Glucose is an important source of energy for all living cells. Glucose is transported across the cells by a family of energy-dependent Na+-Dependent Glucose Transporters (SGLTs) and by facilitative glucose transporters known as glucose transporters (Gluts or SLC2A family). Molecular cloning of SGLTs has identified a family of closely related genes that encodes at least 14 proteins (Glut-1 to Glut-14). The kidneys play a major role in the regulation of glucose levels. Kidneys filter approx. 180 g of glucose per day from the blood, and this is mostly reabsorbed back into the blood in the proximal tubules. Typically, glucose is first absorbed within epithelium by a specific transporter protein, Sodium glucose co transporters (SGLT), in the brush-border membrane and then it is transported out of the cell across the basolateral membranes by a facilitated sugar transporter (GLUTs). At least 3 members of SGLTs (SGLT1-3) have been cloned and characterized from various species, and at lease 6 other poorly studied proteins are also expressed in the kidney. Individual member of this family have identical predicted secondary structures with up to 15 transmembrane domains. SGLT1-3 genes code for protein of approx 659-672 residues (calculated size of ~75 kDa). Both N and C-termini are predicted to be extracellualr. There is approx 60-70% homology between SGLT1-3. SGLTs transport a-methyl-D-glucoside (a-MDG), a non-metabolized model substrate, in Na-dependent manner. SGLT1 does not discriminate a-MDG, glucose, and galactose. SGLT2/3 do not transport D-galactose efficiently.

SGLT1/NAGT or SLC5A1/NAGT (rat/mouse 665 aa; human 664 aa, chromosome 22q13.1, ~75 kDa) is a high affinity, Na+-coupled, intestinal responsible for active glucose transport across the brush border membrane. In the kidney, SGLT1 is expressed in proximal tubule Sq1 segments. It is also expressed in the intestine. Defects in SGLT1 gene have been implicated in congenital glucose-galactose

malabsorption syndrome (GGM). SGLT2/SLC5A2 (rat/mouse 670 aa; human 672 aa, chromosome 16p11.2) is the low affinity, high capacity Na+-glucose transporter located in the S1 segments of proximal tubules. It is ~60% identical with SGLT1. SGLT2 mediates saturable Na-dependent and phlorizin-sensitive glucose transport. In contrast with SGLT1, SGLT2 does not transport D-galactose. Defect in SGLT2 may be associated with renal glycosuria. SGLT3/SLC54 (pig 660 aa; mouse 656/660/616 aa; human 659 aa, chromosome 22), originally named SAAT1 or pSGLT2, was initially identified in LLC-Pk1 cell line derived from pig renal epithelium. It is also low affinity Na-glucose transporter. It is expressed in kidney, intestine, liver, skeletal muscle and spleen. Like SGLT2, SGLT3 has a low affinity for sugars, and is highly selective for D-glucose and low affinity for D-galactose.

A human gene, KST1, encoding a new member of SGLT family has been identified. KST1 (rabbit 674 aa, human 675 aa chromosome 16p12-p11), an ortholog of rabbit kidney SGLT or rkST, is expressed in brain, heart, muscle, kidney, liver and placenta. Due to its wide tissue expression and chromosomal location, mutations in KST1 have been implicated in ICCA (infantile convulsion and choreoathetosis) and BFIC (benign familial infantile convulsions).

Recently, a cDNA (pRS1 from pig and hRS1 from human) has been cloned and expressed that encodes a membrane-associated protein that alters sugar transport by SGLT1 and SMIT. RS1 (mouse 582 aa, pig 623 aa, 617 aa, chromosome 1p36.1, ~67 kDa) is also described as regulatory subunit or beta-subunit of SGLT (RSSG) or regulatory solute carrier protein. RS1 is expressed in renal outer cortex, outer medulla, small intestine, liver, and LLCPK1 cells. Coexpression of hRS1 and SGLT1 in oocytes inhibits SGLT1 expression.