Bile salt-induced disintegration of egg phosphatidylcholine liposomes: a kinetic study based on turbidity changes.

The disintegration kinetics of egg phosphatidylcholine small unilamellar liposomes in various bile salts (nine species) were investigated by monitoring turbidity changes with a stopped-flow apparatus. The pseudo-first-order rate constants obtained as a function of bile salt concentration (up to 25 mM) were analyzed based on a two-step model in which a penetration-saturation step of bile salt into the bilayer and a lamellar-micellar transition step were assumed for the disintegration mechanism of the bilayer. The order of the rate of the penetration-saturation step, which is assumed to be a measure of the disintegration ability, was as follows: SCDOC greater than SDOC greater than STCDOC greater than STDOC greater than STC greater than SC greater than SGCDOC greater than SGDOC greater than SGC. The results indicated that (1) the dihydroxy bile salts have a greater disintegration ability than the corresponding trihydroxy bile salts, (2) the chenodeoxy bile salts have a greater ability than the corresponding deoxy-bile salts regardless of non-conjugated or conjugated form, (3) the taurine conjugates always have a greater ability than the glycine conjugates. The penetration-saturation rate of the bile salts against the lipid bilayer depends considerably on the chemical nature of each bile salt, varying by a factor of about 10(5). In the conjugated bile salts alone, they were in a narrower range of a factor of 10(3). The physical integrity of liposomes can hardly be maintained in the bile salt-rich intestinal tract but the resulting mixed micelles may contribute substantially to solubilization and enhanced delivery of drugs.