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GENERAL CONSIDERATIONS
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D-Glucose and other monosaccharides are hydrophilic substances that cannot easily cross the lipophilic bilayer of the cell membrane. Since carbohydrates are important for supplying energy to essentially all cell types, specific transport mechanisms have evolved. While vesicle-associated glucose transport has been described fairly recently, transporter proteins have been known for years. Such proteins are embedded into the cell membrane and function as hydrophilic pores that allow cellular uptake and release and transcellular transport of monosaccharides.
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Glucose transporter proteins can be divided into 2 groups: (1) sodium-dependent glucose transporters (SGLTs; symporter systems, secondary “active” transporters), which are members of the solute carrier family 5 (SLC5) and couple sugar transport to the electrochemical gradient of sodium, and hence can transport glucose against its own concentration gradient (Fig. 152-1); and (2) facilitative glucose transporters (GLUTs, uniporter systems, “passive” transporters), which are members of the SLC2 family that can transport monosaccharides only along an existing gradient (Fig. 152-2). To date, 5 congenital defects of monosaccharide transport are known (Fig. 152-3). Their clinical picture is the consequence of tissue-specific expression and substrate specificity of the affected transporter.
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