Biochemical, immunological and ultrastructural studies on brush-border membranes of human kidney

Brush border (BB) membranes, isolated from human kidney cortex by density gradient centrifugation, revealed a distinct pattern of structural proteins as could be shown by bio- and immunochemical studies. Marker enzymes such as gamma-glutamyltranspeptidase (GGTP) and alanine-aminopeptidase (AAP) were characterized as extrinsic; alkaline phosphatase (AP) was characterized as an integral constituent of the BB membrane. The surface of the BB membranes exhibited numerous 5 nm particles bound through a linear component to the peripheral BB matrix (negative staining). Increase of AAP and GGTP (30%) activity in the supernatant after proteolytic treatment of BB fragments paralleled selective release of these constituents from the membranes. The surface components were found to be part of BB concanavalin A and wheat germ agglutinin receptor sites. Labelled antisera directed against surface glycoprotein fractions gave a specific immuno fluorescence staining of only the luminal plasma-membrane from the proximal tubule epithelia.