Isolation of Cell Surface Membranes from Cultured C6 Glioblastoma Cells

Plasma membranes were isolated from C6 glioblastoma cells by two methods. In the first method cells were treated with concanavalin A and lysed in hypotonic medium. After partial separation of plasma membranes from other cell material, the lectin was displaced with a‐methyl‐D‐mannoside. In the second method untreated cells or cells iodinated in a lactoperoxidase‐catalyzed reaction were homogenized in isotonic medium. Membrane fractions obtaincd by either homogenization procedure were further purified by rate zonal and equilibrium centrifugations into linear density gradients. Disruption of the glioblastoma cell membrane gives rise to heterogeneous assemblies of mem‐ brane fragments. Two populations of plasma membranes were isolated from untreated and from iodinated cells: a “lighter”)membrane fraction characterized by relatively lower sedimentation velocity and buoyant density, and a “heavier” membrane fraction of relatively faster sedimentation velocity and higher buoyant density. Both fractions showed electrophoretic patterns similar to those of 125 I‐labeled cell surface proteins. Their specific (Na + + K + )‐ATPase activity was seven‐ to eightfold the homogenate activity (recovery, 13.1%). Both fractions were, however, still contaminated by smooth endo‐ plasmic reticulum, as judged from the activity 0: NADPH‐dependent cytochrome c reductase (recovery, 2.4%). It is suggested that plasma membrane fragments present in the two fractions might differ in the organization of their structures, e.g., membrane vesicle intactness and membrane orientation.