title: Subcellular distribution and properties of progesterone (delta4-steroid) 5alpha-reductase in rat medial basal hypothalamus.
abstract: The subcellular distribution and properties of rat hypothalamic progesterone 5 alpha-reductase, which accelerates the conversion of progesterone to 5 alpha-pregnane-3,20-dione, have been investigated by utilizing 3H-labeled substrate and a reverse isotopic dilution assay system. The enxymic activity was associated primarily with a cell debris-membranes fraction deribed from the 100 x g pellet. This fraction contained mainly membrane-like particulates and was free of nuclei. Little or no activity was associated with the purified nuclei. The hypothalamic 5 alpha-reductase was stimulated by NADPH but not by NADH. The reaction proceeded optimally over a pH range of 6.0 to 7.2 and at a temperaturhe substrate specificity of the enzyme for other delta 4-3-ketosteroids and the ability of these steroids to inhibit the 5 alpha reduction of [1,2-3H]progesterone as well as the effect of 17 beta-estradiol were also studied. 20 alpha-hydroxypregn-4-en-3-one was more reactive that progesterone, while testosterone was the least reactive. The estimated Km for 20 alpha-hydroxypregn-4-en-3-one was 8.6 +/- 1.9 x 10(-7) M, and for testosterone, 1.6 +/- 1.4 x 10(-5) M. The inhibition studies indicate that 20 alpha-hydroxypregn-4-en-3-one and 17 beta-estradiol are competitive and noncompetitive inhibitors, respectively, of the 5 alpha reduction of progesterone with Ki of 6.0 +/- 3.0 x 10(-8) M for 20 alpha-hydroxypregn-4-en-3-one and Kii (intercept inhibition constant) of 2.6 +/- 0.7 x 10(-5) M and Kis (slope inhibition constant) of 3.6 +/- 0.6 x 10(-5) M for 17 beta-estradiol. Testosterone is a poor competitive inhibitor of the reaction.
mesh_terms: D004092:20-alpha-Dihydroprogesterone; D000818:Animals; D002458:Cell Fractionation; D004926:Escherichia coli / Q000201:enzymology; D004958:Estradiol; D005260:Female; D006863:Hydrogen-Ion Concentration; D006913:Hydroxysteroid Dehydrogenases / Q000378:metabolism*; D007030:Hypothalamo-Hypophyseal System / Q000201:enzymology*; D007700:Kinetics; D008473:Median Eminence / Q000201:enzymology*; D011374:Progesterone; D011375:Progesterone Reductase / Q000378:metabolism*; D051381:Rats; D013347:Subcellular Fractions / Q000201:enzymology; D013696:Temperature; D013739:Testosterone
chemical_list: D004092:20-alpha-Dihydroprogesterone; D013739:Testosterone; D011374:Progesterone; D004958:Estradiol; D006913:Hydroxysteroid Dehydrogenases; D011375:Progesterone Reductase