Complex formation in sonicated mixtures of β‐lactoglobulin and phosphatidylcholine

Abstract β‐Lactoglobulin, the major whey protein of bovine milk, is secreted via the endomembrane system of the mammary gland. The primary structure of β‐lactoglobulin shares certain characteristics with membrane proteins, although the soluble protein assumes a globular conformation. We have prepared complexes of β‐lactoglobulin and phosphatidylcholines by dissolving both in a helix‐forming solvent (chloroform methanol). The complex is stable when transferred to aqueous solutions and sonicated to form vesicles. Both ionic and hydrophobic interactions appear to be involved in complex formation. We have used spectroscopy (circular dichroism, fluorescence, and nuclear magnetic resonance) and electron microscopy to study these complexes. At pH 3.7, the small, single bilayer vesicles produced by sonication are protected against aggregation by the presence of the protein. As determined by circular dichroism, the proportion of α‐helix in β‐lactoglobulin is increased by complexation with phosphatidylcholine. Circular dichroism and fluorescence spectra show the involvement of at least 1 tryptophan residue in the conformational change. At pH 7.2, β‐lactoglobulin‐phosphatidylcholine vesicles form aggregates as observed by electron microscopy and31P nuclear magnetic resonance spectroscopy. These aggregated vesicles could be resuspended by raising the pH. The ability of the partially unfolded β‐lactoglobulin to interact with lipids is believed to be important to its transport through the endomembrane system.