Inhibition of fetal adrenal 21-hydroxylase by naturally occurring steroids.

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Abstract
Inhibitory effects of naturally occurring steroids on 21-hydroxylase activity were determined by measuring the initial conversion rate of 17-hydroxyprogesterone (17.alpha.-hydroxy-4-pregnene-3, 20-dione) to 11-deoxycortisol (17.alpha., 21-dihydroxy-4-pregnene-3, 20-dione) in incubation experiments with the microsomal fraction (10,000-105,000 .times. g precipitate) of the human fetal adrenal gland. The apparent Km for conversion of 17-hydroxyprogesterone was 13.3 .times. 10-6 M. Human fetal adrenal 21-hydroxylase was inhibited by some of the steroids produced in the feto-placental unit. The following steroids acted as competitive inhibitors of the reaction; progesterone (Ki = 20.0 .times. 10-6 M), 11-deoxycortisol (Ki = 27.5 .times. 10-6 M) and estradiol-17.beta. (Ki = 87.5 .times. 10-6 M). The most potent inhibitor among the estrogens was estradiol-17.beta.

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