title: Formation of 7 alpha- and 7 beta-hydroxylated bile acid precursors from 27-hydroxycholesterol in human liver microsomes and mitochondria.
abstract: In a search for enzymes involved in the formation of bile acids from 27-hydroxycholesterol in humans, the metabolism of this and other side-chain oxygenated steroids was studied in human liver microsomes and mitochondria. The microsomal fraction contained enzyme(s) catalyzing 7 alpha-hydroxylation of 27-hydroxycholesterol and 3 beta-hydroxy-5-cholestenoic acid, whereas the 7 alpha-hydroxylation of cholesterol and 3 beta-hydroxy-5-cholenoic acid was low. Only small amounts of 7 beta-hydroxylated products were formed. Purification and subfractionation of microsomal protein yielded a fraction of cytochrome P-450, which required NADPH and NADPH-cytochrome P-450 reductase and catalyzed 7 alpha-hydroxylation of the side-chain oxygenated 3 beta-hydroxy-delta 5-C27-steroids but was inactive toward cholesterol. Added cholesterol did not inhibit the observed enzymatic activity. The results provide evidence that this enzyme is different from cholesterol 7 alpha-hydroxylase. The mitochondrial fraction contained enzyme(s) that catalyzed an isocitrate-dependent 7 alpha-hydroxylation of 3 beta-hydroxy-5-cholestenoic acid. The activity was much lower with 27-hydroxycholesterol. The mitochondrial fraction also catalyzed the oxidation of the 27-hydroxy group and contained a 3 beta-hydroxy-delta 5-steroid dehydrogenase active on 7 alpha-hydroxylated C27-steroids. The metabolic end product of the reactions catalyzed by these enzymes was 7 alpha-hydroxy-3-oxo-4-cholestenoic acid. A considerable fraction of the 7 alpha-hydroxy-delta 5 intermediates was also converted to the corresponding 7 beta-hydroxysteroids, probably by way of the 7-oxosteroids, suggesting the presence of an epimerizing enzyme in the mitochondrial fraction.
authors.affiliation: Department of Physiological Chemistry, Karolinska Institutet, Stockholm, Sweden.
authors.forename: J
authors.initials: J
authors.lastname: Shoda
authors.forename: A
authors.initials: A
authors.lastname: Toll
authors.forename: M
authors.initials: M
authors.lastname: Axelson
authors.forename: F
authors.initials: F
authors.lastname: Pieper
authors.forename: K
authors.initials: K
authors.lastname: Wikvall
authors.forename: J
authors.initials: J
authors.lastname: Sjövall
mesh_terms: D000328:Adult; D000818:Animals; D001647:Bile Acids and Salts / Q000378:metabolism*; D005260:Female; D006801:Humans; D006900:Hydroxylation; D008297:Male; D008862:Microsomes, Liver / Q000378:metabolism*; D008930:Mitochondria, Liver / Q000378:metabolism*; D010084:Oxidation-Reduction; D011817:Rabbits; D051381:Rats; D017207:Rats, Sprague-Dawley; D013045:Species Specificity; D013552:Swine
chemical_list: D001647:Bile Acids and Salts