The Journal of Membrane Biology

, Volume 114, Issue 2, pp 113–132 | Cite as

Two substrate sites in the renal Na+-d-glucose cotransporter studied by model analysis of phlorizin binding and-d-glucose transport measurements

  • Hermann Koepsell
  • Günter Fritzsch
  • Klaus Korn
  • Andrzej Madrala
Articles

Summary

Time courses of phlorizin binding to the outside of membrane vesicles from porcine renal outer cortex and outer medulla were measured and the obtained families of binding curves were fitted to different binding models. To fit the experimental data a model with two binding sites was required. Optimal fits were obtained if a ratio of low and high affinity phlorizin binding sites of 1:1 was assumed. Na+ increased the affinity of both binding sites. By an inside-negative membrane potential the affinity of the high affinity binding site (measured in the presence of 3 mM Na+) and of the low affinity binding site (measured in the presence of 3 or 90 mM Na+) was increased. Optimal fits were obtained when the rate constants of dissociation were not changed by the membrane potential. In the presence of 90 mM Na+ on both membrane sides and with a clamped membrane potential,KD values of 0.4 and 7.9 μM were calculated for the low and high affinity phlorizin binding sites which were observed in outer cortex and in outer medulla. Apparent low and high affinity transport sites were detected by measuring the substrate dependence ofd-glucose uptake in membrane vesicles from outer cortex and outer medulla which is stimulated by an initial gradient of 90 mM Na+(out>in). Low and high affinity transport could be fitted with identicalKm values in outer cortex and outer medulla. An inside-negative membrane potential decreased the apparentKm ofhigh affinity transport whereas the apparentKm of low affinity transport was not changed. The data show that in outer cortex and outer medulla of pighigh and low affinity Na+-d-glucose cotransporters are present which containlow and high affinity phlorizin binding sites, respectively. It has to be elucidated from future experiments whether equal amounts of low and high affinity transporters are expressed in both kidney regions or whether the low and high affinity transporter are parts of the same glucose transport moleculc.

Key Words

coupled transport glucose transport substrate sites brush-border membrane proximal tubule vesicles phlorizin binding 

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Copyright information

© Springer-Verlag New York Inc 1990

Authors and Affiliations

  • Hermann Koepsell
    • 1
  • Günter Fritzsch
    • 1
  • Klaus Korn
    • 1
  • Andrzej Madrala
    • 1
  1. 1.Max-Planck-Institut für BiophysikFrankfurt (Main) 70Federal Republic of Germany

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