Endocrine Abstracts

Hepatic A-ring reduction of glucocorticoids is enhanced in obesity, perhaps contributing to compensatory activation of the hypothalamic-pituitary-adrenal axis and adrenal androgen excess. One pathway activated is the formation of tetrahydro metabolites by two sequential steps catalysed by 5beta-reductase (5bR) followed by 3alpha-hydroxysteroid dehydrogenases (3HSD). However, regulation of these enzymes is understood poorly. 5bR and 3HSD are also involved in the conversion of cholesterol to bile acids and bile acids inhibit many hepatic enzymes. We therefore investigated whether bile acids may influence the rate of A-ring reduction of glucocorticoids.

Combined 5bR (rate determining step) and 3HSD activities were measured in hepatic cytosol from Wistar rats (male; 9weeks; n=6) as conversion of [³H]₄-corticosterone (1microMolar) to [³H]₄-5beta-tetrahydrocorticosterone in the presence of NADPH by HPLC. Activity of 3HSD alone was measured in tandem as conversion of 5beta-dihydrocorticosterone to 5beta-tetrahydrocorticosterone by GC-MS. Data are mean plus/minus SEM, *=p<0.05 vs control, #=p<0.05 vs chenodeoxycholic acid (CDCA).

Activities of combined 5bR and 3HSD were reduced by addition of a range of bile acids (IC50: 2.67 microMolar CDCA*; 78.1 deoxycholic acid*#; 665 cholic acid*#). CDCA was the most potent and a non-competitive inhibitor of steroid metabolism (Vmax: 27.63plus/minus0.4 nanomoles per milligram protein per hour Control vs 15.43plus/minus0.8 CDCA*)(Km for corticosterone: 107.7plus/minus7.9 microMolar Control vs 119.0plus/minus10.3 CDCA, p not significant). Glycine and taurine conjugates of CDCA were equally as potent as the non-conjugated bile acid (IC50: 9.67 microMolar glycoCDCA*; 6.66 tauroCDCA*). 3HSD alone was inhibited to a lesser degree (p=0.12 CDCA; p=0.08 glycoCDCA; p=0.10 tauroCDCA). Therefore inhibition of the combined reaction is mediated mainly via effects on 5bR.

In summary bile acids act as potent non-competitive inhibitors of 5bR in vitro. Thus fluctuations in concentrations of bile acids within the liver occurring post-prandially or pathologically may have profound effects on the rate of glucocorticoid metabolism catalysed by 5bR.