METABOLISM OF [4-14C]CHOLESTEROL BY HUMAN ADRENAL GLANDS IN VITRO AND ITS INHIBITION BY METYRAPONE

Abstract

ABSTRACT The in vitro conversion of [4-14C] cholesterol1) to steroid hormones was studied in 1 normal, 1 adenomatous and 2 hyperplastic, surgically resected, human adrenals. The degree of overall conversion per gram of tissue was similar (4.4–5.6 %) in NADPH-supplemented homogenates from normal or moderately hyperactive adrenals; a 2-fold increase was found in a markedly hyperfunctioning gland. Only labelled cortisol, corticosterone, 11-deoxycortisol, 11-deoxycorticosterone and androstenedione were isolated from incubations with [4-14C] cholesterol, but cortisol accounted for slightly more than 50 % of overall substrate conversion in the normal adrenal, while 11-deoxycortisol and androstenedione predominated in the abnormal glands. "Apparent 11β-hydroxylase activity" was considerably lower in the abnormal glands than in normal tissue when assessed using [4-14C] cholesterol as substrate, but this deficit was not observed in comparable incubations with 14C-labelled pregnenolone, progesterone or 11-deoxycorticosterone. [4-14C] Cholesterol was a more efficient precursor of androstenedione than either [4-14C] pregnenolone or [4-14C] progesterone. In parallel studies, metyrapone (1.0 mm) depressed the formation of both 11-oxy and 11-deoxymetabolies from [4-14C] cholesterol, overall inhibition ranging from 66 to 86%. The conversion of [4-14C] 11-deoxycorticosterone to corticosterone was inhibited by 93 % under identical conditions. Metyrapone did not, however, impair the overall transformation of either [4-14C]pregnenolone or [4-14C]progesterone, since the inhibition of cortisol and corticosterone biosynthesis was associated with an increment in 11-deoxycortisol (with [4-14C] pregnenolone) or 11-deoxycorticosterone (with [4-14C]progesterone). From these studies it appears likely that the additional site of metyrapone inhibition of steroid biosynthesis suggested by others on the basis of clinical observations involves the rate-limiting, ACTH-regulated conversion of cholesterol to pregnenolone.