BIOPHYSICAL CHARACTERISTICS OF THE PIG-KIDNEY NA+ GLUCOSE COTRANSPORTER SGLT2 REVEAL A COMMON MECHANISM FOR SGLT1 AND SGLT2/

The Na+-dependent, low affinity glucose transporter SGLT2 cloned from pig kidney is 76% identical (at the amino acid level) to its high affi nity homologue SGLT1. Using two-microelectrode voltage clamp, we have characterized the presteady-state and steady-state kinetics of SGLT2 e xpressed in Xenopus oocytes. The kinetic properties of the steady-stat e sugar evoked currents as a function of external Na+ and alpha-methyl -D-glucopyranoside (alpha MG) concentrations were consistent with an o rdered, simultaneous transport model in which Na+ binds first. Na+ bin ding was voltage-dependent and saturated with hyperpolarizing voltages . Phlorizin was a potent inhibitor of the sugar-evoked currents (K-i(P z) approximate to 10 mu M) and blocked an inward Na+ current in the ab sence of sugar. SGLT2 exhibited Na+-dependent presteady-state currents with time constants 3-7 ms. Charge movements were described by Boltzm ann relations with apparent valence approximate to 1 and maximal charg e transfer approximate to 11 nC, and were reduced by the addition of s ugar or phlorizin. The differences between SGLT1 and SGLT2 were that ( i) the apparent affinity constant (K-0.5) for alpha MG (approximate to 3 mM) was an order of magnitude higher for SGLT2; (ii) SGLT2 excluded galactose, suggesting discrete sugar binding; (iii) K-0.5 for Na+ was lower in SGLT2; and (iv) the Hill coefficient for Nat was 1 for SGLT2 but 2 for SGLT1. Simulations of the six-state kinetic model previousl y proposed for SGLT1 indicated that many of the kinetic properties obs erved in SGLT2 are expected by simply reducing the Na+/glucose couplin g from 2 to 1.