The role of malate in ammonia assimilation in cotyledons of radish (Raphanus sativus L.) |
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Authors: | B Dahlbender D Strack |
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Institution: | (1) Botanisches Institut der Universität zu Köln, Gyrhofstrasse 15, D-5000 Köln 41, Federal Republic of Germany |
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Abstract: | The relationships between the metabolism of malate, nitrogen assimilation and biosynthesis of amino acids in response to different nitrogen sources (nitrate and ammonium) have been examined in cotyledons of radish (Raphanus sativus L.). Measurements of the activities of some key enzymes and pulse-chase experiments with 14C]malate indicate the operation of an anaplerotic pathway for malate, which is involved in the synthesis of glutamine during increased ammonia assimilation. It is most likely that the tricarboxylicacid cycle is supplied with carbon through entry of malate, formed via the phosphoenolpyruvate (PEP)-carboxylation pathway, when 2-oxoglutarate leaves the cycle to serve as precursor for an increased synthesis of glutamine via glutamate. This might occur predominantly in the cytosol via the activity of the glutamine synthetase/glutamate synthase (GS/GOGAT) cycle, the NADH-dependent GOGAT being the rate-limiting activity.Abbreviations DTT
dithiothreitol
- EDTA
ethylenediamine-tetraacetic acid
- GDH
glutamate dehydrogenase
- GOGAT
glutamate synthase (glutamine: 2-oxoglutarate aminotransferase)
- GOT
aspartate aminotransferase (glutamate: oxaloacetate transaminase)
- GS
glutamine synthetase
- HPLC
high-performance liquid chromatography
- MCF
extraction medium of methanol: chloroform: 7M formic acid, 12:5:3, by vol.
- MDH
malate dehydrogenase
- MSO
L-methionine, sulfoximine
- PEPCase
phosphoenolpyruvate carboxylase
- TLC
thin-layer chromatography |
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Keywords: | Amino acid metabolism Anaplerotic sequence Glutamate synthase Glutamine synthetase Malate Nitrogen nutrition (ammonium supply) Phosphoenolpyruvate carboxylase Raphanus (seedling development) |
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