3α-Hydroxysteroid Dehydrogenase

Mammalian hydroxysteroid dehydrogenases (HSDs) play pivotal roles in the biosynthesis and inactivation of steroid hormones. In target tissues, they can interconvert potent steroid hormones with their cognate inactive metabolites and regulate the occupancy of steroid hormone receptors. The reactions they catalyze are positional and stereospecific and usually involve the interconversion of a carbonyl with a hydroxyl group on either the steroid nucleus or side chain. cDNA cloning indicates that HSDs belong to two distinct protein phylogenies: the short-chain dehydrogenase/reductase (SDR) family and the aldo-keto reductase (AKR) superfamily. Mammalian 3α-hydroxysteroid dehydrogenase (3α-HSDs or aldo-keto reductase family 1 member C4) are members of the AKR superfamily. They work in concert with the 5α- and 5β-reductases to generate the 3α/5α and 3a/5β-tetrahydrosteroids. These reactions are not innocuous. In steroid target tissues, 3α-HSDs work as molecular switches and control steroid hormone action.

Properties

The most extensively characterized mammalian 3α-HSD is the enzyme from rat liver. Based on the high sequence identity that exists (>60%) with other HSDs that belong to the AKR superfamily, it serves as a good structural template for these HSDs. Rat liver 3α-HSD was originally characterized based on its ability to produce tetrahydro glucocorticoids. It was later found to play an important role in the inactivation of androgens, progestins, and glucocorticoids. 3α-HSD is also a major bile acid-binding protein in rat liver and is involved in the vectorial transport of bile acids from the sinusoidal to the canicular pole of the hepatocyte and may be essential for normal hepatic function. In addition, by oxidizing polycyclic aromatic hydrocarbon (PAH) trans-dihydrodiols to o-quinones, 3α- HSD suppresses the formation of diol-epoxides, which are the most mutagenic metabolites of PAH known. These o-quinones undergo redox cycling to produce reactive oxygen species and o-semiquinone anion radicals. This cycling will lead to the generation of reactive oxygen species multiple times. This mechanism of free radical amplification may contribute to the carcinogenicity of PAH.

Structure

Like aldose-reductase, rat liver 3α-HSD adopts a triosephosphate isomerase or TIM-barrel motif in which there is an alternating α-helix and β-strand arrangement that occurs eight times (α/β), forming a barrel from the β-strands in the core of the structure. The structure is characterized by two large loops at the C-terminal end of the barrel. Using the coordinates for NADPH that exist in the structure of the ADR. NADPH binary complex, the cofactor was modeled into the structure. In this model, the C4 position of the nicotinamide ring is close to four hydrophilic residues (Asp 50, Tyr 55, Lys 84, and His 117) that may comprise a catalytic tetrad. These residues are invariant in HSDs that belong to the AKR superfamily. On the basis of this structure, a catalytic mechanism was proposed in which Tyr 55 was implicated as the general acid, and its effective pKa was lowered by hydrogen bonding with Lys 84, which in turn was salt-linked to Asp 50.

Catalytic Mechanism

3α-HSD is a nonmetalloenzyme and is metal-sensitive because of the presence of the nine free sulfhydryls. The mechanism requires direct hydride transfer from the C4 position of the nicotinamide ring to the ketone at the C3 position of the steroid substrate. The reaction may proceed through the formation of a full or partial carbonium ion. Since there is no metal ion to polarize the acceptor carbonyl, a general acid is invoked to facilitate the reaction. In the oxidation direction the same amino acid may function as a general base. Using stereospecifically labeled NADH it was determined that the 4-pro-R-hydrogen is transferred from the A-face of the pyridine nucleotide cofactor to the β-face of the steroid to force the acceptor carbonyl into the conformation of a 3α-axial alcohol. This stereochemistry of hydride transfer may be conserved in members of the AKR superfamily, since it is shared by human placental aldose-reductase and murine liver 17β-HSD (A-face specific).

Figure 1. Catalytic mechanism for 3α-hydroxysteroid dehydrogenase. (Penning T M.; et al. 1996)

Physiological Function

In the prostate, 3α-HSD regulates the amount of androgen that can bind to the androgen receptor. It catalyzes the interconversion of the potent androgen 5α-dihydrotestosterone (5α-DHT) to 3α-androstanediol, and high levels of 5α-DHT are required for normal and abnormal growth of the prostate. lnhibitors of prostatic 3α-HSD may provide an opportunity for lowering 5α-DHT levels. In brain, 3α-HSD regulates the amount of allosteric agonists that can bind to the GABAa receptor. In this way, 3α-HSD can control Cl- conductance and regulate the anxiolytic and anesthetic actions of 3α-hydroxysteroids. Thus 3α-HSDs regulate the amount of hormone that binds to a steroid hormone receptor, whether this receptor is a member of the nuclear receptor superfamily or a membrane-bound ion-gated channel. Therefore, 3α-HSDs are potential drug targets.

Clinical Significance

Various antidepressants, including the fluvoxamine, SSRIs fluoxetine, sertraline, paroxetine, the SNRI venlafaxine, and mirtazapine, have been found to activate certain 3α-HSD enzymes, resulting in a selective facilitation of 5α-dihydroprogesterone conversion into allopregnanolone. This action has been implicated in their effectiveness in affective disorders, and has resulted in them being described as selective brain steroidogenic stimulants (SBSSs).

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