GLUT Brain Glucose Transporter

GLUT 3 was originally cloned from a human fetal muscle cDNA library (20). It contains 496 amino acid residues (T§b!e.,„3..4) and shows 64% identity with GLUT 1 and

52% identity with GLUT 2. Its amino acid sequence again suggests that its membrane topology is similar to that of GLUT 1 ( Fig 3..3). GLUT 3 mRNA appears to be present in all tissues, but its highest expression is in adult brain, kidney, and placenta. Adult muscle, however, shows only very low levels. In the brain, it is mainly expressed in neurons. A GLUT 3 mRNA is found in fibroblasts and in smooth muscle. Since both of these cell types are found in practically all tissues, the ubiquitous expression of GLUT 3 is understandable. Its affinity for glucose transport is relatively low (K m » 10 mmol/L) but significantly higher than that of GLUT 1 (17 mmol/L). A GLUT 3 is also found in spermatozoa. Such cells undertake glycolysis in the male genital tract, taking up glucose from epididymal fluid.

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