title: Identification of the bile acid binding proteins in human serum by photoaffinity labeling.
abstract: The binding of conjugated and unconjugated bile acids to human serum lipoproteins was investigated by density gradient centrifugation and photoaffinity labeling studies. The binding of bile acids to high-density lipoprotein increased by substitution of the 3 alpha-hydroxy group in cholate and taurocholate by a photolabile 3-azido or 3-azi-function. The affinity of bile acid derivatives to HDL showed the following ranking: 3 beta-azido-7 alpha,12 alpha-dihydroxy-,3,3-azo-7 alpha,12 alpha-dihydroxy- > 3 alpha,7 alpha,12 alpha-trihydroxy-,11 xi-azido-3 alpha,7 alpha,12 xi-trihydroxy- > 11 xi-azido-12-oxo-3 alpha,7 alpha-dihydroxy- > 7,7-azo-3 alpha,12 alpha-dihydroxy-,3 alpha,7 alpha-dihydroxy-,3 alpha,12 alpha-dihydroxy- > 3 alpha-hydroxy-cholan-24-oic acid. Based on the actual serum concentrations of albumin and HDL, a preference of hydrophilic bile acids to HDL is evident, the 3-azido- and 3-azi-derivatives showing a 5-23-fold higher binding to HDL compared to soluble serum proteins. For the identification of the bile acid binding proteins in human blood, photoaffinity labeling with a variety of photolabile conjugated and unconjugated bile acid derivatives was performed with subsequent analysis of radiolabeled serum proteins by one- and two-dimensional gel electrophoresis. In addition to albumin and the apolipoproteins A-I and A-II of high-density lipoproteins (Kramer et al. (1979) Eur. J. Biochem. 102, 1-9), three further proteins in the lipoprotein free serum fraction of M(r) 41,000, 50,000 and 83,000 were specifically labeled. By two-dimensional electrophoresis and by immunoprecipitation these proteins were identified as alpha 1-acid glycoprotein (M(r) 41,000), alpha 1-antitrypsin (M(r) 50,000) and transferrin (M(r) 83,000). No binding of bile acids to haptoglobin, alpha 2-HS-glycoprotein, hemopexin or alpha 1-fetoprotein occurred. In conclusion, these studies show that bile acid derivatives bind to several serum proteins in addition to albumin and furthermore that the substituent in position 3 of the steroid nucleus greatly influences the affinity of bile acids to high density lipoproteins.
authors.affiliation: Hoechst Aktiengesellschaft, Frankfurt am Main, Germany.
authors.forename: W
authors.initials: W
authors.lastname: Kramer
mesh_terms: D000328:Adult; D000345:Affinity Labels; D001391:Azo Compounds; D001647:Bile Acids and Salts / Q000378:metabolism*; D001798:Blood Proteins / Q000378:metabolism*; D002499:Centrifugation, Density Gradient; D004591:Electrophoresis, Polyacrylamide Gel; D006801:Humans; D007525:Isoelectric Focusing; D008075:Lipoproteins, HDL / Q000097:blood*; D008077:Lipoproteins, LDL / Q000097:blood*; D008297:Male; D015394:Molecular Structure; D009961:Orosomucoid / Q000378:metabolism; D011233:Precipitin Tests; D014168:Transferrin / Q000378:metabolism
chemical_list: D000345:Affinity Labels; D001391:Azo Compounds; D001647:Bile Acids and Salts; D001798:Blood Proteins; D008075:Lipoproteins, HDL; D008077:Lipoproteins, LDL; D009961:Orosomucoid; D014168:Transferrin