Solubility of cholesterol in bile salt-lecithin model systems

Abstract

Cholesterol solubility was determined in model systems of unconjugated and conjugated bile salts and lecithin at physiologic concentrations. Conjugated bile salts had somewhat less dissolving power than unconjugated forms, with taurine conjugates showing less power than glycine conjugates. Lecithin increased the dissolving power of each bile salt species proportionally up to a lecithin-bile salt molar ratio of 1.0, at which point the amount of cholesterol dissolved was triple that in the absence of lecithin. At most physiologic ratios of lecithin-bile salt, however, lecithin only doubles the amount of cholesterol dissolved. Lecithin reduces, but does not eliminate, significant differences in the cholesterol dissolving power of both unconjugated and conjugated bile salt species. Mixtures of unconjugated bile salts show a simple additive effect in the absence of lecithin, but when lecithin is present the dissolving power of deoxycholate predominates over cholate, and of cholate over chenodeoxycholate. Mixtures of conjugated bile salt species produce a simple additive effect on dissolving power in both the presence and the absence of lecithin. Our cholesterol saturation curves for glycodeoxycholate and glycocholate at a total bile salt concentration of 150 mM, which we consider representative of the cholesterol dissolving power of human gallbladder bile, showed less dissolving power at lecithin-bile salt ratios of 0.25 to 0.50 than did the curve of Adminrand and Small; our curves were reasonably similar to those of Hegardt and Dam on the trilinear graph. Our studies show that changes in total bile salt concentration encountered in human gallbladder bile do produce significant shifts in the saturation curve.