Ethoxyquin as an inducer and inhibitor of phenobarbital-type cytochrome P-450 in rat liver microsomes

Abstract

The effect of ethoxyquin in vivo and in vitro on drug metabolism in rat liver microsomes was studied. In feeding experiments, a threshold dose of induction was found at 0.05% ethoxyquin for 14 days. At 0.5% ethoxyquin, relative liver weight, cytochrome P-450 content, cytochrome b 5 content, ethylmorphine demethylation, and ethoxycoumarin deethylation were increased by a factor of 1.5 to 2. Aryl hydrocarbon hydroxylase activity was, however, not induced but even decreased by 0.5% ethoxyquin in food. Induction of epoxide hydratase was marked, amounting to 400% of control after 0.5% ethoxyquin. The induced enzyme was similar to the phenobarbital-inducible cytochrome P-450 in its CO spectrum, in its affinity for the ligand metyrapone, in the preferential inhibition of monooxygenase activity by metyrapone and not α-naphthoflavone, and in the increase in the phenobarbital-inducible band after gel electrophoresis of microsomal proteins. Mixed pretreatment with ethoxyquin and 3-methylcholanthrene led to formation of cytochrome P-448. However, monooxygenase activities were slightly lower and epoxide hydratase activity was considerably higher than after treatment with 3-methylcholanthrene alone. Ethoxyquin inhibited benzo( a)pyrene hydroxylation and ethoxycoumarin deethylation in vitro. These reactions were less sensitive to ethoxyquin in 3-methylcholanthrene-stimulated microsomes than in phenobarbital-stimulated microsomes, indicating that ethoxyquin does not only preferentially induce but also preferentially inhibits phenobarbital-type cytochrome P-450.