Solute distributions and trapping efficiencies observed in freeze-thawed multilamellar vesicles

It has recently been observed (Gruner, Lenk, Janoff and Ostro (1985) Biochemistry, in the press) that mechanical dispersion of dry lipid in an aqueous buffer to form multilamellar vesicle (MLV) systems does not result in equilibrium trans-membrane distributions of solute. In particular, the entrapped buffer exhibits reduced solute concentrations. Here we demonstrate that egg phosphatidylcholine MLV systems dispersed in the presence of Mn2+ also exhibit non-equilibrium solute distributions, and that repetitive freeze-thawing cycles can remove such solute heterogeneity. Further, the resulting freeze-thawed MLVs exhibit dramatically enhanced trapped volumes and trapping efficiencies. At 400 mg phospholipid per ml, for example, the trapping efficiencies can be as high as 90%. This is associated with a remarkable change in MLV morphology where large inter-bilayer separations are commonly observed.