The glucuronidation of the food-borne heterocyclic amine 2-amino-1-methyl-6-phenylimidazo-[4,5-b] pyridine (PhIP) was investigated using hepatic microsomes from several species. PhIP-glucuronic acid conjugates were formed in an NADPH-free system using microsomes from rabbit, dog, guinea pig, and human. Rat, hamster, and mouse microsomes were incapable of directly producing PhIP-glucuronides. The PhIP-glucuronide generated with human microsomes could be resolved by reverse-phase HPLC from that produced with rabbit microsomes. In addition, the human PhIP-glucuronide was susceptible to enzymatic hydrolysis by β-glucuronidase, whereas the rabbit PhIP-glucuronide did not undergo β-glucuronidase catalyzed hydrolysis. Fast atom bombardment mass spectrometry of both glucuronides revealed the presence of ions with m/z 401 (M + H+). Rabbit PhIP-glucuronide had a λmax of 316 nm, similar to that of the parent PhIP. By contrast, a spectral shift in UV absorbance was observed for the human PhIP-glucuronide, which had a λmax of 305 nm. 1H-NMR spectroscopy and nuclear Overhauser enhancements established that rabbit PhIP-glucuronide was conjugated at the exocyclic amine nitrogen, whereas human PhIP-glucuronide was conjugated at the N3 imidazole ring nitrogen. Km values for PhIP were 0.2–0.3 mM in both species; however, rabbit microsomes exhibited a 22-fold higher Vmax. Collectively, these studies indicate that human and rabbit liver microsomes form structurally different glucuronides of PhIP and suggest the involvement of multiple isoforms of UDP-glucuronosyltransferase. Further, these data suggest that in certain species, including humans, the direct conjugation of PhIP with glucuronic acid may represent a primary route of PhIP metabolism and detoxication.
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