Enlargement of taurocholate micelles by added cholesterol and monoolein: self-diffusion measurements

Abstract

The effect of solubilized cholesterol and 1-monoolein on the size of micellar aggregates of sodium taurocholate (3alpha,7alpha,12alpha-trihydroxy-5-cholanoyl taurine) has been determined in vitro. Measurements of the self-diffusion coefficient of sodium taurocholate (0.15 m in Na(+)) at 37 degrees C and pH 7.4 led to the conclusion that at concentrations above the critical micelle concentration (6.7 mm) the solutions contain, besides monomeric ions, a single micellar species containing five taurocholate ions. In the presence of cholesterol, much larger micelles are formed, apparently containing one molecule of cholesterol and 25 of taurocholate. These mixed micelles coexist with small micelles of pure taurocholate as well as the taurocholate monomers. The addition of 1-monoolein increases the solubility of cholesterol in the taurocholate solution, but not by reducing the size of the micelle into which the cholesterol will fit: three-component micelles (monoolein-taurocholate-cholesterol) are, if their diffusion coefficients are any guide, still larger than taurocholate-cholesterol micelles. The molar ratio of cholesterol to taurocholate is higher in these solutions than in the absence of monoolein. Comparison with work by other authors on taurodeoxy-cholate-cholesterol micelles suggests that more than 25 molecules of either dihydroxy or trihydroxy bile salts are needed to transport each molecule of cholesterol through an aqueous solution in the absence of other amphipathic molecules.