title: On the saturation of the cholesterol 7 alpha-hydroxylase in human liver microsomes.
abstract: The relationships between cholesterol 7 alpha-hydroxylase activity, pool of free microsomal cholesterol, and degree of substrate saturation of the enzyme were studied in untreated (n = 5), cholesterol-fed (n = 4), and cholestyramine-treated (n = 6) gallstone patients undergoing cholecystectomy. Highly accurate methods based on isotope dilution-mass spectrometry were used for assay of the cholesterol 7 alpha-hydroxylase activity and for determination of the concentration of free cholesterol in the microsomes. The cholesterol-enriched diet increased the cholesterol 7 alpha-hydroxylase activity about twofold. Cholestyramine treatment was associated with a five- to sixfold increase of the cholesterol 7 alpha-hydroxylase activity. The concentration of free microsomal cholesterol remained essentially unchanged. The apparent degree of saturation of the enzyme was calculated to be 85% in the untreated patients, 86% in the cholesterol-fed patients, and 67% in those treated with cholestyramine. A significant negative correlation was obtained between enzyme activity and apparent substrate saturation. It is concluded that the apparent substrate saturation of the cholesterol 7 alpha-hydroxylase in human liver microsomes is high but that availability of cholesterol may limit the enzyme activity to some extent a high bile acid synthesis rates.
authors.affiliation: Department of Medicine, Karolinska Institute, Huddinge University Hospital, Sweden.
authors.forename: K
authors.initials: K
authors.lastname: Einarsson
authors.forename: E
authors.initials: E
authors.lastname: Reihnér
authors.forename: I
authors.initials: I
authors.lastname: Björkhem
mesh_terms: D000328:Adult; D000368:Aged; D001647:Bile Acids and Salts / Q000096:biosynthesis; D002769:Cholelithiasis / Q000188:drug therapy / Q000201:enzymology*; D002790:Cholesterol 7-alpha-Hydroxylase / Q000378:metabolism*; D002791:Cholesterol, Dietary / Q000494:pharmacology; D002792:Cholestyramine Resin / Q000627:therapeutic use; D005260:Female; D006801:Humans; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology*; D008875:Middle Aged; D013250:Steroid Hydroxylases / Q000378:metabolism*
chemical_list: D001647:Bile Acids and Salts; D002791:Cholesterol, Dietary; D002792:Cholestyramine Resin; D013250:Steroid Hydroxylases; D002790:Cholesterol 7-alpha-Hydroxylase