Preferential Inhibition of Mouse Hepatic Coumarin 7‐Hydroxylase by Inhibitors of Steroid Metabolizing Monooxygenases

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Abstract
Etomidate, metomidate and metyrapone, all potent inhibitors of steroid metabolizing monooxygenases, inhibit preferentially coumarin 7‐hydroxylase (COH) amongst several liver microsomal monooxygenase activities from control and pyrazole‐treated D2 mice in vitro. SKF‐525A, an inhibitor of phenobarbital‐inducible monooxygenase activities has a much weaker effect on COH than the other three drugs, even though COH is a phenobarbital‐inducible enzyme. Treatment of mice with eto‐ and metomidate decreases the microsomal COH also in vivo while the other activities remained unchanged (with the exception of 7‐ethoxycoumarin O‐deethylase (ECDE) in case of metomidate). Despite of the decrease in COH no parallel decrease in the amount of microsomal P450Coh (P450 isoenzyme highly active in the 7‐hydroxylation of coumarin) could be found in dot immuno‐binding analysis. These data suggest that among several liver microsomal P450 isoenzymes, metyrapone, eto‐ and metomidate interact preferentially with the P450Coh and that eto‐ and metomidate may alter selectively the catalytic properties of P450Coh leading to decreased enzyme activity. Two different Ks‐values could be found for all three drug in their binding to microsomal cytochrome(s) P450. Based on substrate binding spectra, potassium ferricyanide treatment does not dissociate the complex between reduced P450 and metomidate and does it only partly for etomidate. Furthermore potassium ferricyanide treatment of microsomes does not increase COH after in vivo treatment of mice with eto‐ and metomidate. These data further suggest that the complex between P450Coh and eto‐ and metomidate may be particularly strong and independent from the redox state of the haem iron.