INTERACTION OF METOPIRONE WITH ADRENAL MITOCHONDRIAL CYTOCHROME P-450 . A MECHANISM FOR INHIBITION OF ADRENAL STEROID 11BETA-HYDROXYLATION

The mechanism of Metopirone inhibition of the 11β-hydroxylation of deoxycorticosterone has been studied with an 11β-hydroxylase extracted from an acetone powder of beef adrenal mitochondria. Kinetic data indicate that the inhibition is competitive with substrate deoxycorticosterone, the Ki of Metopirone being 1.0 × 10-7 mole/1. Metopirone inhibition of 11β-hydroxylation results from the interaction of this compound with cytochrome P-450, the oxygenactivating and steroid-binding component of the 11β-hydroxylase system. The decomposition of cytochrome P-450 during incubation of the enzyme preparation at 37° is prevented by Metopirone at concentrations that also inhibit 11β-hydroxylation. A similar stabilization of cytochrome P-450 is exerted by substrate deoxycorticosterone. These results indicate that both Metopirone and deoxycorticosterone bind to cytochrome P-450, preventing its degradation. However, the effects of Metopirone and deoxycorticosterone on cytochrome P-450 are not additive, the recovery of the hemoprotein being no greater after incubation with both Metopirone and deoxycorticosterone than with Metopirone alone. Therefore the two compounds are competing for the same binding site on cytochrome P-450. Moreover, Metopirone, at concentrations that inhibit 11β-hydroxylation, also inhibits the spectral changes produced by interaction of deoxycorticosterone with cytochrome P-450. This inhibition can be substantially overcome by an increase in deoxycorticosterone concentration, confirming the competition between Metopirone and deoxycorticosterone for a binding site on cytochrome P-450. Metopirone itself, at these concentrations, does not induce detectable spectral changes in cytochrome P-450. It is the ability of Metopirone to interfere with the binding of substrate deoxycorticosterone to cytochrome P-450 that results in an inhibition of the 11β-hydroxylation reaction. © 1969, American Chemical Society. All rights reserved.