Radiolabeled hexadecyl cholesteryl ether can serve as an effective marker for liposomes in a variety of studies. This paper demonstrates the use of a cholesteryl ether marker in the assay of phospholipid transfer protein activity and in assessing phospholipid vesicle‐cell interactions. The cholesteryl ether has the advantages of ease of synthesis, metabolic inertness, lipid solubility and nonexchangeability.

The phospholipid exchange proteins of rat liver that catalyze the transfer of phosphatidylcholine and phosphatidylinositol from rat liver microsomes to liposomes, have been purified and characterized.Two proteins were detected with dual specificities catalyzing the transfer of both phosphatidylinositol and phosphatidylcholine. Both proteins showed a strong preference for phosphatidylinositol transferring 8 to 9 times as much of the microsomal phosphatidylinositol pool as the microsomal phosphatidylcholine pool. The two proteins had iso-electric points of 5.1 and 5.3 and were purified 300-fold and 500-fold, respectively.A protein that catalyzed specifically the transfer of phosphatidylcholine, was purified 7000-fold. This protein had an iso-electric point of 8.4 and a molecular weight of approximately 16 000 calculated from Sephadex G-50 chromatography and sodium dodecylsulfate-polyacrylamide gel electrophoresis ; the amino acid composition was determined. An antiserum against this protein was raised in rabbits. Treatment of a rat liver supernatant fraction with the antiserum immunoglobulin fraction demonstrated that 60 % of the phosphatidylcholine transfer activity is due to this protein.The transfer of phospholipids between membranes has been shown to be stimulated by proteins in the 105000xg supernatant of a variety of cells [1,2]. It has been suggested that these proteins play a role in determining the phospholipid composition of eucaryotic cell membranes [3], in membrane biogenesis [2] and possibly in synaptic transmission of nerve impulses [4]. Phospholipid exchange proteins may thus be important to many membrane-associated functions.The transfer of phosphatidylcholine, phosphatidylinositol and phosphatidylethanolamine between membranes has been demonstrated with rat liver [5 -101 and bovine liver supernatant [l 11. Recent studies have indicated that bovine brain and liver supernatant contain one exchange protein specific for phosphatidylcholine, and two exchange proteins both of which transfer phosphatidylinositol and, to a lesser extent, phosphatidylcholine [12]. All three proteins are acidic with iso-electric points between 5 and 6. In the present study we have further characterized the phosphatidylinositol and phosphatidylcholine exchange proteins in rat liver.Here we report the presence of an exchange protein in rat liver with an iso-electric point of 8.4 that transfers specifically phosphatidylcholine. In addition two proteins have been partially characterized with isoelectric points of 5.1 and 5.3 and transfer activities towards both phosphatidylinositol and phosphatidylcholine. While this work was in progress a report appeared showing phosphatidylcholine transfer activity in rat intestinal supernatant [13]. On iso-electric focusing this activity concentrated at both acidic (4.5-5.3) and basic (8-9) iso-electric points. MATERIALS AND METHODS Purgication of Phosphatidylinositol and Phosphatidylcholine Exchange ProteinsLivers were obtained from Wistar rats and stored at -20 "C until used. Prior to use ...

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Proteins that catalyze the transfer of phospholipids between membranes are known to occur in the mammalian lung and may play roles in the pulmonary surfactant system. These roles may include 1) packaging of newly synthesized as well as recycled surfactant phospholipid into lamellar bodies, the intracellular storage form of surfactant phospholipid, 2) extracellular movement of phospholipid between the compartments in the alveolar subphase, and 3) delivery of phospholipid to membrane sites during signal transduction events. The nature and distribution of these proteins are discussed as they relate to the pulmonary surfactant system.