The solubility of cholesterol in aqueous solutions of bile salts and lecithin.

Abstract

The solubility of cholesterol in aqueous solutions of bile salts and lecithin with N/10 phosphate buffer, pH 7.3 has been examined. For the sodium salts of the six bile acids used in the study, the capacity for dissolving cholesterol was found to decrease in the following order: Glycodeoxycholate, Taurodeoxycholate, Glycochenodeoxycholate, Taurochenodeoxycholate, Glycocholate, Taurocholate. The differences with respect to cholesterol dissolving capacity were greatest when no lecithin was present, and decreased as the amount of lecithin increased. Graphical representations (molar ratio bile salt to cholesterol as abscissa, molar ratio lecithin to cholesterol as ordinate) of the limits for solubility of cholesterol in 100 millimolar solutions of each of the six bile salts to which purified egg lecithin is added in amounts varying from zero to approximately 80 millimoles per 100 millimoles bile salt, had the shape of curved lines tending to converge against a point, approximately 3.5, on the vertical axis but without reaching it. (Fig. 1). The starting points of the curves on the horizontal axis (millimoles bile salt required to dissolve 1 millimole of cholesterol in the absence of lecithin) were: for Glycodeoxycholate 19, for Taurodeoxycholate 30, for Glycochenodeoxycholate 34, for Taurochenodeoxycholate 47, for Glycocholate 52, for Taurocholate 62. The graphical representation, as above, of the limit for solubility of cholesterol in a 100 millimolar solution of a mixture of the six bile salts in relative amounts as in human bile, to which lecithin from human bile is added was practically identical to the corresponding curve for a 100 millimolar solution of glycochenodeoxycholate with additions of purified egg lecithin (Fig. 2.). The starting point on the horizontal axis for the curve representing mixed bile salts could be calculated — with fair approximation — from the cholesterol dissolving capacities of the individual bile salts in the mixture. The graphical representations of the limits for solubility of cholesterol in 50 millimolar, 100 millimolar and 200 millimolar aqueous solutions of sodium taurodeoxycholate to which purified egg lecithin was added in varying amounts did not show any certain differences from each other. Two purified egg lecithins of greatly different fatty acid pattern (Table 1) isolated from eggs of hens fed linoleic acid deficient diets and linoleic acid rich diets respectively, did not show any certain difference from each other with respect to cholesterol dissolving capacity when added in varying amounts to a 100 millimolar solution of sodium taurodeoxycholate.