Sodium-glucose transport proteins

solute carrier family 5 (sodium/glucose cotransporter), member 1
Identifiers
Symbol	SLC5A1
Alt. symbols	SGLT1
Entrez	6523
HUGO	11036
OMIM	182380
RefSeq	NM_000343
UniProt	P13866
Other data
Locus	Chr. 22 q13.1

solute carrier family 5 (sodium/glucose cotransporter), member 2
Identifiers
Symbol	SLC5A2
Alt. symbols	SGLT2
Entrez	6524
HUGO	11037
OMIM	182381
RefSeq	NM_003041
UniProt	P31639
Other data
Locus	Chr. 16 p11.2

solute carrier family 5 (low affinity glucose cotransporter), member 4
Identifiers
Symbol	SLC5A4
Alt. symbols	SGLT3, SAAT1, DJ90G24.4
Entrez	6527
HUGO	11039
RefSeq	NM_014227
UniProt	Q9NY91
Other data
Locus	Chr. 22 q12.1-12.3

Sodium-dependent glucose cotransporters (SGLT) are a family of glucose transporter found in the intestinal mucosa (enterocytes) of the small intestine (SGLT1) and the proximal tubule of the nephron (SGLT2 in PCT and SGLT1 in PST). They contribute to renal glucose reabsorption. In the kidneys, 100% of the filtered glucose in the glomerulus has to be reabsorbed along the nephron (98% in PCT, via SGLT2). In case of too high plasma glucose concentration (hyperglycemia), glucose is excreted in urine (glucosuria); because SGLT are saturated with the filtered monosaccharide. One must know that glucose is never secreted by the nephron.

Types

The two most well known members of SGLT family are SGLT1 and SGLT2, which 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	10
SLC5A2	Sodium/GLucose coTransporter 2	SGLT2	predominately in the S1 and S2 segments	1:1	90

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]

Function

First, the Na+/K+ ATPase pump on the basolateral membrane of the proximal tubule, cell actively (requires ATP) transports sodium from this cell into the peritubular capillary. This creates a downhill sodium gradient inside the proximal tubule cell. The SGLT proteins use the energy from this downhill sodium gradient created by the ATPase pump 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.

Discovery of sodium-glucose cotransport

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]

See also

• Cotransport
• Cotransporter
• Glucose-galactose malabsorption
• Renal sodium reabsorption
• SGLT1
• SGLT2

References

1. ^ Wright EM, Hirayama BA, Loo DF (January 2007). "Active sugar transport in health and disease". J. Intern. Med. 261 (1): 32–43. doi:10.1111/j.1365-2796.2006.01746.x. PMID 17222166. 
2. ^ Wright EM (January 2001). "Renal Na(+)-glucose cotransporters". Am. J. Physiol. Renal Physiol. 280 (1): F10–8. PMID 11133510. 
3. ^ Ensembl release 48: Homo sapiens Ensembl protein family ENSF00000000509
4. ^ Miller D, Bihler I (1961). "The restrictions on possible mechanisms of intestinal transport of sugars". In Kleinzeller A. Kotyk A. Membrane Transport and Metabolism. Proceedings of a Symposium held in Prague, August 22–27, 1960. Czech Academy of Sciences & Academic Press. pp. 439–449. 
5. ^ Wright EM, Turk E (February 2004). "The sodium/glucose cotransport family SLC5". Pflugers Arch. 447 (5): 510–8. doi:10.1007/s00424-003-1063-6. PMID 12748858. "Crane in 1961 was the first to formulate the cotransport concept to explain active transport [7]. Specifically, he proposed that the accumulation of glucose in the intestinal epithelium across the brush border membrane was [is] coupled to downhill Na+ transport cross the brush border. This hypothesis was rapidly tested, refined, and extended [to] encompass the active transport of a diverse range of molecules and ions into virtually every cell type." 
6. ^ Boyd CA (March 2008). "Facts, fantasies and fun in epithelial physiology". Exp. Physiol. 93 (3): 303–14. doi:10.1113/expphysiol.2007.037523. PMID 18192340. "p. 304. “the insight from this time that remains in all current text books is the notion of Robert Crane published originally as an appendix to a symposium paper published in 1960 (Crane et al. 1960). The key point here was 'flux coupling', the cotransport of sodium and glucose in the apical membrane of the small intestinal epithelial cell. Half a century later this idea has turned into one of the most studied of all transporter proteins (SGLT1), the sodium–glucose cotransporter." 

External links

• MeSH Sodium-Glucose+Transport+Proteins