The sugar specificity of Na+/glucose cotransporter from rat jejunum

A cDNA for a Na+/glucose cotransporter was cloned from rat jejunum cDNA library. This transporter was expressed in Xenopus oocytes by injection of cRNA synthesized from the cDNA, and the transporter ability was electrophysiologically examined. The cotransporter had a very narrow sugar specificity. Only D-glucose, D-galactose, and some of their derivatives elicited significant electrical responses. These results of sugar specificity were compared with those of the H+/hexose cotransporter of Chlorella. Dose-response relationships of several sugars followed a simple Michaelis-Menten type of kinetics. Both Vm and Km were dependent on the sugars. Not only the affinity of sugars to the cotransporter but also the rate of conformational change of the cotransporter loaded with the sugar and Na+, which translocates them from outside to inside, possibly depends on the sugar structure. The rate-limiting step of the transportation may be the conformational change, i.e., isomerization, of the cotransporter that translocates both the sugar and Na+ from outside to inside.