Endosomes differ from plasma membranes in the phospholipid molecular species composition.

125I-Labeled epidermal growth factor was incorporated into and highly concentrated in endosomes of Chinese hamster V79-UF cells during incubation at 37 degrees C for 8 min after binding to its receptors on the cell surface at 4 degrees C. From the labeled cells, endosomes were isolated by isopycnic centrifugation on a Percoll density gradient and then sucrose density gradients. The isolated endosomes were mostly free from contamination by Golgi, endoplasmic reticulum, lysosome, plasma membrane and mitochondria. Endosome membranes were found to differ from plasma membranes in the phospholipid composition. Sphingomyelin and phosphatidylserine were enriched in endosomes, compared with plasma membranes. Diacylphosphatidylcholine and diacylphosphatidylethanolamine were major phospholipids of the membranes in both organelles. The contents of molecular species of diacylphosphatidylcholine and diacylphosphatidylethanolamine with two monoenoic fatty acids were lower in endosomes than in plasma membranes. The differences in the polar head group and molecular species compositions of phospholipids between endosomes and plasma membranes did not change, regardless of whether or not the proportions of phospholipid molecular species in plasma membranes changed. The significance of the lipids in endosomes is discussed.