Stereospecific reduction of 5β-reduced steroids by human ketosteroid reductases of the AKR (aldo-keto reductase) superfamily: role of AKR1C1-AKR1C4 in the metabolism of testosterone and progesterone via the 5β-reductase pathway

Active sex hormones such as testosterone and progesterone are metabolized to tetrahydrosteroids in the liver to terminate hormone action. One main metabolic pathway, the 5 beta-pathway, involves 5 beta-steroid reductase (AKR1D1, where AKR refers to the aldo-keto reductase superfamily), which catalyses the reduction of the 4-ene structure, and ketosteroid reductases (AKR1C1-AKR1C4), which catalyse the subsequent reduction of the 3-oxo group. The activities of the four human AKR1C enzymes on 5 beta-dihydrotestosterone, 5 beta-pregnane-3,20-dione and 20 alpha-hydroxy-5 beta-pregnan-3-one, the intermediate 5 beta-dihydrosteroids on the 5 beta-pathway of testosterone and progesterone metabolism, were investigated. Product characterization by liquid chromatography-MS revealed that the reduction of the 3-oxo group of the three steroids predominantly favoured the formation of the corresponding 3 alpha-hydroxy steroids. The stereochemistry was explained by molecular docking. Kinetic properties of the enzymes identified AKR1C4 as the major enzyme responsible for the hepatic formation of 5 beta-tetrahydrosteroid of testosterone, but indicated differential routes and roles of human AKR1C for the hepatic formation of 5 beta-tetrahydrosteroids of progesterone. Comparison of the kinetics of the AKR1C1-AKR1C4-catalysed reactions with those of AKR1D1 suggested that the three intermediate 5 beta-dihydrosteroids derived from testosterone and progesterone are unlikely to accumulate in liver, and that the identities and levels of 5 beta-reduced metabolites formed in peripheral tissues will be governed by the local expression of AKR1D1 and AKR1C1-AKR1C3.