Exchange of Various Phospholipids and of Cholesterol between Liposomes in the Presence of Highly Purified Phospholipid Exchange Protein

Abstract

The soluble fraction from beef heart homogenate is known to stimulate phospholipid exchange between cell membrane fractions. A purification procedure, consisting of pH adjustment to 5.1, gel filtration on Sephadex G-75, and isoelectric focusing between pH 4 and 6, yielded two fractions with high phospholipid exchange activities per mg of protein. The fraction with the highest specific activity focused at pH 5.5 and showed only one minor contaminant on polyacrylamide disc gel electrophoresis. The other fraction, which focused at pH 4.7, exhibited a major band on disc gel electrophoresis, and several minor bands. Upon elution from the polyacrylamide gel only the major bands yielded eluates that stimulated phospholipid exchange. The highly purified as well as the crude fractions were used to measure the simultaneous exchange of phosphatidylcholine, phosphatidylethanolamine, and sphingomyelin between antigen-sensitized and nonsensitized liposomes. In all instances phosphatidylcholine exchanged much faster than sphingomyelin, but the amount of sphingomyelin exchanged compared to that of phosphatidylcholine did not differ for the most highly purified and the cruder fractions from beef heart supernatant. This suggests that the exchange of phosphatidylcholine and sphingomyelin may be catalyzed by the same protein or by two proteins which were not separated by the procedures employed here. Little or no exchange of phosphatidylethanolamine was observed with any of the beef heart fractions at various stages of purification. Exchange of cholesterol between liposomes was more rapid than that of phosphatidylcholine in the absence of phospholipid exchange protein. The soluble fraction of beef heart homogenate adjusted to pH 5.1 did not stimulate cholesterol exchange.