Fructose transporter, GLUT

Figure 4.6 Transport of glucose, galactose and fructose in and out of the enterocyte. The transporter GLUT-5 is specific for fructose GLUT-2 is non-specific (see Chapter 6).

Fructose is the fuel provided in the semen. The advantage of this is that it is transported across the plasma membrane of the sperm by the transporter protein that is specific for fructose, the GLUT-5 transporter. Since the cells of the vagina and the uterus do not possess this transporter, fructose uptake and oxidation is restricted to the spermatozoa. 

GLUT-S is the fructose transporter. It was named GLUT-5 before its true function was known. It occurs in the apical membrane of the enterocyte, allowing dietary fructose to be absorbed. GLUT-. also occurs in skeletal muscle, adipocytes. 

For the absorption of carbohydrates, amino acids, and peptides, a variety of transport systems following facilitated diffusion and active mechanisms have been identified on a molecular and functional level. D-Glucose is mainly absorbed via the Na -dependent transporter SGLTl in the brush-border membrane of enterocytes [18-20]. It is transported across the basolateral membrane by facilitated diffusion via the hexose transporter GLUT-2. Besides SGLTl, the Na +-independent transport protein GLUT-5 is localized in the apical enterocyte membrane, recognizing fructose as a substrate [21]. 

Inukai K, Katagiri H, Takata K, Asano T, Anai M, et al. 1995. Characterization of rat GLUTS and functional analysis of chimeric proteins of GLUTl glucose transporter and GLUTS fructose transporter. Endocrinology 136 4850-4857. 

GLUTS Small intestine =—- Primarily a fructose transporter... 

GLUTS, present in the small intestine, functions primarily as a fructose transporter. 

GLUTS intestine, testis, kidney, sperm SLC2A5 Primariiy fructose transport... 

GLUTS Intestinal epithelium Spermatozoa This is actually a fructose transporter. 

Fructose both enters and leaves absorptive epithelial cells by facilitated diffusion, apparently via transport proteins that are part of the GLUT family. The transporter on the luminal side has been identified as GLUT 5. Although this transporter can transport glucose, it has a much higher activity with fructose (see Fig. 27.12). Other fructose transport proteins also may be present. For reasons as yet unknown, fructose is absorbed at a much more rapid rate when it is ingested as sucrose than when it is ingested as a monosaccharide. 

GLUT-2 has high activity in liver to maintain transport close to equilibrium for both uptake and release of glucose (see below). It can also transport galactose, mannose and fructose - all of which can be converted to glucose in the liver. 

Fructose is not co-transported with sodium it enters the cell on a GLUTS transporter. Glucose, galactose and fructose are transported out of the enterocyte through the GLUT2 transporter in the basolateral membrane. 

GLUTS (small intestine) Small intestine, kidney, skeletal muscle, brain, and adipose tissue Transports fructose (not glucose)... 

GLUT 5 A facilitative glucose transporter isoform present in the small intestine and other tissues that will transport fructose (and glucose to a lesser extent) across the plasma membrane. 

B. Because the patient s liver enzymes are normal and her symptoms seem to correlate with her intake of fruit juices, most likely her problem stems from an inability to absorb fructose. Since removal of cow s milk from her diet did not eliminate the problem, a lactase deficiency can be ruled out. GLUT 5 is the primary transporter of fructose in the intestine and a deficiency in this transporter would lead to an inability to absorb fructose in the gut, making it a substrate for bacterial metabolism that produces various gases, including hydrogen, as well as organic acids. 

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