Glutamic Acid metabolism and the photorespiratory nitrogen cycle in wheat leaves: metabolic consequences of elevated ammonia concentrations and of blocking ammonia assimilation

Abstract

The effects of methionine sulfoximine and ammonium chloride on [(14)C] glutamate metabolism in excised leaves of Triticum aestivum were investigated. Glutamine was the principal product derived from [U(14)C]glutamate in the light and in the absence of inhibitor or NH(4)Cl. Other amino acids, organic acids, sugars, sugar phosphates, and CO(2) became slightly radioactive. Ammonium chloride (10 mm) increased formation of [(14)C] glutamine, aspartate, citrate, and malate but decreased incorporation into 2-oxoglutarate, alanine, and (14)CO(2). Methionine sulfoximine (1 mm) suppressed glutamine synthesis, caused NH(3) to accumulate, increased metabolism of the added radioactive glutamate, decreased tissue levels of glutamate, and decreased incorporation of radioactivity into other amino acids. Methionine sulfoximine also caused most of the (14)C from [U-(14)C]glutamate to be incorporated into malate and succinate, whereas most of the (14)C from [1-(14)C]glutamate was metabolized to CO(2) and sugar phosphates. Thus, formation of radioactive organic acids in the presence of methionine sulfoximine does not take place indirectly through "dark" fixation of CO(2) released by degradation of glutamate when ammonia assimilation is blocked. When illuminated leaves supplied with [U-(14)C] glutamate without inhibitor or NH(4)Cl were transferred to darkness, there was increased metabolism of the glutamate to glutamine, aspartate, succinate, malate, and (14)CO(2). Darkening had little effect on the labeling pattern in leaves treated with methionine sulfoximine.