A rapid method for the measurement of cholesterol thermodynamic activity in bile salt-lecithin-cholesterol solutions

Earlier work from this laboratory suggested that the cholesterol (Ch) thermodynamic activity is a more meaningful measure of the degree of Ch supersaturation in human bile than the widely known cholesterol saturation index. An early version of a method for determining thermodynamic activity based on Ch uptake from bile salt (BS)-lecithin (LE) solutions into silicone polymer particles, but requiring 12-24 h for reaching equilibrium, was considered unsatisfactory because Ch nucleation and crystal formation frequently occurred within a few hours. The aim of the present work was to develop a method that would reduce equilibration times to the order of 1 h. Changing the thickness of the silicone film alone did not result in the desired reduction of equilibration times and it was soon deduced that the uptake of Ch by the silicone film from the BS-LE solution was a surface-controlled transport process involving the transport of Ch by negatively charged BS and BS-LE micelles at the interface. Three different approaches were tried to modify the silicone film to make its surface positively charged, thereby reducing and/or eliminating the presumed electrical repulsion barrier for the interfacial transport of Ch. The film was treated with different concentrations of aminopropyl methyl-dimethylsiloxane (AMDS) in cyclohexane, octadecyldimethyl-[3-(trimethoxysilyl)-propyl] ammonium chloride (ODTOP) in methanol, and octadecylamine solution in ethanol. Films treated with 1-1.5% ODTOP and 5-10% AMDS reduced the Ch equilibration times for model BS-LE solutions to < 1 h.