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 C h supersaturation in human bile than the widely known cholesterol satu ration index. An early version of a method for determining thermodynam ic activity based on Ch uptake from bile salt (BS)-lecithin (LE) solut ions into silicone polymer particles, but requiring 12-24 h for reachi ng equilibrium, was considered unsatisfactory because Ch nucleation an d crystal formation frequently occurred within a few hours. The aim of the present work was to develop a method that would reduce equilibrat ion times to the order of 1 h. Changing the thickness of the silicone film alone did not result in the desired reduction of equilibration ti mes 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 in volving the transport of Ch by negatively charged BS and BS-LE micelle s at the interface. Three different approaches were tried to modify th e silicone film to make its surface positively charged, thereby reduci ng and/or eliminating the presumed electrical repulsion barrier for th e interfacial transport of Ch. The film was treated with different con centrations of aminopropyl methyl-dimethylsiloxane (AMDS) in cyclohexa ne, octadecyldimethyl-[3-(trimethoxysilyl)-propyl] ammonium chloride ( ODTOP) in methanol, and octadecylamine solution in ethanol. Films trea ted with 1-1.5% ODTOP and 5-10% AMDS reduced the equilibration times f or model BS-LE solutions to <1 h.