title: Tissue distribution and kinetic characteristics of rat steroid 5 alpha-reductase isozymes. Evidence for distinct physiological functions.
abstract: The enzyme steroid 5 alpha-reductase (5 alpha-reductase) catalyzes the reduction of delta 4,5 double bonds in a variety of substrates and is thought to play both catabolic and anabolic roles in steroid hormone metabolism. Here, we describe the isolation and characterization of a cDNA encoding the rat type 2 isozyme of 5 alpha-reductase and compare the kinetic properties and tissue-specific expression patterns of this isozyme with those of the type 1 isozyme. The type 2 isozyme has apparent Km values in the nanomolar range for steroid substrates, whereas the type 1 isozyme has micromolar affinities. The isozymes differ in their inhibition by various 4-azasteroids with the type 2 isozyme showing exquisite sensitivity (Ki = 40 pM) to 21,21-pentamethylene-4-aza-5 alpha-pregn-1-ene-3,20-dione. Messenger RNAs encoding the type 2 isozyme are more abundant than type 1 mRNAs in most male reproductive tissues, whereas the type 1 mRNAs predominate in peripheral tissues. Both 5 alpha-reductase mRNAs are more efficiently induced by dihydrotestosterone than by testosterone in the regenerating prostate. The differences in substrate affinities and tissue distributions of the 5 alpha-reductase isozymes suggest that type 2 plays an anabolic role and type 1 a catabolic role in the metabolism of androgens and other steroid hormones.
authors.affiliation: Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas 75235.
authors.forename: K
authors.initials: K
authors.lastname: Normington
authors.forename: D W
authors.initials: DW
authors.lastname: Russell
mesh_terms: D013741:3-Oxo-5-alpha-Steroid 4-Dehydrogenase / Q000235:genetics / Q000378:metabolism*; D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D003001:Cloning, Molecular; D004247:DNA / Q000235:genetics; D015971:Gene Expression Regulation, Enzymologic; D006863:Hydrogen-Ion Concentration; D007527:Isoenzymes / Q000235:genetics / Q000378:metabolism; D007700:Kinetics; D008297:Male; D008969:Molecular Sequence Data; D012333:RNA, Messenger / Q000235:genetics; D051381:Rats; D016415:Sequence Alignment; D013256:Steroids / Q000378:metabolism*; D013739:Testosterone / Q000378:metabolism; D014018:Tissue Distribution; D014162:Transfection
chemical_list: D007527:Isoenzymes; D012333:RNA, Messenger; D013256:Steroids; D013739:Testosterone; D004247:DNA; D013741:3-Oxo-5-alpha-Steroid 4-Dehydrogenase