Characterization of the Blood‐Brain Barrier: Protein Composition of the Capillary Endothelial Cell Membrane

Abstract: Microvessels were isolated from canine cerebral cortex, and the composition of the endothelial cell membrane was investigated. Endothelial. cell msmtbranes were separated from the surrounding basement mem brane, solubilized, and subjected to sodium dodecyl sul fate‐polyacrylamide gel electrophoresis in 12% gels Staining with Coomassie Blue revealed a characteristic banding pattern of at least 12 major proteins with ap parent molecular weights between 14,000 and 250,000 When proteins from red blood cell ghosts were run si multaneously, no similarities were observed, except foi proteins at apparent molecular weights of 43,000 (band 3) and 35,000 (band 4). These two proteins migrated exactly to the positions of the erythrocyte proteins actin and gly ceraldehyde 3‐phosphate dehydrogenase, respectively Membrane glycoproteins in gels were also examined by the use of fluorescent lectins. Of the fluorescein isothiocyanate‐conjugated (FITC) lectins tested, only FITC concanavalin A had an affinity for any membrane components. Diazotized [¹²⁵I]iodosulfanilic acid, a membrane‐impermeable reagent, was used to label the internal (lumen) cell surface and the external (antilumen) cell surface. Autoradiography and determination of radioactivity levels in gel slices showed that several proteins were specifically labeled, and that major differences in radioactivity of proteins existed in internal and external labeling experiments. It is concluded that the protein composition of the luminal membrane is different from that of the an‐tiluminal membrane. In addition to the results obtained, the above procedures provide a model system for the further investigation of endothelial cell membrane proteins and the blood‐brain barrier.