5α-Pregnan-17α-ol-3,20-dione is produced by 5α-reduction of 17-OHP. The reaction is catalyzed by SRD5A1[14] and possibly, SRD5A2 enzymes.[7] While the role of the SRD5A1 enzyme in this reaction is well established, it is unclear whether SRD5A2 is also involved.[13] Some authors[2][14] claim that the reduction of 17-OHP to 17OHDHP by SRD5A1 is not "sufficient" or "efficient", as supported by measurements of rat SRD5A2 activity in a 1971 study.[15] In a later study, conducted in 2017, however, it has been shown that recombinant human SRD5A1 and SRD5A2 can catalyze the reduction of 17-OHP at comparable rates to the reduction of progesterone.[16] Given both isozymes may be expressed in fetal tissues of both sexes,[17][18] the action of SRD5A2 in this reaction in humans is yet to be established.[7]
^Hastings C, Hansson V (August 1981). "Kinetic properties of the soluble 3 alpha-hydroxysteroid dehydrogenase from rat testis and epididymis". Journal of Steroid Biochemistry. 14 (8): 705–711. doi:10.1016/0022-4731(81)90005-4. PMID6946263.
^Asahina K, Suzuki K, Aida K, Hibiya T, Tamaoki B (February 1985). "Relationship between the structures and steroidogenic functions of the testes of the urohaze-goby (Glossogobius olivaceus)". General and Comparative Endocrinology. 57 (2): 281–292. doi:10.1016/0016-6480(85)90273-4. PMID3156787.
^Barnard L, Gent R, van Rooyen D, Swart AC (November 2017). "Adrenal C11-oxy C21 steroids contribute to the C11-oxy C19 steroid pool via the backdoor pathway in the biosynthesis and metabolism of 21-deoxycortisol and 21-deoxycortisone". The Journal of Steroid Biochemistry and Molecular Biology. 174: 86–95. doi:10.1016/j.jsbmb.2017.07.034. PMID28774496. S2CID24071400.
^Ellsworth K, Harris G (October 1995). "Expression of the type 1 and 2 steroid 5 alpha-reductases in human fetal tissues". Biochemical and Biophysical Research Communications. 215 (2): 774–780. doi:10.1006/bbrc.1995.2530. PMID7488021.
^Lunacek A, Schwentner C, Oswald J, Fritsch H, Sergi C, Thomas LN, et al. (August 2007). "Fetal distribution of 5alpha-reductase 1 and 5alpha-reductase 2, and their input on human prostate development". The Journal of Urology. 178 (2): 716–721. doi:10.1016/j.juro.2007.03.089. PMID17574609.