Activation of the food-derived mutagen 2-amino-1-methyl-6-phenylimidazo[4, 5-b]pyridine by rabbit and human liver microsomes and purified forms of cytochrome P-450

The specificity of rabbit cytochrome P-450 involved in the mutagenicactivation of 2-amino-1-methyl-6-phenylimid-azo[4, 5-b]pyridine(PhIP) was assessed using control and induced rabbit liver andlung microsomes, and six purified forms of cytochrome P-450.The number of revertants produced/2.5µg PhIP by controlrabbit liver was 260 ± 196/10 µg of microsomalprotein (mean ± SD; n = 3), and this increased to 1265± 248 when 2,3,7,8-tetrachloro-dibenzo-p-dioxin (TCDD)-inducedliver microsomes were used as the activation source in the Amestest. Microsomes form phenobarbital-, rifampicin-and acetone-pretreatedrabbits showed no increase in activity over controls. Controllung microsomes did not activate PhIP to a mutagen, whereasTCDD-induced lung microsomes produced 1443 ± 136 (mean± SD; n=4) Ames/Salmonella revertants/100 µg protein.In reconstitution experiments cytochrome P450 forms 4 and 6were found to be efficient activators of PhIP to a mutagen.Form 6 was 3.1-fold more active than form 4 and produced 4577revertants/10 pmol with a 20-min preincubation step in the Amestest. Cytochrome form 5 produced 17 revertants/10 pmol and forms2, 3b and 3c were not active in metabolizing PhIP to a mutagen.A highly significant statistical correlation existed betweenthe capacity of control and induced liver microsomes to activatePhIP to a mutagen and their cytochrome P-450 form 4 (r = 0.97,r² = 0.94) and form 6 (r = 0.95, r² = 0.90) content. These datastrongly support the involvement of polycyclic hydrocarbon-inducibleforms of cytochrome P450 in the activation of PhIP in the rabbit.Anti-rabbit forms 4 and 6 IgGs recognized proteins in sevenhuman liver microsomes of comparable mol. wt to rabbit cytochromeP-450 forms 4 and 6. However, no correlation existed betweenthe content of these proteins and the capacity of human livermicrosomes to activate PhIP.