Active toward androgens, estrogens, and xenobiotic substrates. Also exhibits low 20 alpha-HSD activity. Shows a-stereospecificity in hydrogen transfer between cofactors and substrates (A-specific). Preferentially catalyzes the reduction of 4-androstenedione, 5-alpha-androstane-3,17-dione, androsterone and dehydroepiandrosterone to testosterone, dihydrotestosterone, 5-alpha-androstane-3-alpha,17-beta-diol and 5-androstene-3-beta,17-beta-diol, respectively. Monomer. Mainly found in liver. Also expressed weakly in kidney. Three forms are detected, probably due to post-translational modifications. Belongs to the aldo/keto reductase family.