Interactions Between Ionized Calcium and Sodium Taurocholate: Bile Salts Are Important Buffers for Prevention of Calcium-Containing Gallstones

Abstract

Abstract Calcium precipitation in bile is a requisite physicochemical event in the pathogenesis of pigment gallstones that, in turn, may serve as a nucleus for precipitation of cholesterol from its supersaturated state to form cholesterol stones. Using the Ca2+-ion electrode, the interactions of calcium with taurocholate anions were studied at 24°C and pH 7.0 over physiologic ranges (0.5–200 mM) of bile salt (BS) and total calcium (0.15–15 mM). All studies were made at a 0.295 M total ionic strength to minimize possible bile salt effects on Ca2+-activity coefficients. It was found that Ca2+ reacts with free taiurocholate anions to form the free acid salt CaBS+, with a formation constant (K'f) of about 794. In micellar solutions, Ca2+ reacted with two taurocholate anions in the micelle to form the neutral salt Ca(BS)20. The K'f for this reaction was low, about 7.6, or roughly 1/100th that of the free acid salt. Upon further addition of calcium to such micellar solutions, Ca(BS)20 appeared to be converted to a micellar acid salt, CaBS+. In any given solution, therefore, three distinct species could be present: (a) free CaBS+, (b) micellar Ca(BS)20, and (c) micellar CaBS+. In the presence of calcium, there was no critical micellar concentration (CMC) for taurocholate, but rather continuous micelle formation along a rectangular hyperbola. Knowledge of overall apparent K'f values, and K'f values for both CABS+ and CA(BS)20 permitted calculation of the relative proportions of free and micellar calcium-taurocholate in any given solution. The term CMC50 is introduced to indicate that at which 50% of calcium is free and 50% is in micellar form. It is concluded that bile acid anions, both free and micellar, constitute an important potential buffer for intraluminal calcium in the biliary tree and intestine. By thus limiting ambient Ca2+ concentration, bile acid anions may protect against calcium precipitation with various anions in bile such as bilirubinate (pigment gallstones), and may also affect the interactions of calcium within the intestinal lumen (absorption of free fatty acids, oxalate, etc.), as well as mucosal absorption of both Ca2+ and bile acid anions.