Purification and Characterization of Hamster Hepatic Microsomal N,O-Acetyltransferase.

A microsomal N,O-acetyltransferase which activates carcinogenic arylacetohydroxamic acids was purified 75-fold from hamster liver sequentially by anion exchange column chromatography, chromatofocusing, gel filtration, and hydroxyapatite column chromatography. The purified enzyme, AT-2, was a glycoprotein with a molecular weight of 60000 and a pI value of 5.4. The N-terminal amino acid sequence of AT-2 was: 60000 and a pI value of 5.4. The N-terminal amino acid sequence of AT-2 was: Asp-Ser-Pro-Ser-Pro-Ile-Arg-Asn-Thr-His-Thr-Gly-Gln-Val-Arg-Gly-Leu-Val- His- Lys-. This sequence was highly homologous to that of the form 2 carboxylesterase of rabbit liver, but not to that of major hepatic microsomal carboxylesterases of hamster and other species. AT-2 catalyzed the hydrolysis of 4-nitrophenyl acetate and the N,O-acetyltransfer of N-hydroxy-2-acetylaminofluorene. Both enzyme activities were strongly inhibited by paraoxon, but not by iodoacetamide. These results demonstrate that this N,O-acetyltransferase is a member of carboxylesterase (EC 3.1.1.1).