Glutamic Acid Metabolism and the Photorespiratory Nitrogen Cycle in Wheat Leaves

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Abstract
The effects of methionine sulfoximine and ammonium chloride on [14C]glutamate metabolism in excised leaves of T. aestivum were investigated. Glutamine was the principal product derived from [U14C]glutamate in the light and in the absence of inhibitor or NH4Cl. Other amino acids, organic acids, sugars phosphates and CO2 became slightly radioactive. Ammonium chloride (10 mM) increased formation of [14C] glutamine, aspartate, citrate and malate but decreased incorporation into 2-oxoglutarate, alanine and 14CO2. Methionine sulfoximine (1 mM) suppressed glutamine synthesis, caused NH3 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 14C from [U-14C]glutamate to be incorporated into malate and succinate; most of the 14C from [1-14C]glutamate was metabolized to CO2 and sugar phosphates. Formation of radioactive organic acids in the presence of methionine sulfoximine does not take place indirectly through dark fixation of CO2 released by degradation of glutamate when ammonia assimilation is blocked. When illuminated leaves supplied with [U-14C]glutamate without inhibitor or NH4Cl were transferred to darkness, there was increased metabolism of the glutamate to glutamine, aspartate, succinate, malate and 14CO2. Darkening had little effect on the labeling pattern in leaves treated with methionine sulfoximine.