After administration to rats of methyl acrylate (I), methyl methacrylate (II) and methyl crotonate (III), urinary mercapturic acids were isolated and identified as the dicarboxylic acids N-acetyl-S-(2-carboxyethyl)cysteine (IV, R = H), N-acetyl-S-(2-carboxypropyl)cysteine (V, R = H) and N-acetyl-S-(1-methyl-2-carboxyethyl)cysteine (VI, R = H) and for a minor part as their monomethyl esters IV (R = CH3) and VI (R = CH3). 2. After a single dose of the acrylates (I), (II) and (III) (0.14 mmol/kg), the excretion of the thioethers amounted to 6.6 +/- 0.6, 0.0, and 2.0 +/- 0.6% dose respectively. 3. After 18 h previous administration of the carboxylesterase inhibitor tri-o-tolyl phosphate (0.34 mmol/kg) the excretion of the thioethers amounted to 40.6 +/- 2.1, 11.0 +/- 3.3, and 16.0 +/- 2.0% dose. 4. For methyl acrylate (I) the ratio of the excreted dicarboxylic acid and monomethyl ester was 20:1. After previous administration of tri-o-tolyl phosphate this ratio was 1:2.
