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Abstract:
Free-flow electrophoresis was used to separate microvilli from the lateral basal plasma membrane of the epithelial cells from rat small intestine. The activities of the marker enzyme for the microvillus membrane, i.e. alkaline phosphatase (EC 3.1.31), was clearly separated from the marker for the lateral-basal plasma membrane, i.e. the (Na+, K+)-ATPase (EC 3.6.1.3). A microvillus membrane fraction was obtained with a high specific activity of alkaline phosphatase (an 8-fold enrichement over the starting homogenate). The lateral-basal plasma membrane fraction contained (Na+, K+)-ATPase (5-fold over homogenate) with some alkaline phosphatase (2-fold over homogenate).
Glucose transport was studied in both membrane fractions. The uptake of D-glucose was much faster than that of L-glucose in either plasma membrane, D-Glucose uptake could be accounted for completely by its transport into an osmotically active space. Interestingly, the characteristics of the glucose transport of the microvillus membrane were different from those of the lateral-basal plasma membrane. In particular: Na+ stimulated the D-glucose transport by the microvillus membrane, but not by the lateral-basal plasma membrane. In addition, the glucose transport of the microvillus membrane was much more sensitive to phlorizin inhibition than that of the lateral-basal plasma membrane.
These experiments thus provide evidence not only for an asymmetrical distribution of the enzymes, but also for differences in the transport properties with respect to glucose between the two types of plasma membrane of the intestinal epithelial cell.
