SAAT1 IS A LOW-AFFINITY NA+/GLUCOSE COTRANSPORTER AND NOT AN AMINO-ACID TRANSPORTER - A REINTERPRETATION

Recently a member of the Na+/glucose (SGLT1) gene family of cotransporters was isolated from a pig renal cell line and was thought to be the neutral amino acid transporter System A. This cDNA (Kong, C. T., Yet, S. F., and Lever, J. E. (1993) J. Biol. Chem. 268, 1509-1512) encodes a 660-amino acid protein with 76% identity to SGLT1. To confirm and extend the kinetic characterization of SAAT1, we have expressed this clone in Xenopus oocytes and measured transport using both radiotracer and electrophysiological techniques. SAAT1 did not stimulate either 50 mu m 2-(methylamino)isobutyrate uptake or 2-(methylamino)isobutyrate-evoked inward Na+ currents, but instead stimulated 50 mu m alpha MG (alpha-methyl-D-glucopyranoside) uptake 27-fold from 2 +/- 1 pmol.h(-1)/oocyte (n = 9) to 55 +/- 6 pmol.h-1/oocyte (n = 9) and alpha MG-evoked inward Na+ currents (I) by up to 1000 nA/oocyte. The apparent affinity constant for alpha MG (K-0.5(alpha MG)) was approximate to 2 mM and was independent of membrane potential from -30 to -150 mV but was voltage-sensitive between -30 and +30 mV. The relative sugar specificity for the transporter was alpha MG greater than or equal to D-glucose >> D-galactose >>> 3-O-methyl-D-glucopyranose, L-glucose. The sugar-evoked currents were Na+-dependent (K-0.5(Na) approximate to 10 mM at -50 mV) and the Hill coefficient was 1. K-0.5(Na) decreased with hyperpolarization of the membrane from -50 to -150 mV. Phlorizin inhibited the alpha MG-evoked current with apparent K-i of 18 mu M at -50 mV. We conclude that the SAAT1 cDNA encodes a renal low affinity Na+(1)/glucose(1) cotransporter and propose that pig SAAT1 be renamed pSGLT2.