INVITRO ANALYSIS OF THE H+-HEXOSE SYMPORTER ON THE PLASMA-MEMBRANE OF SUGAR-BEETS (BETA-VULGARIS L)

The mechanism of hexose transport into plasma membrane vesicles isolated from mature sugarbeet leaves (Beta vulgaris L.) was investigated. The initial rate of glucose uptake into the vesicles was stimulated approximately fivefold by imposing a transmembrane pH gradient (DELTA-pH), alkaline inside, and approximately four-fold by a negative membrane potential (DELTA-psi), generated as a K+-diffusion potential, negative inside. The -fold stimulation was directly related to the relative DELTA-pH or DELTA-psi gradient imposed, which were determined by the uptake of acetate or tetraphenylphosphonium, respectively. DELTA-psi- and DELTA-pH-dependent glucose uptake showed saturation kinetics with a K(m) of 286 micromolar for glucose. Other hexose molecules (e.g. 2-deoxy-D-glucose, 3-O-methyl-D-glucose, and D-mannose) were also accumulated into plasma membrane vesicles in a DELTA-pH-dependent manner. Inhibition constants of a number of compounds for glucose uptake were determined. Effective inhibitors of glucose uptake included: 3-O-methyl-D-glucose, 5-thio-D-glucose, D-fructose, D-galactose, and D-mannose, but not 1-O-methyl-D-glucose, D- and L-xylose, L-glucose, D-ribose, and L-sorbose. Under all conditions of proton motive force magnitude and glucose and sucrose concentration tested, there was no effect of sucrose on glucose uptake. Thus, hexose transport on the sugar-beet leaf plasma membrane was by a H+-hexose symporter, and the carrier and possibly the energy source were not shared by the plasma membrane H+-sucrose symporter.