Multiplicity of cytochrome P-450 species involved in theophylline metabolism in mouse hepatic microsomes.

To ascertain the multiplicity of the cytochrome P-450 (P-450) species participating in the individual metabolic conversion of theophylline by 8-hydroxylation, 3-demethylation and 1-demethylation in mice, kinetics were studied under various conditions using untreated and inducer-treated mouse hepatic microsomes. Eadie-Hofstee plots of 1-demethylation in untreated microsomes exhibited a straight line, whereas those of 8-hydroxylation and 3-demethylation were curved lines. The biphasic kinetics indicated the contribution of two P-450 populations to the respective metabolic pathways; one characterized by high affinity and low capacity, the other by low affinity and high capacity. The high affinity population was efficiently induced by beta-naphthoflavone (beta-NF), and was highly susceptible to inhibition by a specific CYP1A inhibitor. The low affinity population was sensitive to induction by phenobarbital (PB), and was markedly inhibited by preferential inhibitors for PB-inducible P-450 species. The present results indicated that two P-450 populations contributed to the theophylline metabolism in mouse hepatic microsomes, and that the high and low affinity populations corresponded, respectively, to CYP1A, and a PB-inducible P-450 species having a much higher capacity than CYP1A.