Identification of N-acetyl(2,2-dichlorovinyl)- and N-acetyl(1,2-dichlorovinyl)L-cysteine as two regioisomeric mercapturic acids of trichloroethylene in the rat

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Abstract
The regioselectivity of glutathione conjugation to trichloroethylene (TRI) and the metabolism of its cysteine and N-acetyl-L-cysteine conjugates were investigated in the rat. Intraperitoneal (ip) administration of TRI to rats at a dose of 400 mg/kg resulted in excretion in urine of small amounts of the two distinct regioisomers N-acetyl-S-(1,2-dichlorovinyl)-L-cysteine (1,2-DCV-NAC) and N-acetyl-S-(2,2-dichlorovinyl)-L-cysteine (2,2-DCV-NAC). The vicinal (vic) isomer was excreted in a 2 times higher amount (16 nmol) than the geminal (gem) isomer (8 nmol). Intraperitoneal administration of a 1:1 mixture (2.5 .mu.mol/kg each) of the two regioisomers of S-(dichlorovinyl)-L-cysteine (DCVC) to the rat resulted in excretion of the corresponding mercapturic acids in urine, the main fractions being excreted within 8 h after administration. The gem-dichlorovinyl isomer appeared to be acetylated to a higher extent than the 1,2-dichlorovinyl isomer; 73% vs 50% of the dose administered. Intraperitoneal administration of a 1:1 mixture (12.5 .mu.mol/kg each) of the two regioisomers of N-(trideuterioacetyl)-S-(dichlorovinyl)-L-cysteine (DCV-NAC-d3) resulted in excretion of both deuterium-labeled and unlabeled mercapturic acids in urine. The vic isomer was excreted unchanged at a significantly higher percentage, 34% of dose (i.e., still deuterium labeled), than the gem isomer, 17% of the dose. This suggests less efficient metabolism of the vic isomer when compared to the gem isomer. Both regioisomers of DCV-NAC-d3 were excreted in urine unlabeled at 40% of the dose, which indicates that for both isomers deacetylation, followed by reacetylation (resulting in unlabeled DCV-NAC), is an important metabolic pathway. The present data are indicative for different rates of in vivo metabolism of two regioisomers of DCVC and DCV-NAC. Experiments with commercial acylase I also showed a pronounced stereoselective deacetylation: 2,2-DCV-NAC was deacetylated at a 50-fold higher rate than 1,2-DCV-NAC. From the present in vivo data, however, it cannot be concluded which of the enzymes involved in metabolism of cysteine conjugates and mercapturic acids are responsible for the observed stereoselective metabolism in vivo.

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