Studies of the human fetal adrenal glandproperties of 17α-hydroxylase and C17-C20 lyase in the biosynthesis of dehydroepiandrosterone from pregnenolone

Abstract The microsomal fraction of the human fetal adrenal gland showed NADPH dependent enzyme activities of 17α-hydroxylase and C 17 -C 20 lyase. The apparent Michaelis constants ( K M ) of 17α-hydro xylase and C 17 -C 20 lyase for NADPH were 6 × 10 −8 M and 3 × 10 −7 M, respectively. The apparent k m value of 17α-hydroxylase for pregnenolone was 1.3 × 10 −8 M and that of C 17 -C 20 lyase for 17-hydroxypregnenolone was 1.2 × 10 −7 M. These enzymes were inhibited by most of the steroids produced in the feto-placental unit. 5-Pregnene-3β,20β-diol (apparent K i = 1.5 × 10 −8 M), pregnenolone-sulphate (2.4 × 10 −8 M), 5-pregnene-3β,20α-diol (3.1 × 10 −8 M), 17-hydroxypregnenolone (6.1 × 10 −8 M) and progesterone (8.1 × 10 −8 M) inhibited the activity of 17α-hydroxylase toward pregnenolone. The following steroids were the inhibitors of C 17 -C 20 lyase toward 17-hydroxypregnenolone; 5-pregnene-3β, 20β-diol (apparent K i = 0.6 × 10 −7 M), 5-pregnene-3β,20α-diol (1.7 × 10 −7 M), progesterone (3.4 × 10 −7 M), 17,20β-dihydroxy-4-pregnen-3-one (5.6 × 10 −7 M), 17-hydroxyprogesterone (7.6 × 10 −7 M), dehydroepiandrosterone (2.3 × 10 −6 M), 5-androstene-3β,17β--diol (3.2 × 10 −6 M), 16α-hydroxydehydroepiandrosterone (3.2 × 10 −6 M), testosterone (6.2 × 10 −6 M) and estradiol-17β (9.2 × 10 −6 M). The type of inhibition of these steroids appeared to be competitive.