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+, a nd Fe2+ (Me(2+)), have been studied using polarography to determine co ncentration of free aquo ions. Polarography has been proved to be usef ul for studying heterogeneous equilibria. Whereas taurocholate does no t form such complexes (possibly because it does not form small aggrega tes), taurodeoxycholate, taurochenodeoxycholate, and tauroursodeoxycho late (all bearing two OH groups) participate in formation of such comp lexes. 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 c omplex-forming SO3-. The stability of complexes were characterized by values of pK(i), obtained from shifts of half-wave potentials with con centrations of bile salt in excess. The values of pK(j), corresponding to equilibria between Me(2+) in solution and solids, which can be obt ained from decrease of limiting currents with concentration of bile sa lt, are less suitable for comparison, as it is often not possible to r each sufficiently high concentrations of bile salts. Comparison of pK( i) with pK(j) indicates that for some bile salts-Me(2+) combinations t he same complex predominates in solution and in the solid; for others the composition of these complexes differ. For Cu2+ and Cd2+, taurours odeoxycholate forms the most stable complexes. For all three bile salt s studied, Cu2+ formed the least stable complexes. Possibility and con sequences of formation of complexes of bile salts with Me(2+) ions in bile should be kept in mind. (C) 1997 Academic Press.