UDP-Glucuronosyltransferase-mediated Metabolic Activation of the Tobacco Carcinogen 2-Amino-9H-pyrido[2,3-b]indole
作者:Yijin Tang、David M. LeMaster、Gwendoline Nauwelaërs、Dan Gu、Sophie Langouët、Robert J. Turesky
DOI:10.1074/jbc.m111.320093
日期:2012.4
2-Amino-9H-pyrido[2,3-b]indole (A alpha C) is a carcinogenic heterocyclic aromatic amine (HAA) that arises in tobacco smoke. UDP-glucuronosyltransferases (UGTs) are important enzymes that detoxicate many procarcinogens, including HAAs. UGTs compete with P450 enzymes, which bioactivate HAAs by N-hydroxylation of the exocyclic amine group; the resultantN-hydroxy-HAA metabolites form covalent adducts with DNA. We have characterized the UGT-catalyzed metabolic products of A alpha C and the genotoxic metabolite 2-hydroxyamino-9Hpyrido[2,3-b] indole (HONH-A alpha C) formed with human liver microsomes, recombinant human UGT isoforms, and human hepatocytes. The structures of the metabolites were elucidated by H-1 NMR and mass spectrometry. A alpha C and HONH-A alpha C underwent glucuronidation by UGTs to form, respectively, N-2-(beta-D-glucosiduronyl)- 2-amino-9H-pyrido[2,3-b] indole (A alpha C-N-2-Gl) and N-2-(beta-D-glucosidurony1)-2-hydroxyamino-9H-pyrido[2,3-b] indole (A alpha C-HON2-Gl). HONH-A alpha C also underwent glucuronidation to form a novel O-linked glucuronide conjugate, O(beta-D-glucosiduronyl)-2-hydroxyamino-9H-pyrido[2,3-b] indole (A alpha C-HN2-O-Gl). A alpha C-HN2-O-Glisabiologicallyreactivemetabolite and binds to calf thymus DNA (pH 5.0 or 7.0) to form the N-(deoxyguanosin-8-yl)-A alpha C adduct at 20-50-fold higher levels than the adduct levels formed with HONH-A alpha C. Major UGT isoforms were examined for their capacity to metabolize A alpha C and HONH-A alpha C. UGT1A4was the most catalytically efficient enzyme (V-max/K-m) at forming A alpha C-N-2-Gl (0.67 mu l.min(-1).mg of protein(-1)), and UGT1A9 was most catalytically efficient at forming A alpha C-HN-O-Gl (77.1 mu l.min(-1) mg of protein(-1)), whereasUGT1A1was most efficient at forming A alpha C-HON2-Gl (5.0 mu l.min(-1).mg of protein (-1)). Human hepatocytes produced A alpha C-N-2-Gl and A alpha C-(HNO)-O-2- Gl in abundant quantities, but A alpha C-HON2-Gl was a minor product. Thus, UGTs, usually important enzymes in the detoxication of many procarcinogens, serve as a mechanism of bioactivation of HONH-A alpha C.