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Abstract:
We investigated the distribution of the endogenous sodium/D-glucose cotransporter (SGLT1) in polarized Caco-2 cells, a model for enterocytes. A cellular organelle fraction was separated by free flow electrophoresis. As detected by an ELISA assay the major amount of SGLT1 resides in early endosomal fractions and only a minor amount in apical plasma membranes. The distribution-ratio between endosomes and apical membrane was approximately 2:1. Further immunochemical investigation of SGLT1 distribution by epifluorescence and confocal microscopy revealed that SGLT1 is located in small vesicles associated with microtubuli.
Furthermore, the half-life of SGLT1 in Caco-2 cells was determined to be 2.5 d by metabolic labeling followed by immunoprecipitation. Since SGLT1 has a relatively long residence time, it is unlikely that intracellular SGLT1 populations are part of the synthesis/degradation pathway. We, therefore, propose that intracellular compartments containing SGLT1 are part of an endo-/exocytosis process which regulates SGLT1 abundance at the apical cell surface in response to altered physiological demands for D-glucose reabsorption.
(This abstract is as it is printed in the journal).
