Interaction of the Apoproteins of Very Low Density and High Density Lipoproteins with Synthetic Phospholipids

Abstract

The interaction of synthetic dimyristoyl phosphatidylcholine (lecithin) liposomes with isolated apoC-I and apoC-III proteins from very low density [human plasma] lipoproteins was studied by microcalorimetry. Complex formation is a highly exothermal process characterized by a maximal enthalpy of -130 kcal/mol (-544 kJ) apoC-III-1 and -65 kcal/mol apoC-I proteins (-272 kJ). The complex composition determined after its isolation by ultracentrifugal flotation agrees with the value derived from the enthalpy binding curves. The binding of a constant amount of dimyristoyl lecithin to apoprotein mixtures containing various proportions of apoA-I and apoC-III failed to demonstrate the existence of any preferential association between the 2 apoproteins, in contrast with results obtained previously with apoA-I/apoA-II protein mixtures. Finally the various contributions to the enthalpy of binding such as that arising from an increase in apoprotein helicity were evaluated. A classification of the apolipoproteins according to their lipid-binding affinity is proposed as: apoA-II .simeq. apoC-III > apoC-I > apoA-I proteins.