METABOLISM OF THE FOOD-DERIVED MUTAGEN AND CARCINOGEN 2-AMINO-1-METHYL-6-PHENYLIMIDAZO(4,5-B)PYRIDINE (PHIP) BY HUMAN LIVER-MICROSOMES

Animal studies have shown that 2-amino-1-methyl-6-phenylimidazo(4,5-b) pyridine (PhIP) undergoes both activation to a genotoxic metabolite an d detoxication, catalysed by CYP enzymes. In this study, using direct chemical analysis, we have examined PhIP metabolism by the microsomal fraction of human liver for comparison to that occurring in animals. P hIP was incubated with human liver microsomes in the presence of an NA DPH regenerating system and the reaction mixture then analyzed by HPLC . Only one metabolite, identified as N-hydroxy PhIP, was produced. The N-hydroxylation of PhIP by human liver microsomal fraction obeyed Mic haelis - Menten kinetics, with a K-m of 55 mu M and a V-max of 666 pmo l/min/mg protein. Furafylline, a potent and specific inhibitor of CYP1 A2 in man, inhibited this reaction by > 95%, with an IC50 of 0.6 mu M. PhIP inhibited high affinity phenacetin O-deethylase activity of huma n liver microsomes, an activity catalysed specifically by CYP1A2, with an IC50 of about 80 mu M. These data indicate that, in human liver mi crosomes, N-hydroxylation is the only route of oxidative metabolism of PhIP, yielding a genotoxic species, and that this reaction is catalys ed almost exclusively by CYP1A2. Furthermore, the exclusive oxidative activation of PhIP by human liver is in direct contrast to PhIP metabo lism in rodents and non-human primates where oxidative detoxication pr oducts predominate.