METABOLITE POOLS DURING STARCH SYNTHESIS AND CARBOHYDRATE OXIDATION IN AMYLOPLASTS ISOLATED FROM WHEAT ENDOSPERM

Starch synthesis and CO2 evolution were determined after incubating in tact and lysed wheat (Triticum aestivum L. cv. Axona) endosperm amylop lasts with C-14-labelled hexose-phosphates. Amyloplasts converted [U-C -14]glucose I-phosphate (Glc1P) but not [U-C-14]glucose 6-phosphate (G lc6P) into starch in the presence of ATP. When the oxidative pentose-p hosphate pathway (OPPP) was stimulated, both [U-C-14]Glc1P and [U-C-14 ]Glc6P were metabolized to CO2, but Glc6P was the better precursor for the OPPP, and Glc1P-mediated starch synthesis was reduced by 75%. In order to understand the basis for the partitioning of carbon between t he two potentially competing metabolic pathways, metabolite pools were measured in purified amyloplasts under conditions which promote both starch synthesis and carbohydrate oxidation via the OPPP. Amyloplasts incubated with Glc1P or Glc6P alone showed little or no interconversio n of these hexose-phosphates inside the organelle. When amyloplasts we re synthesizing starch, the stromal concentrations of Glc1P and ADP-gl ucose were high. By contrast, when flux through the OPPP was highest, Glc1P and ADP-glucose inside the organelle were undetectable, and ther e was an increase in metabolites involved in carbohydrate oxidation. M easurements of the plastidial hexose-monophosphate pool during starch synthesis and carbohydrate oxidation indicate that the phosphoglucose isomerase reaction is at equilibrium whereas the reaction catalysed by phosphoglucomutase is significantly displaced from equilibrium.