Human hepatic cortisol reductase activities: enzymatic properties and substrate specificities of cytosolic cortisol delta 4-5 beta-reductase and dihydrocortisol-3 alpha-oxidoreductase(s).

The metabolism of cortisol by human liver homogenates has been studied. Cortisol delta 4-reductase and dihydrocortisol-3-oxidoreductase activities were distributed in all subcellular fractions. The products of the soluble enzymes were identified. Cortisol and 5 beta-dihydrocortisol were reduced to 3 alpha,5 beta-tetrahydrocortisol, and 5 alpha-dihydrocortisol was reduced to 3 alpha,5 alpha-tetrahydrocortisol. The soluble enzymes showed a wide range of substrate specificity. The 21 substituted cortisol derivatives were not metabolized. The apparent Km values of cortisol delta 4-5 beta-reductase and dihydrocortisol-3 alpha-oxidoreductase for their substrates (cortisol, 5 alpha-dihydrocortisol, and 5 beta-dihydrocortisol) all ranged from 18 to 27 microM. Dexamethasone inhibited the reduction of all of these substrates and the inhibition was abolished by 21 substitution of the dexamethasone. Testosterone was a competitive inhibitor of the reduction of cortisol, 5 alpha-dihydrocortisol, and 5 beta-dihydrocortisol with a Ki ranging from 11 to 32 microM. NADPH was the preferred cofactor for the cortisol delta 4-5 beta-reductase and dihydrocortisol-3 alpha-oxidoreductase. No end product inhibition was observed.