When primary bovine adrenocortical cells were cultured in a serum- and lipoprotein-free medium, the pattern of steroids synthesized from pregnenolone precursor was altered by prior treatment with ACTH and by prior exposure to pregnenolone. As evidenced by the high performance liquid chromatographic profile of steroids synthesized from pregnenolone, ACTH induced 17-hydroxylase activity, whereas, regardless of ACTH treatment, prior exposure to pregnenolone reduced 21-hydroxylase activity. Direct addition of possible secreted products of pregnenolone–androstenedione, progesterone, and 17-hydroxyprogesterone (17-hydroxypregn-4-ene-3,20-dione)–was found to cause loss of 21-hydroxylase activity measured as 21-deoxycortisol (llβ,17-dihydroxypregn-4-ene-3,20-dione) conversion to cortisol. Several steroids that caused loss of 11β-hydroxylase activity did not affect 21-hydroxylase. Androstenedione suppressed both 21- and llβ-hydroxylase but did not affect 17-hydroxylase or C17,20-lyase activities. The time course of loss of activity on treatment with androstenedione and the effective concentrations were very similar for 21- and llβ-hydroxylase. Other androgens, testosterone and 5α-dihydrotestosterone, also caused loss of 21-hydroxylase activity, but dehydroepiandrosterone was ineffective, and a survey of the effects of a variety of C19 steroids showed that androgenic activity as such was not required. Progesterone and analogs, which are 21-hydroxylase substrates, caused loss of 21-hydroxylase. Since androstenedione and other C19 steroids are pseudosubstrates for 21-hydroxylase, it appears likely that formation of a complex with cytochrome P-45021 is responsible for the loss of enzyme activity. Because the concentration of androstenedione in the adrenal cortex in vivo is in the range here shown to suppress 21-hydroxylase, it is possible that this action occurs in vivo. If so, it may affect adrenal androstenedione synthesis by diverting precursor away from the glucocorticoid pathway.
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