Cholesterol solubilization in aqueous micellar solutions of quillaja saponin, bile salts, or nonionic surfactants

Quillaja saponin in aqueous solution enhanced cholesterol solubility by as much as a factor of 10(3) at room temperature. Increased temperature and [NaCl] increased cholesterol solubility, whereas solubility was greatest at an aqueous pH of 4.6 at 298 K. Although various saponin sources were observed to differ in their abilities to solubilize cholesterol, trends in their solubilization properties with changing aqueous phase parameters were consistent. Surfactant molecules containing fused-ring structures as their hydrophobic portion, such as sodium cholate, sodium deoxycholate, and quillaja saponin, solubilized cholesterol significantly better than the linear hydrocarbon chain surfactants Tween 20 and Triton X-100. Mixtures of surfactants studied were found to exhibit synergistic effects: they formed micelles at lower concentrations than did those formed by the individual surfactants themselves, and they had a better ability to solubilize cholesterol. The knowledge obtained from these studies improves our understanding of cholesterol association with saponin and other types of surfactants and enhances the potential for using saponins for the solubilization and extraction of hydrophobic solutes in various pharmacological and industrial applications.