Studies of the lipid binding characteristics of the apolipoproteins from human high density lipoprotein. II. Calorimetry of the binding of apo AI and apo AII with phospholipids.

The interactions of lysophosphatidylcholine and synthetic 1,2-dimyristoyl-sn-glycerophosphocholine (DMPC) liposomes with the isolated HDL-apolipoproteins, apo AI and apo AII, has been studied by microcalorimetry. Complex formation is a highly exothermal process characterized by a maximal enthalpy of about --200 kcal/mol of apoprotein when added to DMPC at 28 degrees C in 0.05 M sodium carbonate/bicarbonate buffer, pH 9.6. For the apo AI apoprotein, the binding consists of two processes, one endothermal occurring at low phospholipid/protein ratios and one exothermal predominant at higher phospholipid levels. The endothermal process has been attributed to a lipid-induced disaggregation of the apo AI while the exothermal process is similar to the binding of apo AII or apo HDL to phospholipids. The binding of a constant AI and apo AII, demonstrates the existence of a maximal association at a 1 : 1 molar ratio of the apolipoproteins. The sequential binding of DMPC to apo AI and apo AII suggests the existence of cooperativity between the two apoproteins in phospholipid binding as apo AII promotes the incorporation of apo AI into a protein-phospholipid complex.