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 affinity homologue SGLT1. Using two-microelectrode voltage clamp, we have characterized the presteady-state and steady-state kinetics of SGLT2 expressed in Xenopus oocytes. The kinetic properties of the steady-state sugar evoked currents as a function of external Na+ and alpha-methyl-D-glucopyranoside (alpha MG) concentrations were consistent with an ordered, simultaneous transport model in which Na+ binds first. Na+ binding was voltage-dependent and saturated with hyperpolarizing voltages. Phlorizin was a potent inhibitor of the sugar-evoked currents (K-i(Pz) approximate to 10 mu M) and blocked an inward Na+ current in the absence of sugar. SGLT2 exhibited Na+-dependent presteady-state currents with time constants 3-7 ms. Charge movements were described by Boltzmann relations with apparent valence approximate to 1 and maximal charge transfer approximate to 11 nC, and were reduced by the addition of sugar 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 previously proposed for SGLT1 indicated that many of the kinetic properties observed in SGLT2 are expected by simply reducing the Na+/glucose coupling from 2 to 1.