Distribution of mixtures of bile salt taurine conjugates between lecithin-cholesterol vesicles and aqueous media: an empirical model.

Bile salts are surfactants that partition into phospholipid bilayers. When liposomes or membranes are exposed to mixed solutions of bile salts, the more hydrophobic bile salt species associate preferentially with the lipid bilayer. As a consequence, in the aqueous phase, the free monomeric concentration of bile salt declines and the more hydrophilic species become relatively enriched. Above a critical saturating concentration of lecithin-associated bile salt, a phase transition occurs with loss of membrane integrity and formation of mixed micelles. In this paper we present a quantitative model which, for mixed solutions of bile salt taurine conjugates, predicts the distribution of bile salt monomers between large unilamellar vesicles composed of lecithin and cholesterol and the aqueous phase. The model is based on association isotherms for individual bile salts, determined by an ultrafiltration method with empirical curve fitting, and is critically dependent upon the observation that association coefficients of each bile salt are a function of the total bound bile salt/lecithin mole ratio. Given the concentrations of individual bile salts, lecithin and cholesterol, the model permits calculation of the membrane-bound bile salt/lecithin ratio and the concentration of each bile salt remaining free as soluble monomer in the aqueous phase, as well as the overall hydrophilic-hydrophobic balance (hydrophobicity index) of the bile salts remaining free in aqueous solution. Distribution data determined empirically for a variety of mixtures of bile salt taurine conjugates and large unilamellar vesicles of varying cholesterol:lecithin ratio agree closely with predictions. This model may be of value in predicting the physical, biological and toxic properties of mixed bile salt solutions.