Characterization of the adsorption of conjugated and unconjugated bile acids to insoluble, amorphous calcium phosphate.
Recently we showed that supplemental dietary calcium stimulates the intestinal formation of insoluble calcium phosphate and decreases the ratio of dihydroxy to trihydroxy bile acids in human duodenal bile. Because previous in vitro studies indicated that these effects could be due to differential adsorption of bile acids to amorphous calcium phosphate, we characterized the binding of bile acids to calcium phosphate. Freshly formed, amorphous, calcium phosphate bound and thus precipitated glycine-conjugated and unconjugated bile acids, whereas taurine-conjugated bile acids showed little binding. Glycochenodeoxycholic acid hardly adsorbed to other insoluble calcium phosphates, including hydroxyapatite. Adsorption studies using increasing amounts of glycine-conjugated and unconjugated bile acids showed that binding occurred above a bile acid-specific critical minimum concentration, dependent on bile acid hydrophobicity. The simultaneous use of a fluorescent hydrophobic probe indicated that this binding was due to ionic adsorption of monomers of bile acids, followed by their hydrophobic aggregation on the calcium phosphate surface, probably in the form of a bilayer. Finally, using human duodenal bile we found that amorphous calcium phosphate, but not Ca2+, preferentially bound and thus precipitated dihydroxy bile acids. We conclude that freshly formed, amorphous, calcium phosphate is a prerequisite for adsorption of bile acids and that monomers of glycine-conjugated and unconjugated dihydroxy bile acids have a high binding affinity for amorphous calcium phosphate.
Govers
M J
MJ
Department of Nutrition, Netherlands Institute for Dairy Research (NIZO), Ede.
Termont
D S
DS
Van Aken
G A
GA
Van der Meer
R
R
D000327:Adsorption; D001647:Bile Acids and Salts / Q000737:chemistry*; D002130:Calcium Phosphates / Q000737:chemistry / Q000378:metabolism*; D006801:Humans; D012995:Solubility
D001647:Bile Acids and Salts; D002130:Calcium Phosphates; C485828:alpha-tricalcium phosphate; C519480:amorphous calcium phosphate; C485830:tetracalcium phosphate; C485838:calcium phosphate, monobasic, anhydrous; C020243:calcium phosphate; C485829:calcium phosphate, dibasic, anhydrous