Comparison of spontaneous phospholipid transfer between phospholipid vesicles and between phospholipid-bile salt mixed micelles.

Fluorescent-labeled N-(7-nitro-2,1,3-benzoxadiazol-4-yl)phosphatidylethanolamine was used to compare the spontaneous rates of phospholipid transfer between phospholipid vesicles and between phospholipid-bile salt micelles. The half times for transfer between the mixed micelles are 200 to 6,000 times faster than between vesicles, depending on the acyl chain length of the N-(7-nitro-2,1,3-benzoxadiazol-4-yl)phosphatidylethanolamine and the total lipid concentration. A kinetic analysis of the rates of phospholipid transfer between vesicles indicated that transfer occurs predominantly as soluble monomers moving through the water phase. Transfer between mixed micelles was found to occur by the same aqueous diffusion mechanism at low lipid concentrations, but at high lipid concentrations, transfer occurs predominantly during transient micelle collisions. The faster rate of transfer between mixed micelles relative to vesicles results both from the collision-dependent transfer and from an increase in the rate-limiting step for aqueous diffusion-phospholipid dissociation from the micelle surface. The relative contribution of collision-dependent transfer to the overall transfer rate increases with phospholipid acyl chain length and total lipid concentration. For the phospholipid species and mixed micelle concentrations normally found in the biliary tract, the predominant mode of transfer is predicted to occur by the collision-dependent mechanism.