A study of Δ5-3β-hydroxysteroid dehydrogenase in human foetal adrenal glands

Abstract

Abstract. 'Microsomal fractions' of foetal adrenal tissue obtained at 18–24 weeks of gestation were incubated with several radioactive Δ5-3β-hydroxysteroids to study the properties of Δ5-3β-hydroxysteroid dehydrogenase (Δ5-3β-HSD)1. Incubations were performed under CO gas to inhibit hydroxylation. Enzyme activity was measured by the conversion of pregnenolone, 17α-OH-pregnenolone, dehydroepiandrosterone, androstenediol and androstenetriol to the corresponding Δ4-3-oxo-steroids. 17α-OH-pregnenolone was the most preferred substrate for the enzyme among the Δ5-3β-substra-teroid substrates examined. Michaelis constants (Km) of Δ5-3β-HSD were 2 × 10−4 m for pregnenolone, 7.2 × 10−6m for 17α-OH-pregnenolone and 7.8 × 10−6 m for dehydroepiandrosterone. No significant amounts of Δ4-3-oxosteroids were produced from androstenediol and androstenetriol, and Km values for them were not determined. The enzyme preferred NAD+ to NADP+ as a hydrogen receptor and the Km for NAD+ was 1.1 × 10−4 m in the presence of 17α-OH-pregnenolone. The enzyme activity for 17α-OH-pregnenolone was competitively inhibited by dehydroepiandrosterone, suggesting that both Δ5-3βhydroxysteroids were catalyzed at the same site of the enzyme, and also non-competitively by oestriol. Other steroids produced by the foeto-placental unit which inhibited the enzyme activity for 17α-OH-pregnenolone were pregnenolone-3β-yl-sulphate, androstenediol, oestradiol17β, oestrone and oestradiol-17β-yl-sulphate. It appears likely that the formation of Δ4-3-oxo-steroids as precursors for cortisol synthesis in foetal adrenal glands is influenced by the presence of steroids produced by the foeto-placental unit.