AGI May 39/5

Nagata, Katsumi, Naohiro Hori, Kenzo Sato, Kunimasa Ohta, Hideaki Tanaka, and Yasutake Hiji. Cloning and functional expression of an SGLT-1-like protein from the Xenopus laevis intestine. Am. J. Physiol. 276 (Gastrointest. Liver Physiol. 39): G1251–G1259, 1999.—A cDNA encoding an Na1-glucose cotransporter type 1 (SGLT-1)-like protein was cloned from the Xenopus laevis intestine by the 58and 38-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 SGLT1s. 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 myoinositol . D-glucose . D-galactose $ a-methyl-D-glucoside. The current induced by myo-inositol increased with membrane hyperpolarization and depended on external myoinositol and Na1: the apparent Michaelis-Menten constant was 0.25 6 0.07 (SD) mM with myo-inositol, whereas the apparent concentration for half-maximal activation was 12.5 6 1.0 mM and the Hill coefficient was 1.6 6 0.1 with Na1. In conclusion, the cloned protein shares features with both SGLT-1 and the Na1-myo-inositol cotransporter.