Steroid Inhibitory Effects upon Human Adrenal 3βHydroxysteroid Dehydrogenase Activity*

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Abstract
The inhibitory effects of varying concentrations of steroids upon 3β-hydroxysteroiddehydrogenase/Δ5-Δ4 isomerase (3β–HSD) kinetics were studied in human adrenal microsomes. Each enzyme assay was conducted in triplicate at five different concentrations of three substrates (dehydroepiandrosterone, pregnenolone, and 17OH-pregnenolone), using microsomes from at least three donors. Each steroid was screened for possible inhibition at concentrations of 10-−8 and 10−6M and thenstudied in more detail at five different concentrations. The type of inhibition and the inhibition constant (Ki) were determined by analysis of Lineweaver-Burk and Dixon plots,together with replots of the slopes from the Dixon plots. The mean Km (Michaelis-Menten constant)for the three substrates was 0.42 ± 0.04 (SE) μmol/liter (n = 73). Each steroidtested, including Δ5-3β-hydroxysteroids, estrogens, and several Δ4–3–ketosteroids, with the exception of cortisol, caused significant inhibition of 3β-HSD activity, and in each case the steroid appeared to behave as acompetitive inhibitor. In most casesthe Ki value was approximately 10−7 M. At micromolar concentrations several steroids, notably estrone and estradiol, caused almost total inhibition of adrenal 3β-HSD activity. Comparison of thecalculated Kj values with available data concerning changes in intraadrenal steroid concentrations during childhood suggests that these changes would be sufficient to cause a relativedecline in 3β-HSD activity during adrenarche. Although postnatal circulating steroid concentrations would appear to be insufficient to influence adrenal steroidogenesis, the high serum levels of placental steroids during fetallife would be expected to cause marked 3β-HSD inhibition.