Pp. 22-45 (24)
Glucose is obtained directly from the diet, principally following the hydrolysis of ingested disaccharides and polysaccharides, and by the synthesis from other substrates in organs such as the liver. Glucose derived from the diet is transferred from the lumen of the small intestine, and both dietary glucose and glucose synthesized within the body have to be transported from the circulation into target cells. These processes involve the transfer of glucose across plasma membranes. Because for its hydrophilicity, glucose cannot penetrate the lipid bilayer, and specific carrier proteins are required to its diffusion. These transporters comprise two structurally and functionally distinct groups, whose members have been identified over the past two decades, namely: the Na+-dependent glucose co-transporters (SGLT, members of a larger family of Na+-dependent transporters, gene name SLC5A) and the facilitative Na+-independent sugar transporters (GLUT family, gene name SLC2A). The number of known glucose transporters has expanded considerably over the past few years. These various transporters exhibit different substrate specificities, kinetic properties and tissue expression profiles. The number of distinct genes products, together with the presence of several different transporters in certain tissues and cell, indicates that glucose delivery into cells is a process of considerable complexity.