MECHANISTIC STUDIES ON THE CONJUGATION OF METHYL ISOCYANATE WITH N-ACETYL-CYSTEINE IN RATS

In order to investigate the utility of selected thiols as scavengers o f MIG, we first assessed the chemical stability of SMG, AMCC and SMC b y measuring their rates of reaction in vitro with thiorphan. The initi al rates of carbamoylation of thiorphan (0.5 mM) by the above conjugat es (0.5 mM) in aqueous buffer at pH 7.4 and 37 degrees C were 2.51, 0. 76 and 8.47 mu mol L(-1) min(-1), respectively, indicating that the me rcapturate AMCC was the most stable of the three MIC conjugates. In li ght of these results, studies were conducted to examine the effect of pretreatment with N-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 ei ther N-acetyl-D-cysteine (D-NAC) or N-trideuteroacetyl-L-cysteine (d(3 )-L-NAC) in order to explore the mechanism by which MIC undergoes conj ugation to AMCC in vivo. The results indicated that exogenous NAC effe ctively enhances the urinary excretion of MIC in the form of AMCC, and that it does so largely by direct conjugation with the isocyanate, ra ther than via biosynthesis to GSH.