Converts testosterone (T) into 5-alpha-dihydrotestosterone (DHT) and progesterone or corticosterone into their corresponding 5-alpha-3-oxosteroids. It plays a central role in sexual differentiation and androgen physiology. a 3-oxo-5alpha-steroid + NADP(+) = a 3-oxo-Delta(4)-steroid + H(+) + NADPH Optimally active at acidic pHs. Expressed in high levels in the prostate and many other androgen-sensitive tissues. Individuals with Thr-49 have an increased risk of prostate cancer (PubMed:10501358). The enzyme with Thr-49 has a higher in vitro V(max) than the Ala-49 enzyme (PubMed:10501358). The disease is caused by variants affecting the gene represented in this entry. Belongs to the steroid 5-alpha reductase family. 5-alpha reductase entry