Abstract
We recently reported that the glucose transporter isoform, GLUT5, is expressed on the brush border membrane of human small intestinal enterocytes (Davidson, N. O., Hausman, A. M. L., Ifkovits, C. A., Buse, J. B., Gould, G. W., Burant, C. F., and Bell, G. I. (1992) Am. J. Physiol. 262, C795-C800). To define its role in sugar transport, human GLUT5 was expressed in Xenopus oocytes and its substrate specificity and kinetic properties determined. GLUT5 exhibits selectivity for fructose transport, as determined by inhibition studies, with a Km of 6 mM. In addition, fructose transport by GLUT5 is not inhibited by cytochalasin B, a competitive inhibitor of facilitative glucose transporters. RNA and protein blotting studies showed the presence of high levels of GLUT5 mRNA and protein in human testis and spermatozoa, and immunocytochemical studies localize GLUT5 to the plasma membrane of mature spermatids and spermatozoa. The biochemical properties and tissue distribution of GLUT5 are consistent with a physiological role for this protein as a fructose transporter.
Highlights
GLUT2 is restricted to the basolateral membranes of the absorptive cells of the small intestine (Thorens et al, 1990a) thereby making it unlikely that GLUT2 mediates the
Phyeiol. 262, C795-CSOO).To define its role (Carruthers,1990).In addition we show that GLUT5 is highly in sugar transport, human GLUT5 was expressed in expressed in human spermatozoa, and immunohistochemical Xenopus oocytes and its substrate specificity and ki- studies suggest that GLUT5 is expressed only in the most netic properties determined
GLUT5 Transports Fructose-Xenopus oocytes injected with synthetic human GLUT5 mRNA showed a stimulation of ['4C]fructose uptake over that of water-injected oocytes (Figs. 1and 2 A )
Summary
GLUT2 is restricted to the basolateral membranes of the absorptive cells of the small intestine (Thorens et al, 1990a) thereby making it unlikely that GLUT2 mediates the. We have recently demonstrated thathehuman facilitative glucose transporter isoform, GLUT5, is expressed at high levels on apical membranes of the absorptive epithelialcells of the small intestine (Davidson et al, 1992). We recentlyreportedthat the glucose transporter uptake (Kayano et al, 1990)led us to testwhether this protein isoform,GLUTS, is expressed onthebrushborder could mediate fructose transport.
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