Physical properties of isolated complexes of human and bovine AI apolipoproteins with L α dimyristoyl phosphatidylcholine

Human or bovine A-I apolipoproteins in solution form complexes with sonicated L-α-dimyristoyl phosphatidylcholine at 23 and 37°, but not at 8°, suggesting a strong dependence of the interaction on the physical state of the lipid (phase transition temperature 23°). Complexes were isolated by gel filtration on a Sepharose 4B column and were subsequently analyzed for protein and lipid content, molecular weight, and physical state of the lipid portion. The average stoichiometry of all complexes, regardless of the initial concentrations or ratios of protein and lipid, was constant: 90±20 mol of phospholipid/mol of protein monomer, suggesting a highly cooperative interaction. Sedimentation equilibrium experiments indicated homogeneous macromolecular preparations and gave molecular weights around 235,000 (±15%) for the complexes, with the human and bovine apo-A-I proteins contributing 77,000 (±10%), i.e. about 3 protein subunits per complex. The lipid portion of the complexes retained some characteristics of a bilayer: it had a broad phase transition with a midpoint at 25.5° as reported by the fluorescence polarization of the lipophilic probe diphenylhexatriene. Above the phase transition temperature the mobility of the phospholipids in the complexes with both apo-A-I proteins was considerably decreased relative to the pure L-α-dimyristoyl phosphatidylcholine dispersion; below the phase transition temperature the opposite was true, i.e. the protein fluidized the lipids. The results indicate that apo-A-I proteins interact stoichiometrically with L-α-dimyristoyl phosphatidylcholine vesicles above the gel to liquid crystalline transition temperature of the lipid, promoting the destruction of vesicles and the formation of well defined particles of the general size of high density serum lipoproteins.