Glucuronidation of 2-hydroxyamino-1-methyl-6-phenylimidazo[4,5-b]pyridine by human microsomal UDP-glucuronosyltransferases: identification of specific UGT1A family isoforms involved

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine is a heterocyclic aromatic amine found in cooked meats and dietary exposure to PhIP has been implicate d in the etiology of colon cancer in humans. PhIP, along with other heteroc yclic aromatic amines, requires metabolic activation to exhibit genotoxic e ffects, PhIP is initially oxidized by the activity of cytochrome P4501A2 to produce 2-hydroxyamino-1-methyl-6-phenylimidazo[4,5-b]pyridine (N-OH-PhIP) , a reaction occurring primarily in the liver. Whereas subsequent biotransf ormation of N-OH-PhIP via acetylation or sulfation can produce reactive ele ctrophiles that readily bind to DNA, N-glucuronidation, catalyzed by UDP-gl ucuronosyltransferases (UGTs), functions as a detoxification mechanism. Alt hough hepatic glucuronidation of N-OH-PhIP has been well characterized, the extrahepatic metabolism of this compound is poorly understood. Studies in our laboratory now indicate that the intestinal tract, and particularly the colon, is a significant site of glucuronidation of N-OH-PhIP, When assays were performed with microsomes prepared from the mucosa of the intestinal t ract, it was determined that glucuronidation of N-OH-PhIP occurs throughout the intestinal tract, with activity approximately three times higher in th e colon as that found in the upper intestine. Glucuronidation rates from co lon microsomes showed considerable interindividual variability and incubati on with N-OH-PhIP yielded two glucuronides, HPLC analysis showed that the p redominant product formed is the N-OH-PhIP-N-2-glucuronide, while the N3-gl ucuronide accounts for <10% of the total glucuronidation product. These rat es approach the rates found in human liver microsomes, demonstrating the si gnificance of extrahepatic metabolism of this food-borne carcinogen. Subseq uent assays with human recombinant UGTs demonstrated that at least four hum an UGT isoforms, all from the UGT1A subfamily, are capable of catalyzing th e biotransformation of N-OH-PhIP, Members of the UGT2B family available for this study did not conjugate N-OH-PhIP, although immunoinhibition studies in human liver microsomes strongly suggest the involvement of a UGT2B isofo rm(s) in this organ.