A two-step model of disintegration kinetics of liposomes in bile salts.

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Abstract
The disintegration kinetics of egg phosphatidylcholine small unilamellar liposomes (SUV) in sodium deoxycholate (SDOC) and sodium cholate (SC) was investigated as a mimic of the disintegration behavior of orally administered liposomes in the intestine. The disintegrating action of the bile salts was followed by measuring turbidity changes with a stopped-flow apparatus, from which the pseudo-first-order disintegration rate constant (kobs) was calculated as a function of bile salt concentration (up to 25 mM). The disintegration rate of the SUV in SDOC was considerably higher than that in sc, and the kobs increased with a tendency to reach a plateau in SDOC but not in SC. At 20 mM bile salts, the disintegration half-life of the SUV (1.5 mM phosphorus) was 0.003 s in SDOC and was 0.69 s in SC. A model in which the penetration-saturation step of bile salt molecules and the subsequent lamellar-micellar transition step are involved was applied to analyses of the disintegration kinetics. The results showed that SDOC molecules penetrate into the bilayer structure faster than SC molecules by a factor of 5 .times. 102. This is probably due to the difference of molecular surface available for hydrophobic interaction.