Effects of Metyrapone on Pregnenolone Biosynthesis and on Cholesterol-Cytochrome P-450 Interaction in the Adrenal1

The effects of the steroid 11²- hydroxylase inhibitor metyrapone on the conversion of cholesterol-4-14C to labeled pregnenolone have been examined in time-course incubation studies using a preparation of rat adrenal mitochon-dria. Metyrapone inhibited the cholesterol →pregnenolone reaction in a dose-related manner, the effect varying from 17 to 78‰ with 0.025 to 1.0 mM inhibitor concentrations, respectively. No effect of this agent on the conversion of 20α-hydroxycholesterol to pregnenolone was apparent. Studies of the effects of metyrapone on the changes in difference spectra resulting from cholesterol-cytochrome P-450 interaction with a preparation of acetone-extracted submitochondrial particles of bovine adrenal cortex demonstrated that metyrapone competitively inhibited the binding of cholesterol to the hemoprotein. It thus appears that inhibition by metyrapone of the two major enzyme systems of adrenal mitochondria has similar characteristics. (Endocrinology94: 1451, 1974)