Alteration of membrane phospholipid bilayer organization in human erythrocytes during drug-induced endocytosis

Our plan was to evaluate the potentially important role of phospholipids in erythrocyte shape alterations by determining if their orientation was altered during endocytosis. Stomatocytosis and endocytosis were induced in normal intact human erythrocytes by incubation with three agents: primaquine, vinblastine, and chlorpromazine, each of which has its own requirements and time course for producing endocytosis. The organization of the phospholipid bilayer was assessed by measuring the extent of degradation of phophatidylcholine (PC), phophatidylethanolamine (PE), phosphatidylserine (PS), and sphingomyelin (SM) produced by exposure of erythrocytes to a nonpenetrating protease-free phospholipase A₂ alone or in combination with a purified sphingomyelinase as well. The induction of stomacytosis did not change this orientation. However, correlating with the onset of endocytosis but not its extent, there was an increase in PE degradation, which could be detected regularly only by use of phospholipase A₂ alone. Use of the combination of phospholipase A₂ and sphingomyelinase showed that the extent and course of endocytosis was paralled by an apparent movement of PC and SM from the outer to the inner half of the lipid bilayer. Since no further PE was hydrolyzed and because no PS was ever degraded, this inward movement of PC and SM did not represent the establishment of complete symmetry in the membrane. By adjusting the experimental design it was possible to implicate the endocytic process, and not insertion of drug in the membrane, as the cause of the alterations in phospholipid organization seen. Our findings indicate that the phospholipid orientation is very closely involved in the endocytosis process and that specific states of phospholipid asymmetry may be related to identifiable membrane events.