Metabolic Activation of Pyrolysate Arylamines by Human Liver Microsomes; Possible Involvement of a P‐448‐H Type Cytochrome P‐450

Metabolic activating capacity of human livers for carcinogenic heterocyclic arylamines has been studied using a Salmonella mutagenesis test. A large individual variation was observed among 15 liver samples in the capacities of activation of Glu‐P‐1(2‐amino‐6‐methyldipyrido[1,2‐α:3′,2′‐ d ]imidazole), IQ (2‐amino‐3‐methylimidazo[4,5‐ f ]quinoline) and MeIQx (2‐amino‐3,8‐dimethyl‐3 H ‐imidazo[4,5‐ f ]quinoxaline). The average numbers of revertants induced by the three heterocyclic arylamines were nearly the same or rather higher in the presence of hepatic microsomes from human than those from rat. In high‐performance liquid chromatography, formation of N‐hydroxy‐Glu‐P‐1 was detected and accounted for more than 80% of the total mutagenicity observed in the human microsomal system with Glu‐P‐1, indicating that, similarly to experimental animals, N‐hydroxylation is a major activating step for heterocyclic arylamines in human. Addition of flavone or 7,8‐benzoflavone to human liver microsomes showed effective inhibition of the mutagenic activation of Glu‐P‐1, although the treatment rather enhanced microsomal benzo[ a ]pyrene hydroxylation in human livers. Mutagenic activation of Glu‐P‐1 by human liver microsomes was also decreased by the inclusion of anti‐rat P‐448‐H IgG, and was well correlated with the content of immunoreactive P‐448‐H in livers, suggesting the involvement of a human cytochrome P‐450, which shares immunochemical and catalytic properties with rat P‐448‐H, in the metabolic activation of heterocyclic arylamines in human livers.