INtestinal sugar transport: studies with isolated plasma membranes.

The luminal and the contraluminal plasma membranes of small intestinal epithelial cells were isolated separately and in the form of membrane vesciles. In the luminal membrane distinct transport systems exist for D-fructose and for D-glucose and glucalogs as evidenced by stereospecificity for the substrate, counter- and cotransport phenomena, and inhibitor specificity. D-Glucose is taken up against a concentration gradient when energy is supplied in the form of an electrochemical Na+ gradient between medium and intravesicular membrane space. The contraluminal membrane also possesses a glucose transport system. However, it is sensitive to different inhibitors than the glucose transport in the luminal membrane. D-Glucose uptake by the contraluminal membrane is not enhanced by Na+ gradients and can be characterized as "facilitated diffusion." The studies with the isolated plasma membranes allow a description of transcellular D-glucose transport in terms of the translocation events across single membranes.