Interactions of cholesterol esters with phospholipids: cholesteryl myristate and dimyristoyl lecithin

The ternary phase diagram of cholesteryl myristate--dimyristoyl lecithin--water has been determined by polarizing light microscopy, scanning calorimetry, and x-ray diffraction. Hydrated dimyristoyl lecithin forms a lamellar liquid--crystalline phase (L alpha) at temperatures greater than 23 degrees C into which limited amounts of cholesteryl myristate (less than 5 wt. %) can be incorporated. The amount of cholesterol ester incorporated is dependent upon the degree of hydration of the L alpha phase. Below 23 degrees C dimyristoyl lecithin forms ordered hydrocarbon chain structures (L beta' and P beta') which do not incorporate cholesterol ester. Comparison with other phospholipid--cholesterol ester--water phase diagrams suggests the following general principles: i) the incorporation of cholesterol ester occurs only into liquid crystalling phospholipid bilayers, ii) the extent of incorporation is temperature-dependent, with increasing amounts of cholesterol ester being incorporated at higher temperatures, and iii) unsaturated cholesterol esters induce increased disordering of the phospholipid bilayers.