Many biochemical studies indicated that the glucose transporter in liver cells was distinct from that of the red cells. Moreover, adult liver cells had only very low levels of GLUT 1 mRNA. Cloning of the second glucose carrier, GLUT 2, was accomplished by screening rat and human cDNA libraries with a cDNA probe for GLUT 1. GLUT 2 has 55% identity in amino acid sequence with GLUT 1, and it displays the same topologic organization in the cell membrane as predicted for GLUT 1. Human GLUT 2 contains 524 amino acids (T§ble.„,.3...4), compared with rat GLUT 1 of 522 residues, and they show 82% identity in amino acid sequences, an excellent example of conservation of structure between species. GLUT 2 is preferentially expressed in liver (sinusoidal membranes), kidney (tubule cells), small intestine (enterocytes), and the insulin-secreting b cells of the pancreas.
In the liver cell, GLUT 2 has a low affinity for glucose (K m = 17 mmol/L) and shows symmetric transport, i.e., a similar Km for influx and efflux. This high-capacity, low-affinity transporter is useful for rapid glucose efflux following gluconeogenesis. GLUT 2 can also transport galactose, mannose, and fructose ( 19). The ability to transport fructose is only seen with GLUT 2 and GLUT 5 (see below).
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