Kinetic Analysis of the Interaction of the Phosphatidylcholine Exchange Protein with Unilamellar Vesicles and Multilamellar Liposomes

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Abstract
The mode of action of the phosphatidylcholine exchange protein from bovine liver was studied using unilamellar vesicles and multilamellar liposomes, both of which membranes contain phosphatidylcholine and phosphatidic acid. The protein-mediated exchange of phosphatidylcholine between vesicles and liposomes fit the kinetic model previously presented. Kinetic analysis of the exchange rates indicate that the apparent dissociation constant of the exchange protein-vesicle complex decreases with an increasing phosphatidic acid content of the vesicles. Both vesicles and liposomes of 10 mol% phosphatidic acid show the same dissociation constant, but the formation and the disruption of the protein-membrane complex was 50-100 times higher for the vesicles than for liposomes. This implies that the exchange protein can discriminate between vesicles and liposomes. Equilibrium gel chromatography on a column of Bio Gel A-5m confirmed that the exchange protein binds more strongly to vesicles of an increased phosphatidic acid content. The protein-mediated exchange of phosphatidylcholine in the vesicle-liposome system demonstrates a pH optimum at 4.0-5.5. The kinetic analysis at pH 5.0 as compared to pH 7.4 indicates that the enhanced exchange at pH 5.0 can be accounted for solely by an altered interaction of the exchange protein with the liposomes.