GLUT 5 was isolated from human (21), rat, and rabbit enterocyte cDNA libraries. It consists of 501 amino acid residues (Ta.bie.,,,3,4), and has only 42%, 40%, 39%, and 42% identity with GLUTs 1, 2, 3, and 4, respectively. It is said to be primarily expressed in the jejunum (both in the brush border and basolateral membrane), but its mRNA has been detected, albeit at low levels, in human kidney, skeletal muscle, and adipocytes, microglial cells, and the blood-brain barrier. GLUT 5 appears to transport glucose poorly and is really the transporter for fructose. It is found in high concentrations in mature human spermatozoa ( 22), which are known to use fructose as an energy source (human seminal fluid contains high concentrations of fructose, which is manufactured by the seminal vesicles). In oocytes injected with GLUT 5 mRNA to effect GLUT 5 expression, the Km for fructose uptake was 6 to 11 mmol/L.
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