Expanded substrate screenings of human and Drosophila type 10 17β-hydroxysteroid dehydrogenases (HSDs) reveal multiple specificities in bile acid and steroid hormone metabolism: characterization of multifunctional 3α/7α/7β/17β/20β/21-HSD

Abstract

17β-Hydroxysteroid dehydrogenases (17β-HSDs) catalyse the conversion of 17β-OH (-hydroxy)/17-oxo groups of steroids, and are essential in mammalian hormone physiology. At present, eleven 17β-HSD isoforms have been defined in mammals, with different tissue-expression and substrate-conversion patterns. We analysed 17β-HSD type 10 (17β-HSD10) from humans and Drosophila, the latter known to be essential in development. In addition to the known hydroxyacyl-CoA dehydrogenase, and 3α-OH and 17β-OH activities with sex steroids, we here demonstrate novel activities of 17β-HSD10. Both species variants oxidize the 20β-OH and 21-OH groups in C21 steroids, and act as 7β-OH dehydrogenases of ursodeoxycholic or isoursodeoxycholic acid (also known as 7β-hydroxylithocholic acid or 7β-hydroxyisolithocholic acid respectively). Additionally, the human orthologue oxidizes the 7α-OH of chenodeoxycholic acid (5β-cholanic acid, 3α,7α-diol) and cholic acid (5β-cholanic acid). These novel substrate specificities are explained by homology models based on the orthologous rat crystal structure, showing a wide hydrophobic cleft, capable of accommodating steroids in different orientations. These properties suggest that the human enzyme is involved in glucocorticoid and gestagen catabolism, and participates in bile acid isomerization. Confocal microscopy and electron microscopy studies reveal that the human form is localized to mitochondria, whereas Drosophila 17β-HSD10 shows a cytosolic localization pattern, possibly due to an N-terminal sequence difference that in human 17β-HSD10 constitutes a mitochondrial targeting signal, extending into the Rossmann-fold motif.