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Sodium-dependent glucose cotransporters are a family of glucose transporter found in the intestinal mucosa of the small intestine (SGLT1) and the proximal tubule of the nephron (SGLT2 and SGLT1). They contribute to renal glucose reabsorption.
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SGLT1 and SGLT2 are members of the SLC5A gene family.
| Gene | Protein | Acronym | Tissue distribution in proximal tubule[1] |
Na+:Glucose Co-transport ratio |
Contribution to glucose reabsorption (%)[2] |
|---|---|---|---|---|---|
| SLC5A1 | Sodium/GLucose coTransporter 1 |
SGLT1 | S3 segment | 2:1 | 98 |
| SLC5A2 | Sodium/GLucose coTransporter 2 |
SGLT2 | predominately in the S1 and S2 segments |
1:1 | 2 |
Including SGLT1 and SGLT2, there are total seven members in the human protein family SLC5A, several of which may also be sodium-glucose transporters.[3]
These proteins use the energy from a downhill sodium gradient to transport glucose across the apical membrane against an uphill glucose gradient. Therefore, these co-transporters are an example of secondary active transport. (The GLUT uniporters then transport the glucose across the basolateral membrane, into the peritubular capillaries.) Both SGLT1 and SGLT2 are known as symporters since both sodium and glucose are transported in the same direction across the membrane.
In August 1960, in Prague, Robert K. Crane presented for the first time his discovery of the sodium-glucose cotransport as the mechanism for intestinal glucose absorption.[4]
Crane's discovery of cotransport was the first ever proposal of flux coupling in biology.[5][6]
Co-transport proteins of mammalian cell membranes had eluded efforts of purification with classical biochemical methods until the late 1980's. These proteins had proven difficult to isolate since they contain hydrophilic and hydrophobic sequences and exist in membranes only in very low abundance (<0.2% of membrane proteins). The rabbit form of SGLT1 was the first mammalian co-transport protein ever to be cloned and sequenced and this scientific break-through was reported in 1987. To circumvent the difficulties with traditional isolation methods, Swiss-born biochemist Matthias Hediger and his collaborators at UCLA used a novel technique of expression cloning. They size-fractionated large amounts of rabbit intestinal mRNA with a preparative gel electrophoresis device developed by Hediger. These size fractions were then sequentially injected into Xenopus oocytes to ultimately find the RNA species that induced the expression of sodium-glucose cotransport.[7]
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