Calcium binding to bile salts.

Calcium binding to bile salt monomers and micelles is an important issue with respect to the possible (but rare) precipitation of calcium bile salts in the gallbladder. In the present work the binding of Ca2+ to six bile salts was measured in solutions containing 2 to 100 mM bile salts by means of a calcium-sensitive dye, murexide, which determines the ionic calcium concentration. In solutions containing bile salt at concentration higher than 20 mM most, if not all, of the bound Ca2+ is associated with micellar surfaces. The results were analyzed by employing a model which combines specific binding with electrostatic equations and accounts for the system being a closed one. The analysis of Ca2+ binding data considered explicitly the presence of Na+ ions and yielded intrinsic binding coefficients for Ca2+ and Na+ which were utilized to explain and predict binding results for various concentrations of Ca2+, Na+ and bile salts. The calculations indicate that in saline solutions most of the surface sites were bound by Na+, whereas less than 10% were bound by Ca2+ even in the presence of 8 mM Ca2+. The binding of Ca2+ to bile salt micelles increases with pH. An increase in temperature results in reduced binding affinity of Ca2+ to the bile salt micelles.