Abstract
Inhibitory effects of naturally occurring steroids on 21-hydroxylase activity were determined by measuring the initial conversion rate of 17-hydroxyprogesterone (17α-hydroxy-4-pregnene-3, 20-dione) to 11-deoxycortisol (17α, 21-dihydroxy-4-pregnene-3, 20-dione) in incubation experiments with the microsomal fraction (10, 000-105, 000 ×g precipitate) of the human fetal adrenal gland. The apparent Michaelis constant (Km) for conversion of 17-hydroxyprogesterone was 13.3×10-6M.
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=20.0×10-6M), 11-deoxycortisol (Ki=27.5×10-6 M) and estradiol-17β(Ki=87.5×10-6M). The most potent inhibitor among the estrogens was estradiol-17β.
