Kinetic analysis of adrenal 3 beta-hydroxysteroid dehydrogenase activity during human development.
Kinetic analyses of microsomal 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD) activity in adrenal glands from 11 individuals, aged 1-60 yr, were carried out to determine whether changes in substrate or cofactor affinity (Km) or cellular content, as reflected in maximal velocity, could explain the changes in adrenal delta 5-3 beta-hydroxysteroid secretion that occur in late childhood and puberty. The Km values for the cofactor NAD+ were similar regardless which substrate, dehydroepiandrosterone (DHA), pregnenolone, or 17-hydroxypregnenolone (17OH-delta 5P), was used. The Km values for DHA (0.3 microM), pregnenolone (0.4 microM), and 17OH-delta 5P (0.3 microM) were similar and within the intraadrenal concentration ranges for DHA and 17OH-delta 5P previously reported. Each substrate was a competitive inhibitor for the others, with close similarity between affinity and inhibition constants. These observations point to the presence of a single 3 beta-HSD, rather than several substrate-specific variants. There was no change in substrate Km with age; the maximal velocity was lower (0.1-0.6 nmol/mg X min) in a single 1-yr-old infant than in later life, but there was no significant change (mean, 2.9-4.6 nmol/mg X min for the three substrates) between values at 12 and 60 yr. This suggests that ACTH-mediated induction of 3 beta-HSD may be low in infancy and higher in adults, while in vivo studies point to a reduction in actual 3 beta-HSD activity during this period. The likely explanation for this paradox between enzyme levels and final activity is that 3 beta-HSD is progressively inhibited during late childhood and puberty by rising intraadrenal concentrations of various delta 4-3-ketosteroids.
Byrne
G C
GC
Perry
Y S
YS
Winter
J S
JS
D006907:17-alpha-Hydroxypregnenolone / Q000378:metabolism; D015096:3-Hydroxysteroid Dehydrogenases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D000293:Adolescent; D000311:Adrenal Glands / Q000201:enzymology*; D000328:Adult; D000375:Aging; D002648:Child; D003687:Dehydroepiandrosterone / Q000378:metabolism; D005260:Female; D006801:Humans; D007223:Infant; D007700:Kinetics; D008297:Male; D008861:Microsomes / Q000201:enzymology; D008875:Middle Aged; D009243:NAD / Q000378:metabolism; D011284:Pregnenolone / Q000378:metabolism
D009243:NAD; D006907:17-alpha-Hydroxypregnenolone; D003687:Dehydroepiandrosterone; D011284:Pregnenolone; D015096:3-Hydroxysteroid Dehydrogenases