Interactions of cholesterol/cholesteryl esters with dimyristoylphosphatidylcholine bilayers.

The limited solubility of cholesteryl esters in polar lipids is important as it determines their access to polar surfaces of lipoproteins, their interaction with plasma membranes and their phase formation with phospholipids. Variations in cholesteryl ester concentrations in membranes may be related to the physiological role of the membranes. In lysosoma1 membranes cholesteryl ester concentrations are approx. 8% (w/w), but in Schwann cell membranes the concentrations are negligible [ I ] . Abnormalities in cholesterol and cholesteryl ester composition have been found in membranes of cancerous tissue [2] and during the pathogenesis of atherosclerosis 131. One approach to unravelling how changes in lipid composition may be related to pathogenicity is to study the physical properties of lipids in membranes using differential scanning calorimetry (DSC). This method gives data on the gel-to-liquid-crystalline phase transition enthalpy ( A H ) and the size of the co-operative unit (CU) which is the number of molecules in the non-dominant phase linked together by the nearest neighbour molecules. The CU is equal to the Van’t Hoff enthalpy divided by the calorimetric enthalpy 141. These parameters are associated with the interaction of cholesterol and its esters in the dimyristoylphosphatidylcholine (DMPC) model membrane used. For the sample preparation for DSC measurements, DMPC (5O0/o, w/w) was dispersed in 25 mM-Tris, 1 mMEDTA, pH 7.2 buffer at 35°C. Bilayers were obtained by vigorous mixing followed by repeatedly forcing the mixture through a syringe needle ( 1 mm internal diameter) attached to two syringes. DSC measurements were carried out at a heating rate of 0.5 K/min using buffer as a reference. Where 5-30 mol% cholesterol or its esters (oleate, linoleate or laurate) were used, they were mixed with DMPC in the buffer. Cholesterol or its esters ( < 5 mol%) were dissolved in chloroform and the solution was evaporated before the addition of buffer and DMPC. The results demonstrated (Fig. 1 a) that the A H of the DMPC transition increased with concentrations up to 1 mol% ([’< 0.01) but at higher concentrations (1-3 mol%) the effects were less marked for the unsaturated esters, whereas for the saturated ester there was a small decrease in Aff . These findings suggest an increase in the disorder or fluidity of the acyl chains of DMPC with the unsaturated esters. Saturated esters caused an increase in disorder of the chains up to 1 mol% followed by a small ordering effect at higher concentrations of the ester. The significance of this is that membranes at physiological temperatures are more likely to be maintained in the fluid state because most of the cholesteryl esters of biological membranes are unsaturated. It is in this state that many reactions may occur. Results also indicated that the size of CU decreased with low concentrations ( < 1 mol%) of all three esters in DMPC ( I ’ = < 0.01). At higher concentrations (1-3 mol%) the