Interaction of phloretin with the anion transport protein of the red blood cell membrane.

Phloretin is an inhibitor of anion exchange and glucose and urea transport in human red cells. Equilibrium binding the kinetic studies indicate that phloretin binds to band 3, a major integral protein of the red cell membrane. Equilibrium phloretin binding has been found to be competitive with the binding of the anion transport inhibitor, 4,4'-dibenzamido-2,2'-disulfonic stilbene (DBDS), which binds specifically to band 3. The apparent binding (dissociation) constant of phloretin to red cell ghost band 3 in 28.5 mM citrate buffer, pH 7.4, 25 degree C, determined from equilibrium binding competition, is 1.8 +/- 0.1 microM. Stopped-flow kinetic studies show that phloretin decreases the rate of DBDS binding to band 3 in a purely competitive manner, with an apparent phloretin constant of 1.6 +/- 0.4 microM. The pH dependence of equilibrium binding studies show that it is the charged, anionic form of phloretin that competes with DBDS binding, with an apparent phloretin inhibition constant of 1.4 microM. The phloretin binding and inhibition constants determined by equilibrium binding, kinetic and pH studies are all similar to the inhibition constant of phloretin for anion exchange. These studies suggest that phloretin inhibits anion exchange in red cells by a specific interaction between phloretin and band 3.