Transfer of [14C]phosphatidylcholine between liposomes and human plasma high density lipoprotein. Partial purification of a transfer-stimulating plasma factor using a rapid transfer assay.

A simple method was developed for the rapid determination of [14C]phosphatidylcholine transfer from small unilamellar liposomes to human plasma HDL, based on the selective precipitation of liposomes by heparin and MnCl2. The assay was utilized to monitor the progress in the partial purification of a phospholipid transfer factor from human plasma. The purification procedure included ultracentrifugation at d = 1.25 g/ml, hydrophobic chromatography on phenyl-Sepharose, affinity chromatography on heparin-Sepharose and gel filtration. The partially purified protein(s) catalyzed the net transfer of phospholipid from small unilamellar phosphatidylcholine liposomes to isolated HDL. The transfer of [14C]phosphatidylcholine from liposomes consisting of phosphatidylcholine/phosphatidylserine/cholesterol (molar ratio, 4:1:5) to HDL was stimulated without affecting the permeability barrier of the liposomal membranes and is, therefore, taken to represent exchange with HDL phospholipid rather than net transfer.