THE GLUT5 HEXOSE TRANSPORTER IS ALSO LOCALIZED TO THE BASOLATERAL MEMBRANE OF THE HUMAN JEJUNUM

The intestine is a major site of expression of the human GLUT5 hexose transporter, which is thought to be localized exclusively to the brush border membrane (BBM) where its major role is likely to be in the absorption of fructose. In this study we present novel biochemical and morphological evidence showing that the GLUTS transporter is also expressed in the basolateral membrane (BLM) of the human intestine. BBM and BLM were isolated by fractionation of human jejunum. BBM were enriched with alkaline phosphatase activity by over 9-fold relative to a crude jejunal homogenate and contained immunoreactive sucrase isomaltase and GLUTS proteins. By contrast the BBM fraction was substantially depleted of immunoreactive al subunits of the Na,K-ATPase and GLUT2 glucose transporters which were abundantly present in the BLM fraction. This BLM fraction was enriched by over 11-fold in potassium-stimulated phosphatase activity relative to the crude homogenate; BLM also reacted to immunological probes for GLUTS but showed no observable reactivity with antibodies directed against sucrase-isomaltase. Quantitative immunoblotting revealed that the BBM and BLM contained near equal amounts of GLUTS per mg of membrane protein. Immunogold localization of GLUTS on ultrathin sections of human jejunum showed that GLUTS was present in both apical BBM and BLM. This gold labelling was absent when antiserum was pre-incubated with the antigenic peptide corresponding to a specific C-terminal sequence of human GLUTS. Quantitative analyses of the number of gold particles per unit length of BBM and BLM indicated that the mean density of gold labelling was marginally greater in the BLM (0.399 gold particles/mu m) than in the BLM (0.293 gold particles/mu m). The localization of GLUTS in the BLM of the human jejunum may suggest that it specifically participates in the transfer of fructose across the basal membrane of the enteroctye.