The kinetic parameters, steroid substrate specificity and identities of reaction products were determined for four homogeneous recombinant human 3α-hydroxysteroid dehydrogenase (3α-HSD) isoforms of the aldo-keto reductase (AKR) superfamily. The enzymes correspond to type 1 3α-HSD (AKR1C4), type 2 3α(17β)-HSD (AKR1C3), type 3 3α-HSD (AKR1C2) and 20α(3α)-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 3α-, 17β- and 20α-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 kcat/Km values for substrates that exceeded those obtained with other isoforms by 10–30-fold. In the reduction direction, all isoforms inactivated 5α-dihydrotestosterone (17β-hydroxy-5α-androstan-3-one; 5α-DHT) to yield 5α-androstane-3α,17β-diol (3α-androstanediol). However, only AKR1C3 reduced ∆4-androstene-3,17-dione to produce significant amounts of testosterone. All isoforms reduced oestrone to 17β-oestradiol, and progesterone to 20α-hydroxy-pregn-4-ene-3,20-dione (20α-hydroxyprogesterone). In the oxidation direction, only AKR1C2 converted 3α-androstanediol to the active hormone 5α-DHT. AKR1C3 and AKR1C4 oxidized testosterone to ∆4-androstene-3,17-dione. All isoforms oxidized 17β-oestradiol to oestrone, and 20α-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 kcat/Km allows this enzyme to form 5α/5β-tetrahydrosteroids robustly. AKR1C3 was most prominent in the prostate and mammary glands. The ability of AKR1C3 to interconvert testosterone with ∆4-androstene-3,17-dione, but to inactivate 5α-DHT, is consistent with this enzyme eliminating active androgens from the prostate. In the mammary gland, AKR1C3 will convert ∆4-androstene-3,17-dione to testosterone (a substrate aromatizable to 17β-oestradiol), oestrone to 17β-oestradiol, and progesterone to 20α-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 3α-androstanediol which has been implicated as a parturition hormone. The major isoforms in the brain, capable of synthesizing anxiolytic steroids, are AKR1C1 and AKR1C2. These studies are in stark contrast with those in rat where only a single AKR with positional- and stereo-specificity for 3α-hydroxysteroids exists.

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