Reactions of small aggregates of taurine conjugates of dihydroxy bile salts with divalent transition metal ions

Formation of slightly soluble complexes between the small aggregates (dimers to tetramers) of bile salts and divalent cations, Cu2+, Cd2+, and Fe2+ (Me(2+)), have been studied using polarography to determine concentration of free aquo ions. Polarography has been proved to be useful for studying heterogeneous equilibria. Whereas taurocholate does not form such complexes (possibly because it does not form small aggregates), taurodeoxycholate, taurochenodeoxycholate, and tauroursodeoxycholate (all bearing two OH groups) participate in formation of such complexes. Nevertheless, these complexes are much weaker than those formed with parent, unconjugated bile salts. These differences can be due to differences in stacking as well as to exchange of COO- for the less complex-forming SO3-. The stability of complexes were characterized by values of pK(i), obtained from shifts of half-wave potentials with concentrations of bile salt in excess. The values of pK(j), corresponding to equilibria between Me(2+) in solution and solids, which can be obtained from decrease of limiting currents with concentration of bile salt, are less suitable for comparison, as it is often not possible to reach sufficiently high concentrations of bile salts. Comparison of pK(i) with pK(j) indicates that for some bile salts-Me(2+) combinations the same complex predominates in solution and in the solid; for others the composition of these complexes differ. For Cu2+ and Cd2+, tauroursodeoxycholate forms the most stable complexes. For all three bile salts studied, Cu2+ formed the least stable complexes. Possibility and consequences of formation of complexes of bile salts with Me(2+) ions in bile should be kept in mind. (C) 1997 Academic Press.