Kinetics of Na+-dependent D-glucose transport.

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 Km and Vmax from uptake measurements into a vesicle population that is heterogeneous with respect to size (surface to volume ratio). The Km for D-glucose was 14 mM and independent of the Na+-concentration, while the Vmax was strongly Na+-dependent and increased 15-fold between 1 and 100 mM Na+. The Km of Na+ for activation of the Vmax was 18 mM. The calculated KI values for phlorhizin were 2.7 and 1.9 micrometer when determined under active and equilibrating D-glucose flux conditions, respectively.