Thermodynamics of lipid-protein association. Enthalphy of association of apolipoprotein A-II with dimyristoylphosphatidylcholine-cholesterol mixtures

Apolipoprotein A-II spontaneously associates with dimyristoylphosphatidylcholine (DMPC)-cholesterol mixtures to give products whose composition is a sensitive function of temperature and cholesterol content. At most temperatures, the lipid-to-protein stoichiometry of the product recombinant increases with increasing mol% cholesterol. Up to about 18 mol% cholesterol, the complexes have the same average sterol/DMPC ratio as that of the starting mixtures. At 24 mol% cholesterol or higher, no detectable lipid/protein complex formed. At 37°C, the lipid-to-protein stoichiometry is essentially constant, irrespective of the cholesterol content and subsitution of unsaturated phospholipids for DMPC. The enthalpy of lipid-protein association is a function of cholesterol content and, at 25°C, increases linearly with the mol% cholesterol in the reaction mixture until it becomes endothermic between 15 and 20 mol% cholesterol. The results fit a model in which cholesterol is excluded from phospholipids in the 'boundary' layer, which is perturbed by the protein. At high cholesterol concentrations, the formation of a recombinant is thermodynamically unfavorable.