PIXE and PIGE studies of ion diffusion through phospholipid bilayers containing cholesterol

Abstract

PIXE and PIGE, using an external proton beam from a Van de Graaff accelerator, have been employed to study the effects of cholesterol on the passage of the alkali ions Li, Na, K, Rb and Cs through saturated phosphatidylcholine membranes. These techniques make it possible to study the simultaneous diffusion of several ions, which is a distinct advantage over more conventional tracer and ion electrode methods. Cholesterol, at 30 mol%, was incorporated into multilamellar vesicles which enclosed the five ions dissolved as chloride salts. Ion leakage was induced by incubating both these and pure lipid vesicles for 30 min at a series of temperatures spanning the gel to liquid-crystal phase transition. Vesicles were subsequently trapped on Nuclepore filters and analyzed by PIXE and PIGE to detect the remaining ions. Cholesterol at 30 mol% was shown to markedly reduce the large increase in ion permeability which occurs at the melting temperature of the lipid.