Fructose transporter

GLUT2 is a glucose/fructose transport facilitator expressed in liver, small intestine, kidney, and pancreatic p-cells. GLUT2 has low-affinity for glucose (Km= 60 mM) and fructose (ivm=65 mM), and is an essential part of the glucose sensor of pancreatic (3-cells which controls insulin secretion and biosynthesis. 

In a previous work, using D-fructose pyran- and furan- forms as inhibitors of D-fructose transport in CHO (Chinese Hamsters Ovary)-GLUT5 cells, Rollin, Holman and co-workers established that both ring forms were tolerated. The approach used was to block each hydroxyl function with allylic ether it was concluded that two sites, 0-2 (pyranose and furanose) and 0-6 (furanose) could be modified and addressed a visualization of vital interactions with the protein. These interactions were considered to occur because the D-fructofuranose form is relatively symmetrical for that reason, the binding site can arise either in anomeric center side or on the other side of the molecule. Hence D-fructopyranose appears to present to GLUT5 transporter by hydroxyl 3, 4, 5 recognition (Fig. 3). 

The in vitro bioassays allowed to determine the inhibition constant of D-fructose transport by the CHO cells. This measure is carried out by competition with radioactive D-fructose. The study put in evidence that pentose-OZT derivatives are not recognized by the protein transporter. Only the ketohexose-OZT derivatives expressed some inhibition of GLUT5. These inhibition constants showed to be much effective with L-Sor derivatives than with D-Fru derivatives and even better than D-fructose itself (Kt = 15.5 mM) (Table 2). 

Table 6.1 Glucose (and the fructose) transporters in the plasma membrane of human cells and tissues...

Burant CF, Takeda J, Brot-Laroche E, Bell GI, Davidson NO (1992), Fructose transporter in human spermatozoa and small intestine is GLUT5, J. Biol. Chem. 267 14523-14526. 

A protein family involved in transporting hexoses into mammalian tissues Facilitative glucose transporter, which is insulin-sensitive A fructose transporter, catalyzes the uptake of fructose Genetically modified... 

GLUT5, present in the small intestine, functions primarily as a fructose transporter. 

K. Sigiist-Nelson and U. Hopfer, A distinct D-fructose transport system in isolated brush border membrane, Biochim. Biophys. Acta, 367 247-2M (1974). H. Sezakt and T. Kimura, Carrier-mediated transport in drug absorption. Topics in Pharmaceutical Sciences (D. D. Breimer and P Speiser, eds.), Elsevier, Amsterdam, 133-142, 1983. 

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. 

Shu, R., David, E., and Ferraris, R. P (1997). Dietary fructose enhances intestinal fructose transport and CLUT5 expression in weaning rats. Am. Physiol. 272, G446-C4.53. 

Salivary a-amylase is a protein that contributes to the enamel pellicle (Sect. 12.1.3). More importantly, it attaches bacteria, especially streptococci, to teeth surfaces. Thus, following a meal rich in carbohydrates, amylopectin, amylase, and glycogen are digested to maltose at the surface of many oral bacteria. The maltose is taken into the cytosol by a phosphoenolpyruvate transporter homologous to the fructose transporter of S. mutans. Within these bacteria, the maltose is digested to two molecules of glucose 6-phosphate and metabolized to lactic acid. Thus, twice as much acid is produced per mole maltose than per mole sucrose and it contributes to tooth demineralization even if less sucrose is consumed. 

Fructose transport is distinct from glucose-galactose transport and requires a specific saturable membrane carrier (facilitated diffusion). 

Corpe CP, Bovelander FJ, Munoz CM, Hoekstra JH, Simpson lA, et al. 2002. Cloning and functional characterization of the mouse fructose transporter, GLUT5. Biochim Biophys Acta 1576 191-197. 

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... 

Brot-Laroche E. Differential regulation of the fructose transporters GLUT2 and GLUT5 in the intestinal cell line Caco-2. Proc Nutr Soc 1996 Mar 55(lB) 201-208. 

Existence of an active-transport system for D-glucose in mammalian intestine has been recognized for some time, but the mechanism of D-fructose transport is still controversial. Part of the controversy can be attributed to species differences in transport systems. Although the rate of uptake of D-fructose is lower than that observed for actively transported sugars, such as D-glucose and D-galactose (see Table I),... 

The D-fructose transport-system seems to be highly sugar-specific, although a decrease in uptake occurs in the presence of D-sorbose, D-glucose, D-galactose, 3-O-methyl-D-glucose, and sucrose. Little effect on uptake is shown by the presence of D-arabinose, D-fucose, D-man-nose, L-sorbose, D-tagatose, and D-xylose.17... 

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