Isolation from bovine brain of a fraction containing capillaries and a fraction containing membrane fragments of the choroid plexus.

Combined differential and density gradient centrifugation was used for the isolation of a capillary-rich fraction from the cerebral cortex and a brush border containing fraction from the bovine choroid plexus. The activities of gamma-glutamyl transpeptidase and several other marker enzymes were monitored during the fractionation procedure. Electron microscopic examination showed a membrane-rich fraction in the choroid plexus high in the gamma-glutamyl transpeptidase and 5'-nucleotidase activities. From the brain cortex, a capillary-rich fraction was obtained which was high in gamma-glutamyl transpeptidase and alkaline phosphatase activities. A histochemical examination showed gamma-glutamyl transpeptidase activity localized in the capillary walls.