A Flux Model of Glycolysis and the Oxidative Pentosephosphate Pathway in Developing Brassica napus Embryos

Developing oilseeds synthesize large quantities of triacylglycerol from sucrose and hexose. To understand the fluxes involved in this conversion, a quantitative metabolic flux model was developed and tested for the reaction network of glycolysis and the oxidative pentose phosphate pathway (OPPP). De...

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Bibliographic Details
Published in:The Journal of biological chemistry 2003-08, Vol.278 (32), p.29442-29453
Main Authors: Schwender, Jörg, Ohlrogge, John B., Shachar-Hill, Yair
Format: Article
Language:English
Subjects:
Amino Acids - chemistry
Amino Acids - metabolism
Brassica napus
Carbohydrate Metabolism
Carbon - chemistry
Cell Wall - metabolism
Cytosol - metabolism
Fatty Acids - metabolism
Gas Chromatography-Mass Spectrometry
Glucose - chemistry
Glucose - metabolism
Glucose-6-Phosphate - metabolism
Glycolysis
Hexoses - chemistry
Kinetics
Lipid Metabolism
Magnetic Resonance Spectroscopy
Models, Biological
NADP - metabolism
Oxygen - metabolism
Pentose Phosphate Pathway
Pentosephosphates - chemistry
Sucrose - chemistry
Sucrose - metabolism
Sugar Phosphates - metabolism
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Summary:Developing oilseeds synthesize large quantities of triacylglycerol from sucrose and hexose. To understand the fluxes involved in this conversion, a quantitative metabolic flux model was developed and tested for the reaction network of glycolysis and the oxidative pentose phosphate pathway (OPPP). Developing Brassica napus embryos were cultured with [U-13C6]glucose, [1-13C]glucose, [6-13C]glucose, [U-13C12]sucrose, and/or [1,2-13C2]glucose and the labeling patterns in amino acids, lipids, sucrose, and starch were measured by gas chromatography/mass spectrometry and NMR. Data were used to verify a reaction network of central carbon metabolism distributed between the cytosol and plastid. Computer simulation of the steady state distribution of isotopomers in intermediates of the glycolysis/OPPP network was used to fit metabolic flux parameters to the experimental data. The observed distribution of label in cytosolic and plastidic metabolites indicated that key intermediates of glycolysis and OPPP have similar labeling in these two compartments, suggesting rapid exchange of metabolites between these compartments compared with net fluxes into end products. Cycling between hexose phosphate and triose phosphate and reversible transketolase velocity were similar to net glycolytic flux, whereas reversible transaldolase velocity was minimal. Flux parameters were overdetermined by analyzing labeling in different metabolites and by using data from different labeling experiments, which increased the reliability of the findings. Net flux of glucose through the OPPP accounts for close to 10% of the total hexose influx into the embryo. Therefore, the reductant produced by the OPPP accounts for at most 44% of the NADPH and 22% of total reductant needed for fatty acid synthesis.
ISSN:0021-9258
1083-351X
DOI:10.1074/jbc.M303432200