Substrate specifity of the hexose carrier in the plasmalemma of Chenopodium suspension cells probed by transmembrane exchange diffusion.

Substrate specifity of the proton-driven hexose cotransport carrier in the plasmalemma of photoautotrophic suspension cells of Chenopodium rubrum L. has been studies through the short-term perturbation of (14)C-labelled efflux of 3-O-methyl-D-glucose. Efflux, occurring exclusively via carrier-mediated exchange diffusion, is trans-stimulated by the substrate and trans-inhibited by the glucose-transport inhibitors phlorizin (K 1/2=7.9 mM) and its aglucon phloretin (K 1/2=84 μM); with both inhibitors, 3-O-methyl-D-glucose efflux may be blocked completely. Trans-stimulation of efflux (up to fourfold) by a variety of the D-enantiomers of neutral hexoses, including glucose (K 1/2=48 μM), 3-O-methyl-D-glucose (K 1/2=139 μM), and fructose (K 1/2=730 μM), but not by, for instance, D-allose, and L-sorbose, shows that carrier-substrate interaction critically involves the axial position at C-1 and C-3, respectively. We suggest that substrate binding by the Chenopodium hexose carrier involves both hydrophobic interaction with the pyran-ring and hydrogen-ion bonding at C-1 and C-3 of the D-glucose conformation.