Molecular Composition of the Submicrosomal Membrane Lipid of Rat Brain

Rough-surfaced and light and heavy smooth-surfaced microsomes were isolated from rat brain by means of discontinuous sucrose gradient centrifugation. Electron microscopically, the rough-surfaced microsomes were characterized by vesicles with ribosomes and the light and heavy smooth-surfaced microsomes by fairly homogeneous membrane features without ribosomes. The rough-surfaced microsomal membranes were distinguished by the absence of glycolipids, such as ganglioside, cerebroside, and sulfatide. Cerebroside was exclusively recovered in the light smooth-surfaced microsomal membranes. Ganglioside and Na,K-ATPase were contained in larger amounts in the heavy smooth-surfaced microsomal membranes than in the light smooth-surfaced microsomal membranes in terms of protein content. Among the three submicrosomal membranes, cholesterol and phospholipid were found in the largest amounts in the light smooth-surfaced microsomal membranes, where the molar ratio of cerebroside-cholesterol-phospholipid was about 1:10:10. The membranes of rough- and smooth-surfaced microsomes were very similar in regards to the composition of phospholipid classes, although the fatty acid composition of the former contained a greater proportion of unsaturated fatty acids than that of the latter. When the membrane proteins were analyzed by sodium dodecyl sulfate gel electrophoresis, some differences were observed between the light and heavy smooth-surfaced microsomal membranes.