The Preference of Cholesterol for Phosphatidylcholine in Mixed Phosphatidylcholine-phosphatidylethanolamine Bilayers

The following phosphatidylethanolamines were studied by differential scanning calorimetry: 1,2-dipalmitoleoyl-, 1,2-dioleoyl-, 1,2-dilauroyl-, 1,2-dielaidyl-, 1,2-dimyristoyl- and 1,2-dipalmitoyl-sn-glycero-3-phosphoryl-ethanolamine. The saturated and trans-unsaturated species underwent thermotropic phase transitions at temperatures about 20-30 degrees C higher than the corresponding phosphatidylcholines but the enthalpy changes were nearly identical. The transition temperatures for the cis-unsaturated species were about the same as those of the corresponding phosphatidylcholines but here the enthalpy change was markedly decreased as compared with the phosphatidylcholines. Freeze-fracture electron microscopy revealed phase changes from a lamellar to a hexagonal phase for 1,2-dipalmitoleoyl- and 1,2-dioleoyl-sn-glycero-phosphorylethanolamine at 20 and 0 degrees C respectively. At these temperatures no transitions were apparent in the calorimeter scan. Incorporation of increasing amounts of cholesterol into phosphatidylethanol-amine bilayers gradually decreased the enthalpy changes of the phase transition in the same manner as was demonstrated before for phosphatidylcholine/cholesterol mixtures. This was studied both for 1,2-dipalmitoleoyl- and 1,2-dimyristoyl-sn-glycerophosphorylethanolamine. In an equimolar mixture of 1,2-dioleoyl- and 1,2-dipalmitoylphosphoryl-ethanolamine, which showed phase separation, cholesterol preferentially decreased the transition of the lowest melting component. In equimolar mixtures of phosphatidylethanolamines and phosphatidylcholines, which showed phase separation, cholesterol preferentially abolished the transition of the phosphatidylcholine component present. This occurred both in experiments where the phosphatidylcholine was the lowest melting and where it was the highest melting component present in the mixture. These experiments strongly suggest that in phosphatidylcholine-phosphatidylethanolamine mixtures at temperatures where both components are in the liquid-crystalline state cholesterol is preferently associated with the phosphatidylcholine component in the mixture.