Bioactivation of Heterocyclic Aromatic Amines by UDP Glucuronosyltransferases

2-Amino-9H-pyrido[2,3-b]indole (A alpha C) and 2-amino-l-methyl-6-phenylimidaz6[4,5-b]pyridine (PhIP) are Carcinogenic heterocyclic aromatic amines (HAA) that arise during the burning of tobacco and cooking of meats. UDP-glucuronosyltrarisfeinses (UGT) detoxicate many procarcino,gens and their metabolites. The genotoxic N-hydroxylated metabolite of AaC,. 2-hydroxyamino-9H-pyrido[2,3-b]indole (HONH-A alpha C), undergoes glucuronidation to form the isomeric glucuronide (Glue) conjugates N-2-(beta-D-glucosidurony1)-2-hydroxyamino-9H-pyrido[2,3-b]indole (A alpha C-HON2-Gluc) and 0 (beta-D-glucosiduronyl)-2-hydr oxyamin o-9Hpyrido [2,3-b]indole (A alpha C-HN2-O-Gluc). AaC-HON2-Gluc is a stable metabolite but A alpha C-HN2,O-Gluc is a biologically reactive intermediate, which covalently adducts to DNA at levels that are 20-fold higher than HONH-AaC. We measured the rates of formation of AaC-HON2-Gluc and A alpha C-HN2-O-Gluc in human organs: highest activity occurred with liver and kidney microsomes,. and lesser activity was found with colon and rectum microsomes. A alpha C-HN2-O-Glue formation was largely diminished in liver and kidney microsomes, by nifiumic acid, a selective inhibitor UGT1A9. In contrast, AaC-HON2-Gluc formation was less affected and other UGT contribute to N-2-glucuronidation of HONH-AaC. UGT were reported to catalyze the formation of isomeric Gluc conjugates at the N-2 and N-3 atoms of 2-hydroxyamino-1-methyl-6-phenylimidazo[4,5-b]pyridine (HONH-PhIP), the genotoxic metabolite of PhIP. However, we found, that the N3-Gluc of HONEI-PhIP also, covalently bound to DNA at higher levels than HONH-PhIP. The product ion spectra of this. Glue conjugate acquired by ion trap mass spectrometry revealed that the Gluc moiety was linked to the oxygen atom of HONH-PhIP and not the N-3 imidazole atom of the oxime tautomer of HONH-PhIP as was originally proposed. UGT1A9, an abundant UGT isoform expressed in human liver and kidney, preferitially forms the O-linked Gluc conjugates of HONH-AaC and HONEI-PhIP as opposed to their detoxicated N-2-Gluc isomers. The regioseleetive O-glucuronidation of HONH-AaC and HONH-PhIP, by UGT1A9, is a mechanism of bioactivatiOn of these ubiquitous HAAs.