Journal of Lipid Research, Vol 38, 1217-1228, Copyright © 1997 by Lipid Research, Inc.
Distribution of mixtures of bile salt taurine conjugates between lecithin-cholesterol vesicles and aqueous media: an empirical model
DM Heuman
Department of Medicine, Medical College of Virginia, USA.
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.