A new rat liver phospholipid exchange protein.

The soluble fraction from several mammalian tissue homogenates is known to stimulate phospholipid exchange between cell membrane fractions or artificial vesicles. All phospholipid exchange proteins purified to data exhibit an acidic isoelectric point. Using an assay that measures the transfer of [32P] phosphatidylcholine from liposomes to beef heart mitochondria, we report the presence of a new phospholipid exchange protein with a basic isoelectric point (8.4) in rat liver cytosol. A purification procedure, consisting of pH adjustment to 5.1, gel filtrations on Sephadex G 75 and DE 52 cellulose, isoelectric focusing between a pH of 5 and 10, and gel filtration on Sephadex G-50, yielded a fraction with high phosphatidylcholine exchange activity per mg of protein. This fraction exhibits a major band and two minor bands on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The molecular weight of the major band (18 700) is close to that for basic exchange protein fraction obtained by gel filtration (17 000). The distribution of basic and acidic exchange proteins differs markedly in various tissues and animal species. About 50 and 35% of phosphatidylcholine exchange activity from rat liver and rat intestine respectively are due to basic phospholipid exchange proteins. In contrast, no basic exchange protein was found in beef heart and only a small amount in beef liver. In the latter organ, less than 10% of phosphatidylcholine exchange activity was due to a basic phospholipid exchange protein fraction.