Uptake and phosphorylation of glucose and fructose in Daucus carota cell suspensions are differently regulated

Cell suspensions of Daucus carota L, were grown in batch culture on 50 mM s ucrose, 100 mM glucose or 100 mM fructose. Sucrose was rapidly converted ex tra-cellularly into equimolar amounts of glucose and fructose, and glucose was then taken up preferentially. This impaired uptake of fructose could pa rtially be explained by the eight-fold lower affinity of the hexose carrier in the plasmamembrane for fructose compared to glucose. However, cells gro wn on fructose as the sole carbon source showed a shorter lag phase and sho wed more biomass production compared to glucose-grown cells, indicating tha t conversion of glucose and fructose were also differently regulated. Ninet y-five % of the glucose phosphorylating activity was membrane-associated an d most probably confined to mitochondria; therefore, it might be present in a respiratory 'compartment' making glucose a better substrate for respirat ion than fructose. The soluble fraction contained the majority of the fruct okinase activity. This activity was hypothesized to be more or less randoml y distributed through the cytosol; in this soluble 'compartment' a pool of fructose-6-phosphate is formed. Concomitantly, via glucose-6-phosphate (G-6 -P) and glucose-phosphate (G-1-P), it is converted into UDPG-glucose, resul ting in structural cell components. The observed transient obstruction of t he conversion of G-1-P into UDP-glucose in fructose-grown cells, leading to G-1-P accumulation, might be a result of both an altered equilibrium maint ained by phosphoglucomutase, interconverting G-6-P and G-1-P and low levels of nucleotide triphosphates. Low nucleotide triphosphate production, conne cted with a low initial respiration rate, might be caused by the ten-fold l ower affinity of the membrane-associated phosphorylating enzymes for fructo se compared to glucose. Our results were taken to indicate that two separat e pools of glycolytic intermediates exist in D. carota cells: one distribut ed throughout the cytosol and one surrounding the mitochondria. (C) 2000 Ed itions scientifiques et medicales Elsevier SAS.