Comparison of phosphatidylethanolamine and phosphatidylcholine vesicles produced by treating cholate-phospholipid micelles with cholestyramine

Abstract

We have previously reported the preparation and characterization of unilamellar phosphatidylcholine vesicles from cholate-phospholipid micelles treated with the bile-salt sequestrant cholestyramine (Ventimiglia, J.B., Levesque, M.C., and Chang, T.Y. (1986) Anal. Biochem. 157, 323-330). We now describe a slightly modified procedure for forming unilamellar vesicles consisting of phosphatidylethanolamine, and the characterization of the resultant vesicles by gel exclusion chromatography. In contrast to phosphatidylcholine vesicles, the formation of phosphatidylethanolamine vesicles is highly pH dependent; pH 9.2 is superior to pH 8.1 or pH 7.1. Via the dialysis step, the final pH of the vesicles could be altered to be at 8.1 or at 7.1, although decreasing the pH from 9.2 resulted in the loss of approx. 20% of the total lipid as large aggregates. Residual cholate was still present in the resultant vesicles after cholestyramine treatment; the low levels of cholate, removable by dialysis, was found to stabilize the phosphatidylethanolamine vesicles formed at pH 8.1. These results suggest that the majority of the amino groups of the phosphatidylethanolamine molecules should either be in the deprotonated form, or be neutralized and/or restricted by the anionic cholate monomers in order to facilitate the vesicle formation. Phosphatidylethanolamine vesicles were found to be much more permeable to small ions than phosphatidylcholine vesicles. The incorporation of phosphatidylserine, but not phosphatidylinositol, into the phosphatidylethanolamine vesicles at 10% resulted in decreased permeability of the bilayer against the cobalt ion influx, suggesting cooperative and complementary packing of phosphatidylethanolamine and phosphatidylserine molecules within the bilayer.