title: Side chain hydroxylations in biosynthesis of cholic acid. 25- and 26-Hydroxylation of 5beta-cholestane-3alpha, 7alpha, 12alpha-triol by reconstituted systems from rat liver microsomes.
abstract: 25- and 26-Hydroxylation of 5beta-cholestane-3alpha, 7alpha, 12alpha-triol was studied with reconstituted systems from rat liver microsomes consisting of partially purified cytochrome P -450, NADPH-cytochrome P -450 reductase, a phospholipid, and an NADPH -generating system. Cytochrome P -450 was prepared either by sodium cholate treatment and ammonium sulfate fractionation or by subtilisin and sodium deoxycholate treatment followed by DEAE-cellulose chromatography. No side chain hydroxylation was observed when cytochrome P-450 was omitted. With ammonium sulfate-fractionated cytochrome P-450 25- and 26-hydroxylation was stimulated 5- to 8-fold by addition of NADPH-cytochrome P-450 reductase. With subtilisin-treated cytochrome P-450 an almost absolute requirement for NADPH-cytochrome P-450 reductase was observed. Omission of lipid did not reduce the rate of hydroxylation. Centrifugation of the cytochrome P-450 preparation at 100,000 X g for 1 hour just before incubation increased markedly lipid dependency. A significant difference between 25- and 26-hydroxylation was observed with respect to substrate saturation. The stimulatory effect of phenobarbital treatment on 25-hydroxylation and the inhibitory effect of this treatment on 26-hydroxylation were associated with the cytochrome P-450 fraction. The use of increasing amounts of sodium cholate in the solubilization of cytochrome P -450 resulted in a gradual decrease of 25-hydroxylase activity and a gradual increase of 26-hydroxylase activity. 25- and 26-Hydroxylase activities were separated partially by chromatography of subtilisintreated cytochrome P-450 fraction on DEAE-cellulose. The question whether different species of cytochrome P-450 are involved in 25- and 26-hydroxylation is discussed.
mesh_terms: D000818:Animals; D002776:Cholestanes / Q000378:metabolism*; D002793:Cholic Acids / Q000096:biosynthesis*; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D003579:Cytochrome Reductases / Q000378:metabolism; D007700:Kinetics; D008297:Male; D008862:Microsomes, Liver / Q000378:metabolism*; D006899:Mixed Function Oxygenases / Q000378:metabolism*; D051381:Rats; D012995:Solubility; D013381:Subtilisins
chemical_list: D002776:Cholestanes; D002793:Cholic Acids; D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; D003579:Cytochrome Reductases; D013381:Subtilisins