GLUT Liver Glucose Transporter

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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