Lipids of plasma membranes from rat thymic lymphoid cells: deficiency of sphingomyelin.

Plasma membranes were isolated from normal thymocytes of Wistar-King-A rats and from Moloney virus-induced rat thymic leukemias (RML11 and RML30 cells) using a simplified method developed by us. All the isolated plasma membranes were electron-microscopically pure and enriched in the specific activities of (Na+ + K+)-ATPase, Mg2+-ATPase and 5'-nucleotidase in comparison with those of the corresponding whole cell homogenates. These plasma membranes as well as the original cells were analyzed for phopholipid composition and contents of phospholipid, cholesterol and plasmalogen. There was no difference in the phospholipid composition among the three plasma membranes. However, all the plasma membranes were deficient in sphingomyelin, namely, 1.8% for the normal thymocytes, 2.2% for the RML11 cells and 1.9% for the RML30 cells as percentage of the total phospholipid phosphorus. The contents of phospholipid (mumol per mg protein), cholesterol (mumol per mg protein) and plasmalogen (mol% to phospholipid) of the plasma membranes from both lines of malignant cells were lower than those of the normal thymocyte membranes. The molar ratio of cholesterol to phospholipid of the malignant cell membranes was also lower than that of the normal membranes, because in the former membranes the degree of decrease in the cholesterol content was higher than that in phospholipid content.