7α- and 7β-hydroxy-epiandrosterone as substrates and inhibitors for the human 11β-hydroxysteroid dehydrogenase type 1

The human 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) catalyzes both the NADP(H)-dependent oxido-reduction of cortisol and cortisone and the inter-conversion of 7α- and 7β-hydroxy-dehydroepiandrosterone (DHEA) through a 7-oxo-DHEA intermediate. As shown with human liver and intestine fractions, 7α-hydroxy-epiandrosterone (7α-hydroxy-EpiA) and 7β-hydroxy-EpiA were readily inter-converted with no evidence for a 7-oxo-EpiA intermediate. Whether this inter-conversion resulted from action of the 11β-HSD1 or from an unknown epimerase is unresolved. Furthermore, whether these steroids could inhibit the cortisol–cortisone oxido-reduction remains a question. The recombinant human 11β-HSD1 was used to test these questions. NADP+ supplementation only provided the production of 7β-hydroxy-EpiA out of 7α-hydroxy-EpiA with a Vmax/KM ratio at 0.1. With NADPH supplementation, both 7α-hydroxy-EpiA and 7β-hydroxy-EpiA were formed in low amounts from 7β-hydroxy-EpiA and 7α-hydroxy-EpiA, respectively. These inter-conversions occurred without a trace of the putative 7-oxo-EpiA intermediate. In contrast, the 7-oxo-EpiA substrate was efficiently reduced into 7α-hydroxy-EpiA and 7β-hydroxy-EpiA, with Vmax/KM ratios of 23.6 and 5.8, respectively. Competitive and mixed type inhibitions of the 11β-HSD1-mediated cortisol oxidation were exerted by 7α-hydroxy-EpiA and 7β-hydroxy-EpiA, respectively. The 11β-HSD1-mediated cortisone reduction was inhibited in a competitive manner by 7-oxo-EpiA. These findings suggest that the active site of the human 11β-HSD1 may carry out directly the epimeric transformation of 7-hydroxylated EpiA substrates. The low amounts of these steroids in human do not support a physiological importance for modulation of the glucocorticoid status in tissues.