Taurocholate- and taurochenodeoxycholate-lecithin micelles: The equilibrium of bile salt between aqueous phase and micelle

The equilibrium of bile salt between aqueous phase and mixed micelle was studied in solutions of pure bile salt and lecithin comparing taurocholate and taurochenodeoxycholate. The relationship between bile salt concentration in the aqueous phase and the ratio of bile salt/lecithin in the mixed micelle was determined by equilibrium dialysis on serial dilutions of these solutions. Extrapolation of this relationship to zero mixed-micellar bile salt permitted calculation of the critical micelle concentration (CMC) of the mixed micelle. For taurocholate, taurochenodeoxycholate, and an equimolar mix of these two bile salts, the mixed micelle CMC's were 3.1 mM, 0.47 mM, and 0.89 mM respectively. In the most concentrated solutions, aqueous phase bile salt concentration surpassed the CMC of the simple bile salt micelle by more than four-fold indicating the presence of simple micelles as well as mixed micelles. At all dilutions taurochenodeoxycholate had a much greater affinity for the mixed micelle than did taurocholate. This last finding may be the reason for the superior cholesterol solubilizing capacity of taurochenodeoxycholate-lecithin solutions compared to taurocholate-lecithin solutions.