Dependence on Phospholipid Composition of the Fraction of Cholesterol Undergoing Spontaneous Exchange between Small Unilamellar Vesicles

Spontaneous cholesterol exchange between small unilamellar vesicles comprised of different phospholipids and their binary mixtures has been studied in order to understand the factors involved in the establishment and maintenance of intracellular cholesterol distributions. Exchange was performed from neutral donor vesicles containing different cholesterol concentrations, traces of [₃H] cholesterol, and [₁₄C]cholesteryl oleate as a nonexchangeable marker. The acceptor vesicles, in 10-fold excess, had the same composition, but 15 mol % phosphatidylglycerol was included to permit chromatographic separation. Data were best fitted by a single exponential and a base value. In donor vesicles containing only one phospholipid, the kinetic rate constants agreed with data reported previously; however, the base values were larger than the expected equilibrium value of 9.09%. The size of this nonexchangeable pool and the exchange rate were found to depend on the type of phospholipid. In binary phospholipid donor systems, well above the transition temperatures of the lipid components, the exchange parameters were preferentially closer to those of one component according to the order POPC > DMPC > DPPC > bovine brain SPM.