The effect of cytochrome b₅ on progesterone metabolism in the ovine adrenal

Abstract

The production of glucocorticoids and mineralocorticoids in the endoplasmic reticulum (ER) of the mammalian adrenal cortex are, to a great extent, regulated by the relative activities of the steroid 21-hydroxylase (P450c21) and steroid 17α-hydroxylase (P450c17) enzymes. Progesterone can be 17α-hydroxylated to yield 17α-hydroxyprogesterone which, under certain conditions and in certain species, can be further lyased to adrostenedione by the same enzyme. P450c21 can 21-hydroxylate 17α-hydroxyprogesterone to yield cortisol but also converts progesterone to corticosterone. Cytochrome b₅ (cyt b₅) can also participate in the regulation of adrenal microsomal steroid hydroxylase activities by changing the rates of the P450c21 and P450c17 reactions or by affecting the 17α-hydroxylation:17,20-lyase ratio of progseterone, by P450c17. We investigated the metabolism of progesterone by sheep adrenal microsomes to indentify the products of the different steroid hydroxylase activities in the ER and to investigate the influence of cyt b₅ on progesterone metabolism using purified ovine cyt b₅ and anti-cyt b₅. The P450c17-activity in sheep adrenal microsomes is inibited by the addition of purified cyt b₅ while anti-cyt b₅ IgG stimulates the 17α-hydroxylation of progesterone. No 17,21-lyase-activity towards progesterone could be detected in sheep adrenal microsomes.