EFFECT OF ACTH ON ADRENAL MITOCHONDRIAL CYTOCHROME P‐450 IN THE RAT *

One action of ACTH on the adrenal cortex is to increase the rate of cholesterol side-chain cleavage (SCC).l-a Apparently this is the rate-limiting step for corticosteroidogenesis in adrenal zona fasciculata-reticularis cells.' Studies by Koritz and Kumar' support the concept that cholesterol metabolism to pregnenolone is the rate-limiting step, and these authors suggest that ACTH controls steroidogenesis by determining the rate of efflux of pregnenolone from mitochondria and thus the extent of feedback inhibition by pregnenolone of its own synthesis.6 Faresee has shown that ACTH and cyclic AMP, when added to adrenal sections, will increase pregnenolone synthesis from endogenous precursors, and these studies suggest that the accumulation of pregnenolone does not significantly affect cholesterol SCC. Johnson and colleagues' have found an increased rate of pregnenolone formation from endogenous cholesterol in adrenal mitochondria isolated from hypophysectomized rats, following treatment with ACTH. Mitochondria1 cholesterol SCC and steroid 1 1 P-hydroxylation both involve the participation of cytochrome P-450,8-" and these two forms of cytochrome P-450 have been ~ e p a r a t e d . ~ . ' ~ The separated cholesterol SCC cytochrome P-4.50 is isolated as a high-spin heme protein, which, by analogy with the soluble cytochrome P-450 from Pseudornonus putidu," suggests that it is substrate(cholesterol)-bound. Recently'"'" it has been shown that adrenal mitochondria from ether-stressed rats have an increased rate of pregnenolone formation, and that this increased activity of cholesterol SCC is associated with an increase in the