«5α-reductases, also known as 3-oxo-5α-steroid 4-dehydrogenases, are enzymes involved in steroid metabolism. They participate in 3 metabolic pathways: bile acid biosynthesis, androgen and estrogen metabolism. There are three isoenzymes of 5α-reductase, SRD5A1, SRD5A2, and SRD5A3, which vary in different tissues with age.
5α-Reductases catalyze the following chemical reaction:
Thus, the two substrates of these enzymes are a 3-oxo-5α-steroid and acceptor, whereas its two products are 3-oxo-Δ4-steroid and a reduced acceptor. For example:» (wikipedia)