetyl-l-cysteine (l-NAC; 500 mg kg⁻¹, i.p.) on the urinary elimination of AMCC in rats dosed with MIC (15 mg kg⁻¹, i.p.). In separate experiments, groups of rats were pretreated with eitherN-acetyl-d-cysteine (d-NAC) orN-trideuteroacetyl-l-cysteine (d₃-l-NAC) in order to explore the mechanism by which MIC undergoes conjugation to AMCCin vivo. The results indicated that exogenous NAC effectively enhances the urinary excretion of MIC in the form of AMCC, and that it does so largely by direct conjugation with the isocyanate, rather thanvia biosynthesis to GSH.