Metabolism of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine by human cytochrome P4501A1, P4501A2 and P4501B1

While the metabolic activation of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) by N-hydroxylation has been well documented, the relativeroles of the human cytochrome P450 (CYP) enzymes that catalyze thisreaction have not been established. Previous studies indicated that themutagenic activation product, 2-hydroxyamino-PhIP (N2-OH- PhIP), isproduced primarily by CYP1A2, and to a lesser extent by CYP1A1. We recentlyreported that human CYP1B1 also produces N2-OH-PhIP (Carcinogenesis, 18,1793-1798, 1997). In the present study, we examined PhIP metabolism bymicrosomes containing recombinant human CYP1A1, 1A2 or 1B1 expressed in Sf9insect cells and compared the kinetic values for PhIP metabolite formation.PhIP metabolites were analyzed by high pressure liquid chromatography withfluorescence and absorbance detection. Vmax values for N2-OH-PhIP formationwere 90, 16 and 0.2 nmol/min/nmol P450, and the apparent Km values were 79,5.1 and 4.5 microM for human CYP1A2, 1A1 and 1B1, respectively. The non-mutagenic metabolite, 4'-hydroxy-PhIP, was also formed by all three CYPenzymes with Vmax values of 1.5, 7.8 and 0.3 nmol/ min/nmol P450 andapparent Km values of 43, 8.2 and 2.2 microM for human CYP1A2, 1A1 and 1B1,respectively. Although the Vmax for N2-OH-PhIP production was highest forCYP1A2, the catalytic efficiency (Vmax/Km) of CYP1A1 was greater than thatof CYP1A2. These results suggest that, for humans, extrahepatic CYP1A1 maybe more important than previously thought for the metabolic activation ofthe dietary carcinogen PhIP.