Uptake and phosphorylation of glucose and fructose in Daucus carota cell suspensions are differently regulated |
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| Institution: | 1. Department of Physiology and Neurobiology, Eötvös Loránd University, Budapest, Hungary;2. Department of Plant Anatomy, Eötvös Loránd University, Budapest, Hungary;3. Centre for Synaptic Plasticity, School of Physiology, Pharmacology and Neuroscience, University of Bristol, Biomedical Sciences Building, University Walk, Bristol BS8 1TD, UK;1. Department of Biotechnology, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden;2. Instituto de Investigaciones Fármaco Bioquímicas, Av. Saavedra No. 2224, La Paz, Miraflores, Bolivia;1. Division of Bioscience and Biotechnology, The United Graduate School of Agricultural Sciences, Tottori University, Tottori 680-8553, Japan;2. Department of Life Science and Biotechnology, Faculty of Life and Environmental Science, Shimane University, Matsue, Shimane 690-8504, Japan;1. Department of Chemistry, Fudan University, Shanghai 200433, China;2. Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, China;3. College of Hydrology and Water Resources, Hohai University, Nanjing 210098, China;4. Department of Municipal Engineering, School of Civil and Hydraulic Engineering, Hefei University of Technology, Hefei 230009, China;1. Institute of Molecular Plant Sciences, School of Biological Sciences, University of Edinburgh, Edinburgh EH9 3BF, UK;1. Department of Chemistry, Simon Fraser University, Burnaby, BC V5A 1S6, Canada;2. Department of Medicinal Chemistry, National Institute of Pharmaceutical Education and Research (NIPER), S.A.S. Nagar, Punjab 160062, India;3. Department of Chemistry, University of Saskatchewan, 110 Science Place, Saskatoon, Saskatchewan S7N 5C9, Canada |
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| Abstract: | Cell suspensions of Daucus carota L. were grown in batch culture on 50 mM sucrose, 100 mM glucose or 100 mM fructose. Sucrose was rapidly converted extra-cellularly into equimolar amounts of glucose and fructose, and glucose was then taken up preferentially. This impaired uptake of fructose could partially be explained by the eight-fold lower affinity of the hexose carrier in the plasmamembrane for fructose compared to glucose. However, cells grown on fructose as the sole carbon source showed a shorter lag phase and showed more biomass production compared to glucose-grown cells, indicating that conversion of glucose and fructose were also differently regulated. Ninety-five % of the glucose phosphorylating activity was membrane-associated and most probably confined to mitochondria; therefore, it might be present in a respiratory ‘compartment’ making glucose a better substrate for respiration than fructose. The soluble fraction contained the majority of the fructokinase activity. This activity was hypothesized to be more or less randomly 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-1-phosphate (G-1-P), it is converted into UDPG-glucose, resulting in structural cell components. The observed transient obstruction of the 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 maintained by phosphoglucomutase, interconverting G-6-P and G-1-P and low levels of nucleotide triphosphates. Low nucleotide triphosphate production, connected with a low initial respiration rate, might be caused by the ten-fold lower affinity of the membrane-associated phosphorylating enzymes for fructose compared to glucose. Our results were taken to indicate that two separate pools of glycolytic intermediates exist in D. carota cells: one distributed throughout the cytosol and one surrounding the mitochondria. |
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