Stimulation of Ammonia and 2-Oxoglutarate-Dependent O2 Evolution in Isolated Chloroplasts by Dicarboxylates and the Role of the Chloroplast in Photorespiratory Nitrogen Recycling

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Intact chloroplasts isolated from spinach (Spinacia oleracea L.) leaves showed a light-dependent O2 evolution (5.5 ± 0.75 micromoles per milligram chlorophyll per hour) when supplied with ammonia and 2-oxoglutarate. This (ammonia, 2-oxoglutarate)-dependent O2 evolution was stimulated 2- to 4-fold by the dicarboxylates, malate, succinate, fumarate, glutarate, and l-tartarate. Evolution of O2 in the presence of malate was dependent on the presence of both 2-oxoglutarate and NH4Cl; malate with only either 2-oxoglutarate and NH4Cl alone did not support O2 evolution. Furthermore, in the presence of malate, the amount of O2 evolved was solely dependent on the amount of NH4Cl or 2-oxoglutarate added and malate did not affect the ratio of O2 evolved to NH4Cl or 2-oxoglutarate consumed. Studies with inhibitors (2-(3,4-dichlorophenyl)-1,1-dimethyl urea, methionine sulfoximine, and azaserine) indicated that the above activity was directly linked to glutamine synthetase and glutamate synthase activity in the chloroplast and was not caused by the metabolism of malate. The Vmax/2 of (ammonia, 2-oxoglutarate)-dependent O2 evolution was reached at 32 micromolar NH4Cl and 6 millimolar (approximately) 2-oxoglutarate in the absence of malate, and at 22 micromolar NH4Cl and 73 micromolar 2-oxoglutarate when malate (3 millimolar) was present.

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