In vitro studies on the dissolution rate of cholesterol monohydrate crystals in micellar bile acid solutions showed that the addition of lecithin decreases the dissolution rate even though lecithin increases the equilibrium solubility of cholesterol in these solutions. The reduction in rates caused by lecithin was attributed to a large crystal-solution interfacial barrier. An effective permeability coefficient for the interfacial barrier was calculated to be around 1.5 x 10(-5) centimeter per second for the transport of cholesterol molecules.
Experiments on the transport of cholesterol, desmosterol and hydroxycholesterol from aqueous sodium taurocholate-lecithin and aqueous sodium cholatelecithin micelle systems into hexadecane have been carried out as a function of lecithin to bile salt concentration ratios. These data strongly support the previously proposed hypothesis that the kinetics of transport of cholesterol, desmosterol and hydroxycholesterol is (I) interfacially controlled and C2) involves a two-step process in which there is first a collision of the solute-mlcelle complex with the oil-water interface and this is followed by the release of the solute from the micelle in a largely polar environment at the interface. The experimental evidence for this mechanism is that (I) effective interfacial barrier permeability coefficients for sterol transport has been found to be independent of bile salt-lecithin concentrations, when the ratio of lecithin to the bile salt was kept constant, and (2) when the lecithin-bile salt ratio was increased, keeping the bile salt concentration constant, it was found that the interfacial barrier permeability coefficients for the sterols decreased.
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