In enzymology, a 3alpha-hydroxysteroid dehydrogenase (B-specific) () is an enzyme that catalyzes the chemical reaction
The two substrates of this enzyme are androsterone and oxidised nicotinamide adenine dinucleotide (NAD<sup>+</sup>). Its products are 5ñ-androstane-3,17-dione, reduced NADH, and a proton. The alternative cofactor nicotinamide adenine dinucleotide phosphate is also used by this enzyme.
The enzyme belongs to the family of oxidoreductases, specifically those acting on the CH-OH group of donor with NAD<sup>+</sup> or NADP<sup>+</sup> as acceptor, more specifically it is part of the group of hydroxysteroid dehydrogenases. The systematic name of this enzyme class is 3alpha-hydroxysteroid:NAD(P)<sup>+</sup> oxidoreductase (B-specific). Other names in common use include hydroxyprostaglandin dehydrogenase, 3alpha-hydroxysteroid oxidoreductase, and sterognost 3alpha. This enzyme participates in 3 metabolic pathways: bile acid biosynthesis, c21-steroid hormone metabolism, and androgen and estrogen metabolism.
As of late 2007, 7 structures have been solved for this class of enzymes, with PDB accession codes , , , , , , and .