Human 3α-hydroxysteroid dehydrogenase isoforms (AKR1C1-AKR1C4) of the aldo-keto reductase superfamily:: functional plasticity and tissue distribution reveals roles in the inactivation and formation of male and female sex hormones

The kinetic parameters, steroid substrate specificity and identities of reaction products were determined for four homogeneous recombinant human 3 alpha-hydroxysteroid dehydrogenase (3 alpha-HSD) isoforms of the aldo-keto reductase (AKR) superfamily. The enzymes correspond to type 1 3 alpha-HSD (AKRIC4), type 2 3 alpha(17 beta)-HSD (AKR1C3), type 3 3 alpha-HSD (AKR1C2) and 20 alpha(3 alpha)-HSD (AKR1C1), and share at least 84% amino acid sequence identity. All enzymes acted as NAD(P)(H)-dependent 3-, 17- and 20-ketosteroid reductases and as 3 alpha-, 17 beta- and 20 alpha-hydroxysteroid oxidases. The functional plasticity of these isoforms highlights their ability to modulate the levels of active androgens, oestrogens and progestins. Salient features were that AKR1C4 was the most catalytically efficient, with k(cat)/K-m values for substrates that exceeded those obtained with other isoforms by 10-30-fold. In the reduction direction, all isoforms inactivated 5 alpha-dihydrotestosterone (17 beta-hydroxy-5 alpha-androstan-3-one; 5 alpha-DHT) to yield 5 alpha-androstane-3 alpha,17 beta-diol (3 alpha-androstanediol). However, only AKR1C3 reduced Delta(4)-androstene-3,17-dione to produce significant amounts of testosterone. All isoforms reduced oestrone to 17 beta-oestradiol, and progesterone to 20 alpha-hydroxy-pregn-4-ene-3,20-dione (20 alpha-hydroxyprogesterone). In the oxidation direction, only AKR1C2 converted 3a-androstanediol to the active hormone 5 alpha-DHT. AKR1C3 and AKR1C4 oxidized testosterone to Delta(4)-androstene-3,17-dione. All isoforms oxid ized 17 beta-oestradiol to oestrone, and 20 alpha-hydroxyprogesterone to progesterone. Discrete tissue distribution of these AKR1C enzymes was observed using isoform-specific reverse transcriptase-PCR. AKR1C4 was virtually liver-specific and its high k(cat)/K-m allows this enzyme to form 5 alpha/5 beta-tetrahydrosteroids robustly. AKR1C3 was most prominent in the prostate and mammary glands. The ability of AKR1C3 to interconvert testosterone with Delta(4)-androstene-3,17-dione, but to inactivate 5 alpha-DHT, is consistent with this enzyme eliminating active androgens from the prostate. In the mammary gland, AKR1C3 will convert Delta(4)-androstene-3,17-dione to testosterone (a substrate aromatizable to 17 beta-oestradiol), oestrone to 17 beta-oestradiol, and progesterone to 20 alpha-hydroxyprogesterone, and this concerted reductive activity may yield a. pro-oesterogenic state. AKR1C3 is also the dominant form in the uterus and is responsible for the synthesis of 3 alpha-androstanediol which has been implicated as a parturition hormone. The major isoforms in the brain, capable of synthesizing anxiolytic steroids, are AKR1C1 sand AKR1C2. These studies are in stark contrast with those in rat where only a single AKR with positional- and stereospecificity for 3 alpha-hydroxysteroids exists.