Experimental evidence of phosphoenolpyruvate resynthesis from pyruvate in illuminated leaves

Day respiration is the cornerstone of N assimilation since it provides carbon skeletons to primary metabolism for glutamate and glutamine synthesis. However, recent studies have suggested that the tricarboxylic acid pathway (TCAP) is rate-limiting and mitochondrial pyruvate dehydrogenation is partly inhibited in the light. Pyruvate may serve as a carbon source for amino acid (e.g. alanine) or fatty acid synthesis but pyruvate metabolism is not well documented and neither is the possible resynthesis of phosphoenolpyruvate (PEP). Here, we examined the capacity of pyruvate to convert back to PEP using 13C and 2H labeling in illuminated cocklebur (Xanthium strumarium) leaves. We show that the intramolecular labeling pattern in glutamate, 2-oxoglutarate and malate after 13C-3-pyruvate feeding was consistent with 13C-redistribution from PEP via the PEP-carboxylase reaction. Furthermore, the deuterium loss in glutamate after 2H3-13C-3-pyruvate feeding suggests that conversion to PEP and back to pyruvate washed out 2H atoms to the solvent. Our results demonstrate that in cocklebur leaves, PEP resynthesis occurred as a flux from pyruvate, ca. 0.5‰ of the net CO2 assimilation rate. This is likely to involve pyruvate Pi dikinase and the fundamental importance of this flux for PEP and Pi homeostasis is discussed.