Kinetics of Na+‐dependent D‐glucose transport

Abstract

The kinetic parameters of the Na⁺-dependent glucose transport system have been determined in isolated membrane vesicles for D-glucose, Na⁺, and phlorhizin. The D-glucose flux measurements were carried out by the equilibrium exchange procedure at constant external and internal Na⁺ concentrations and zero potential. Equations were developed to extract information about K_m and V_max from uptake measurements into a vesicle population that is heterogeneous with respect to size (surface to volume ratio). The K_m for D-glucose was 14 mM and independent of the Na⁺-concentration, while the V_max was strongly Na⁺-dependent and increased 15-fold between 1 and 100 mM Na⁺. The K_m of Na⁺ for activation of the V_max was 18 mM. The calculated K_I values for phlorhizin were 2.7 and 1.9 μM when determined under active and equilibrating D-glucose flux conditions, respectively.