On the in vitro 'active transport' of aldopentoses by the rat jejunum

Na(+)-dependent transport of D-xylose by bovine intestinal brush border membrane vesicles (BBMV) is inhibited by various pentoses and hexoses

To detect whether pentoses and hexoses occurring in rumen bacteria or in hemicellulose ingested with feed and partly released in the small intestine have an affinity for the Na(+)-dependent glucose transporter of the bovine intestinal brush border membrane (BBM), we investigated whether these monosaccharides inhibit Na(+)-dependent transport of 14C-labelled D-xylose across the BBM using brush border membrane vesicles (BBMV) isolated from the mid-jejunum of cows. We used D-xylose as the transport substrate, because it has a low affinity for the Na(+)-dependent glucose transporter and thus its uptake into BBMV is more efficiently competitively inhibited by other sugars than that of D-glucose. D-Ribose, D-mannose and L-rhamnose occurring in rumen bacteria significantly inhibited Na(+)-dependent uptake of D-xylose into BBMV, but their inhibitory effect was less than that of D-glucose, D-xylose and phlorizin. This also applied to L-arabinose (and D-arabinose), which is, like D-xylose and D-galactose, a constituent of hemicellulose, and to 2-deoxy-D-glucose. Of all monosaccharides tested, only D-fructose did not affect Na(+)-dependent D-xylose transport. It is concluded that some pentoses and hexoses occurring in rumen bacteria (D-ribose, D-mannose and L-rhamnose) or hemicellulose (L-arabinose and D-xylose) have a low affinity for the Na(+)-dependent glucose transporter of the bovine BBM and may therefore be absorbed from the jejunum when released in the small intestine.

A comparative study of specificity of the intestinal Na+/sugar cotransport among vertebrates

Kt values for various monosaccharides were determined from sugar-induced increments of the transmural potentials in isolated small intestines of the goldfish, bullfrog, turtle, quail, guinea pig, rat and rabbit, and specificity patterns of the Na+/sugar cotransporters were compared among these animal species. Absolute requirement of the D-pyranose ring structure was seen in all animals. Requirements of C2-OH and C6 were strong, but not absolute, and OH groups on C3, C4, C6 and the O-atom of the pyranose ring were also suggested to participate, in some degree, in the interaction with the carrier. Comparison of the disaccharide-evoked potentials revealed that there were considerable species differences in activities of trehalase, sucrase and lactase among animals examined, but the differences were relatively small for maltase activity.