Interaction of phosphatidylcholine with bovine serum albumin. Specificity and properties of the complexes.

Phosphatidylcholine dispersed on Celite was rapidly solubilized by neutral bovine serum albumin solutions. Stable protein-lipid complexes were isolated by Agrose gel filtration or by ultracentrifugal flotation in high density solvents, and the physicochemical properties of the complexes were investigated in terms of the stoichiometry of binding, effect of fatty acid ligands on phosphatidylcholine binding, effect of high ionic strength on the stability of the complexes, intrinsic fluorescence and circular dichroism spectra, and sedimentation velocity coefficients. Complexes containing from 2 to 30 phosphatidylcholine molecules per protein molecule were observed; however, no saturation of binding sites could be detected in this range of molar ratios. Oleic acid binding by serum albumin prevents interaction of the protein with phosphatidylcholine, indicating possible competition of these ligands at low contents of the phospholipid. For molar ratios of up to 10 phosphatidylcholine molecules per serum albumin, binding is primarily due to hydrophobic interactions that have no effect on the overall shape and secondary structure of the native protein except for local modifications at tryptophan residues, whose fluorescence becomes quenched and blue shifted on phosphatidylcholine binding. Similar phosphatidylcholine uptake experiments performed with a series of globular proteins indicated that the lipid extraction from Celite surfaces is a non-specific process, accelerated by several other proteins (e.g. aldolase, egg albumin, chymotrypsinogen, soybean trypsin inhibitor, and the major apolipoprotein from bovine serum high density lipoprotein). Formation of stable protein-lipid complexes, however, was only observed with bovine serum albumin, which in contrast to the other proteins is known to have affinity binding sites for anions with hydrophobic side chains.