This family consists of 3-oxo-5-alpha-steroid 4-dehydrogenases, EC:1.3.99.5 Also known as Steroid 5-alpha-reductase, the reaction catalysed by this enzyme is: 3-oxo-5-alpha-steroid + acceptor <=> 3-oxo-delta(4)-steroid + reduced acceptor. The Steroid 5-alpha-reductase enzyme is responsible for the formation of dihydrotestosterone, this hormone promotes the differentiation of male external genitalia and the prostate during fetal development [2]. In humans mutations in this enzyme can cause a form of male pseudohermaphorditism in which the external genitalia and prostate fail to develop normally [2]. A related enzyme is also found in plants is Swiss:Q38944 (DET2) a steroid reductase from Arabidopsis. Mutations in this enzyme cause defects in light-regulated development [1].

1: A role for brassinosteroids in light-dependent development of Arabidopsis Li J, Nagpal P, Vitart V, McMorris TC, Chory J; Science 1996;272:398-401. PMID:8602526

2: Characterization and chromosomal mapping of a human steroid 5 alpha-reductase gene and pseudogene and mapping of the mouse homologue. Jenkins EP, Hsieh CL, Milatovich A, Normington K, Berman DM, Francke U, Russell DW; Genomics 1991;11:1102-1112. PMID:1686016

3: Deletion of steroid 5 alpha-reductase 2 gene in male pseudohermaphroditism. Andersson S, Berman DM, Jenkins EP, Russell DW; Nature 1991;354:159-161. PMID:1944596