Utilization and transport of glucose in Olea europaea cell suspensions

Cell suspensions of Olea europaea var. Galega Vulgar grown in batch culture with 0.5% (w/v) glucose were able to transport D-[C-14] glucose according to Michaelis-Menten kinetics associated with a first-order kinetics. The monosaccharide carrier exhibited high affinity (K(m)approximate to50muM) and was able to transport D-glucose, D-fructose, D-galactose, D-xylose, 2-deoxy-D-glucose and 3-O-methyl-D-glucose, but not D-arabinose, D-mannitol or L-glucose. D- [C-14] glucose uptake was associated with proton uptake, which also followed Michaelis-Menten kinetics. The transport of 3-O-methyl-D-glucose was accumulative (40-fold, at pH 5.0) and the protonophore carbonyl cyanide m-chlorophenylhydrazone strongly inhibited sugar accumulation. The results were consistent with the involvement of a monosaccharide. proton symporter with a stoichiometry of 1:1. When cells were grown with 3% (w/v) glucose, the uptake of D- [C-14] glucose followed first-order kinetics and monosaccharide:proton symporter activity was not detected. The value obtained for the permeability coefficient of hexoses in O. europaea cells supported the hypothesis that the first-order kinetics observed in 0.5% and 3% sugar-grown cells was produced exclusively by passive diffusion of the sugar. The results indicate that in O. europaea cells sugar levels have a regulatory effect on sugar transport, because the activity for monosaccharide transport was repressed by high sugar concentrations.