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Sodium glucose cotransporters (SGLTs) mediate the translocation of carbohydrates across the brush border membrane of different organs such as intestine kidney and brain. The human SGLT5 (hSGLT5) in particular is localized in the kidney were it is responsible for mannose and fructose reabsorption from the glomerular filtrate as confirmed by more recent studies on hSGLT5 knockout mice. Here we characterize the functional properties of hSGLT5 expressed in a stable T Rex HEK 293 cell line using biochemical and electrophysiological assays. We confirmed that hSGLT5 is a sodium/mannose transporter that is blocked by phlorizin. Li(+) and H(+) ions were also able to drive mannose transport and transport was electrogenic. Our results moreover indicate that substrates require a pyranose ring with an axial hydroxyl group ( OH) on carbon 2 (C 2). Compared with Na(+)/glucose cotransport the level of function of Na(+)/mannose cotransport in rat kidney slices was low.

Glucose is reabsorbed from the glomerular filtrate in the proximal segment of the renal tubule in two stages. The first stage is uphill transport across the brush border membrane by Na(+)-glucose cotransport and the second stage is downhill transport across the basolateral membrane by facilitated diffusion. Genes for both a renal Na(+)-glucose cotransporter (SGLT1) and a renal facilitated glucose transporter (GLUT2) have been cloned and sequenced. To examine whether SGLT1 and GLUT2 colocalize to the same tubular epithelial cells in rat kidney, double-immunoperoxidase studies with dual chromogens and paraformaldehyde perfusion-fixed frozen sections of rat kidney were performed. Antipeptide antisera were prepared against rat GLUT2 (amino acids 510-522) and rabbit SGLT1 (amino acids 402-420). Proximal tubules were identified immunocytochemically with an antiserum raised against a synthetic peptide corresponding to the 21 amino acids at the COOH-terminal of the heavy chain of rat gamma-glutamyl transpeptidase, which is a proximal tubule-specific enzyme. The anti-GLUT2 antiserum strongly stained the basolateral membrane of 46% of cortical tubules, whereas the SGLT1 antiserum stained the brush border of 56% of the cortical tubules. The gamma-glutamyl transpeptidase antiserum also stained the brush border of 51% of the cortical tubules. GLUT2 and SGLT1 colocalized to 40% of cortical epithelium, but 16% of cortical epithelial cells were immunopositive for brush border SGLT1 and immunonegative for basolateral GLUT2. These gamma-glutamyl transpeptidase staining results suggest that at least 50% of the tubules in the cortex are proximal tubules and that SGLT1 and GLUT2 colocalize to most proximal tubules. The fact that SGLT1 antiserum immunoreacted with tubules unreactive to the GLUT2 antiserum suggests that either the SGLT1 epitope is conserved on a related brush border protein or that there is another GLUT transporter responsible for the exit of sugar from these proximal tubule cells.