Bifunctional enzyme involved in steroid-hormone metabolism and cholesterol biosynthesis (PubMed:6946726, PubMed:9658408). Catalyzes the NADP(H)-dependent reduction of estrogens and androgens and regulates the biological potency of these steroids (PubMed:9658408) (By similarity). Converts estrone (E1) to a more potent estrogen, 17beta-estradiol (E2) (PubMed:9658408). Converts dihydrotestosterone (DHT) to an inactive form. Participates also in the post-squalene cholesterol biosynthesis, as a 3-ketosteroid reductase (By similarity). 17beta-estradiol + NADP(+) = estrone + H(+) + NADPH a 3beta-hydroxysteroid + NADP(+) = a 3-oxosteroid + H(+) + NADPH 4alpha-methyl-5alpha-cholest-7-en-3beta-ol + NADP(+) = 4alpha-methyl-5alpha-cholest-7-en-3-one + H(+) + NADPH 4alpha-methyl-5alpha-cholest-8-en-3-one + H(+) + NADPH = 4alpha-methyl-5alpha-cholest-8-en-3beta-ol + NADP(+) 3-dehydro-4alpha-methylzymosterol + H(+) + NADPH = 4alpha-methylzymosterol + NADP(+) H(+) + NADPH + zymosterone = NADP(+) + zymosterol 5alpha-cholest-8-en-3-one + H(+) + NADPH = 5alpha-cholest-8-en-3beta-ol + NADP(+) 5alpha-androstane-3beta,17beta-diol + NADP(+) = 17beta-hydroxy-5alpha-androstan-3-one + H(+) + NADPH 5alpha-androstane-3alpha,17beta-diol + NADP(+) = 17beta-hydroxy-5alpha-androstan-3-one + H(+) + NADPH Steroid biosynthesis; estrogen biosynthesis. Steroid biosynthesis; zymosterol biosynthesis; zymosterol from lanosterol: step 5/6. Binds to the short form of prolactin receptor. Most abundant in ovaries of pregnant animals. Phosphorylated. Belongs to the short-chain dehydrogenases/reductases (SDR) family. ERG27 subfamily.