Cysteine S-conjugate N-acetyltransferase from rat kidney microsomes

An acetyltransferase from rat kidney microsomes that catalyzes the N-acetylation of thioethers of L-cysteine has been solubilized, stabilized, and separated from hydrolytic enzymes active against both the acetylated product, a mercapturic acid, and acetyl coenzyme A. Efficiency of catalysis varies with the lipophilicity of the substituent at sulfur in the order, ethyl less than propyl less than benzyl less than butyl, as predicted by the Hansch pi constants. Although L-tryptophan is acetylated at a very low rate, acetylation is not detectable for L-cysteine, L-methionine, L-serine, L-leucine, L-phenylalanine, or L-glutamic acid. The properties and substrate specificity reported here, along with previous studies on enzyme distribution, suggest that cysteine S-conjugate N-acetyltransferase is responsible for the final step in mercapturic acid biosynthesis.