Interaction of bile salt monomer and cholesterol in the aqueous phase

The purpose of the present study was to evaluate the possible interaction of bile salt monomer and cholesterol in the intermicellar aqueous phase. Cholesterol and taurocholate monomer concentrations in the intermicellar aqueous phase were determined using 0-20 mM taurocholate solutions saturated with cholesterol. Maximal solubilities of cholesterol in aqueous solutions having various concentrations of taurocholate, especially below its intermicellar monomer concentration (critical micellar concentration), were determined and compared with the intermicellar cholesterol concentration. The intermicellar monomer concentration of taurocholate was constant (6 mM) and independent of taurocholate concentrations. The cholesterol concentration in the intermicellar aqueous phase gradually increased, depending upon taurocholate concentrations, and became constant (1,3 microM) above 10 mM taurocholate. The solubility of cholesterol increased linearly with the taurocholate concentration even below the critical micellar concentration, and was 0.3 microM at 6 mM taurocholate, which was approx. 20-times higher than the aqueous solubility of cholesterol, but a fifth of the maximal intermicellar cholesterol concentration. The results indicate that the higher cholesterol concentration in the intermicellar aqueous phase compared to its aqueous solubility can be primarily ascribed to the interaction of cholesterol with bile salt monomers possibly forming bile salt-cholesterol dimers, and partly to the sustaining forces induced by numerous micelles.