A cDNA encoding an Na⁺-glucose cotransporter type 1 (SGLT-1)-like protein was cloned from the Xenopus laevis intestine by the 5'- and 3'-rapid amplification of cDNA ends method. The deduced amino acid sequence was 673 residues long, with a predicted mass of 74.1 kDa and 52-53% identity to mammalian SGLT-1s. This gene was expressed in the small intestine and kidney, reflecting a tissue distribution similar to that of SGLT-1. The function of the protein was studied using the two-microelectrode voltage-clamp technique after injection of cRNA into Xenopus laevis oocytes. Perfusion with myo-inositol elicited about twofold larger inward currents than perfusion with D-glucose. The order of the substrate specificity was myo-inositol > D-glucose > D-galactose  -methyl-D-glucoside. The current induced by myo-inositol increased with membrane hyperpolarization and depended on external myo-inositol and Na⁺: the apparent Michaelis-Menten constant was 0.25 ± 0.07 (SD) mM with myo-inositol, whereas the apparent concentration for half-maximal activation was 12.5 ± 1.0 mM and the Hill coefficient was 1.6 ± 0.1 with Na⁺. In conclusion, the cloned protein shares features with both SGLT-1 and the Na⁺-myo-inositol cotransporter.

sodium-glucose cotransporter; sodium-myo-inositol cotransporter; Xenopus laevis oocytes; voltage clamp

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