Bovine apolipoproteins C. II. Isolation and partial physicochemical characterization of complexes with L-alpha-dimyristoyl phosphatidylcholine.

Complexes were made between L-alpha-dimyristoyl phosphatidylcholine (DMPC) and four of the seven bovine apolipoproteins C described in the preceding paper (Patterson, B.W. and Jonas, A. (1980) Biochim. Biophys, Acta 619, 572-586). Reaction mixtures were fractionated by gel filtration chromatography and isopycnic density gradient ultracentrifugation. Selected complexes were further analyzed by sedimentation equilibrium ultracentrifugation and examined for phospholipid bilayer phase transition properties as reported by the fluorescence polarization of a lipophilic probe. Two bovine apolipoproteins C (D2, D3) were able to form complexes with DMPC of virtually the same size, stoichiometry, and density over a wide range of initial lipid/protein molar ratios (200 : 1 to 10 : 1). At very high initial molar ratios (200 : 1 and 100: 1), an additional lipid-enriched complex was formed with these apolipoproteins. Complexes made with D4 were less discrete, having a size, stoichiometry, and density dependent on the initial lipid : protein ratios used. Isolated complexes were smaller than intact DMPC vesicles, representing a break-down product of vesicular structure. The sizes (molecular weights around 2-3 x 10(5)), hydrated densities (1.06-1.12 g/ml), and weight percentage protein compositions (30-40%) of bovine C apolipoprotein-DMPC complexes are comparable to the corresponding parameters for intact bovine HDL. Isolated bovine C apolipoprotein-DMPC complexes retain some phospholipid bilayer structure as indicated by their phase transition behavior. However, the phase transition is considerably broadened and shifted to a higher temperature in the complexes compared to pure lipid. The results obtained are comparable to the phospholipid binding properties of human C apolipoproteins and are consistent with various oblate ellipsoidal models suggested for apolipoprotein-DMPC complexes.