Role of acidic phospholipids in intermembrane sterol transfer.

A liposomal membrane model system was used to examine the effect of acidic phospholipids on spontaneous intermembrane cholesterol transfer. The spontaneous exchange of sterol between small unilamellar vesicles (SUV) containing 35 mol% sterol was monitored with a recently developed assay (Nemecz, G., Fontaine, R.N. and Schroeder, F. (1988) Biochim. Biophys. Acta 943, 511-541), not requiring separation of donor and acceptor membrane vesicles. Acidic phospholipids (2.5-30 mol%) increased the initial rate of spontaneous exchange of sterol by 5-89%, depending on the specific phospholipid. The stimulation of spontaneous sterol transfer by acidic phospholipids was suppressed by high ionic strength, CaCl2 and low pH. The results suggest that negatively charged phospholipids may fluidize sterol-poor domains in SUV membranes and thereby play an important role in the mechanism whereby sterols desorb from membranes into the aqueous medium.