Phosphatidylcholine and phosphatidylinositol exchange proteins in pig liver

The pH 5.1 supernatant fraction of pig liver homogenate stimulated the transfer of [32P]phosphatidylcholine and [32P]phosphatidylinositol from donor liposomes to acceptor liposomes. Purification of the proteins which catalyzed the exchanges yielded five partially purified preparations. A phospholipid exchange protein was purified 63-fold in one of the preparations and the protein was found to be specific for phosphatidylcholine exchange. The molecular weight and isoelectric point of the protein was estimated to be 19,000 and 5.6, respectively. The other four preparations contained partially purified phospholipid exchange proteins which catalyzed the exchanges of phosphatidylinositol as well as phosphatidylcholine. The molecular weights and the isoelectric points of the phosphatidylcholine-phosphatidylinositol exchange proteins were estimated to be 19,000 and 4.7, and 19,000 and 4.8 for the proteins in the second preparation, and 21,000 and 4.5 for the protein in the third preparation. These parameters in the other two preparations were tentatively estimated to be 24,000 and 5.6, and 18,000 and 4.6. Neither sulfhydryl-binding reagents nor trypsin digestion inactivated the activity of the phosphatidylcholine specific exchange protein. Sulfhydryl-binding reagents and trypsin digestion inactivated all the phosphatidylinositol exchange activities of the partially purified exchange proteins and the phosphatidylcholine exchange activities which were presumably associated with the phosphatidylinositol exchange proteins.