Bifunctional enzyme involved in steroid-hormone metabolism and cholesterol biosynthesis (  , ). Catalyzes the NADP(H)-dependent reduction of estrogens and androgens and regulates the biological potency of these steroids. Converts estrone (E1) to a more potent estrogen, 17beta-estradiol (E2) ( ). Converts dihydrotestosterone (DHT) to its inactive form 5a-androstane-3b,17b-diol ( ). Converts moderately progesterone to 3beta-hydroxypregn-4-ene-20-one, leading to its inactivation (, ). Additionally, participates in the post-squalene cholesterol biosynthesis, as a 3-ketosteroid reductase ( ).
Does not have enzymatic activities toward E1 and DHT.
Subcellular locations: Endoplasmic reticulum membrane
Highly expressed in adrenal gland, liver, lung and thymus. Expressed in breast, ovaries, pituitary gland, pregnant uterus, prostate, kidney, lymph node, small intestine, spinal cord and trachea. Weakly expressed in all other tissues tested.
Expressed in eye ciliary epithelial cells and neuroendocrine cells.