Phlorizin inhibits hexose transport across the plasmalemma of Riccia fluitans

The effect of phlorizin has been tested on hexose transport and hexose-induced changes of electrical potential (ψm) and conductance (g m) across the plasmalemma of rhizoid and thallus cells of the aquatic liverwort Riccia fluitans. The decrease of ψm (depolarization) and g m induced by 1 mM 3-oxymethyl-D-glucose (3-OMG) is substantially inhibited by simultaneous addition of 2 mM phlorizin, whereas no significant response was observed when phlorizin was added alone or several minutes after the sugar. Current-voltage data show that the 3-OMG-generated electrical inward current of 0.016 A m(-2) drops to 0.010 A m(-2) when phlorizin is present. Uptake as well as efflux of [(14)C]-3-OMG is strongly and reversibly inhibited by phlorizin between 0.2 and 5 mM. The results are consistent with our hypothesis that the hexose carrier has one binding site with competitive inhibition of glucose uptake by phlorizin (k i=0.08 mM). The electrical data indicate that phlorizin affects an ψm step of the carrier transport cycle.