N-ammonia assimilation, 2-oxoglutarate transport, and glutamate export in spinach chloroplasts in the presence of dicarboxylates in the light |
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Authors: | Woo K C Boyle F A Flugge I U Heldt H W |
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Institution: | Plant Environmental Biology Group, Research School of Biological Sciences, Australian National University, P.O. Box 475, Canberra City, A.C.T. 2601 Australia. |
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Abstract: | The direct incorporation of 15NH4Cl into amino acids in illuminated spinach (Spinacia oleracea L.) chloroplasts in the presence of 2-oxoglutarate plus malate was determined. The amido-N of glutamine was the most highly labeled N-atom during 15NH4 assimilation in the presence of malate. In 4 minutes the 15N-label of the amido-N of glutamine was 37% enriched. In contrast, values obtained for both the N-atom of glutamate and the amino-N of glutamine were only about 20% while that of the N-atom of aspartate was only 3%. The addition of malate during the assimilation of 15NH4Cl and Na15NO2 greatly increased the 15N-label into glutamine but did not qualitatively change the order of the incorporation of 15N-label into all the amino acids examined. This evidence indicates the direct involvement of the glutamine synthetase/glutamate synthase pathway for ammonia and nitrite assimilation in isolated chloroplasts. The addition of malate or succinate during ammonia assimilation also led to more than 3-fold increase in 14C]2-oxoglutarate transport into the chloroplast as well as an increase in the export of 14C]glutamate out of the chloroplast. Little 14C]glutamine was detected in the medium of the chloroplast preparations. The stimulation of 15N-incorporation and 14C]glutamate export by malate could be directly attributed to the increase in 2-oxoglutarate transport activity (via the 2-oxoglutarate translocator) observed in the presence of exogenous malate. |
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