Biochemical characterization of coumarin 7-hydroxylase activity in chick embryo liver microsomes

Coumarin occurs naturally in the diet and can induce and inhibit cytochrome P450 enzymes. Hepatic coumarin 7-hydroxylase activity is the major pathway for coumarin metabolism in humans but not in rats, most strains of mice, or other laboratory animals. Coumarin 7-hydroxylase activity and the effects of chemical inhibitors and inducers on this activity were studied in 19-day-old chick embryo liver microsomes. Activity was between 35 and 75 nmol/mg protein/hr which is approximately 2-fold higher than reported for human liver microsomes. The pH optimum was 7.8 and the K_m determined by both an ether extraction and a high performance liquid chromatography method was 7.3 ± 0.9 (±SD) μM. Substrate inhibition was evident at coumarin concentrations above 250 μM (activities at 1000 and 4000 μM coumarin were 84 and 40% of V_max, respectively). The K_i, values (±SD) for inhibitors of microsomal coumarin 7-hydroxylase activity in vitro were: α-naphthoflavone, 46.9 ± 19.8 nM; metyrapone, 0.8 ± 0.9 μM; aniline, 12.3 ± 8.2 μM; cimetidine, 70.9 ± 27.9 μM; N-nitrosodimethylamine, 0.7 ± 0.9 mM; and dimethyl sulfoxide, 7.9 ± 1.9 mM. Treatment of chick embryos with pyrazole (40 μmol) increased coumarin 7-hydroxylase by 50% at 24 hr, but this activity was unaffected by treatment of embryos with 3-methylcholanthrene (2 μmol) or glutethimide (8 μmol). Thus, hepatic coumarin 7-hydroxylase activity in 19-day-old chick embryos is higher than in most laboratory animals and has similar biochemical properties as the enzyme in humans and mice. The chick embryo liver may be a useful system for studies on the biochemical effects of coumarin and the regulation of cytochrome P450-dependent coumarin 7-hydroxylase.