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- Title
Glutamate synthesis in barley roots: the role of the plastidic glucose-6-phosphate dehydrogenase.
- Authors
Esposito, Sergio; Massaro, Graziella; Vona, Vincenza; Di Martino Rigano, Vittoria; Carfagna, Simona
- Abstract
Evidence is provided for a close link between glutamate (Glu) synthesis and the production of reducing power by the oxidative pentose phosphate pathway (OPPP) in barley (Hordeum vulgare L. var. Alfeo) root plastids. A rapid procedure for isolating organelles gave yields of plastids of over 30%, 60% of which were intact. The formation of Glu by intact plastids fed with glutamine and 2-oxoglutarate, both substrates of glutamate synthase (GOGAT), depends on glucose-6-phosphate (Glc-6-P) supply. The whole process exhibited an apparent Km Glc-6-P of 0.45 mM and is abolished by azaserine, a specific inhibitor of GOGAT; ATP caused a decrease in the rate of Glu formation. Glucose and other sugar phosphates were not as effective in supporting Glu synthesis with respect to Glc-6-P; only ribose-5-phosphate, an intermediate of OPPP, supported rates equivalent to Glc-6-P. Glucose-6-phosphate dehydrogenase (Glc6PDH) rapidly purified from root plastids showed an apparent Km Glc-6-P of 0.96 mM and an apparent Km NADP+ of 9 µM. The enzyme demonstrated high tolerance to NADPH, exhibiting a KiNADPH of 58.6 µM and selectively reacted with antibodies against potato plastidic, but not chloroplastic, Glc6PDH isoform. The data support the hypothesis that plastidic OPPP is the main site of reducing power supply for GOGAT within the plastids, and suggest that the plastidic OPPP would be able to sustain Glu synthesis under high NADPH:NADP+ ratios even if the plastidic Glc6PDH may not be functioning at its highest rates.
- Subjects
PHOTORECEPTORS; DEHYDROGENASES; GLUCOSE; PHYSIOLOGICAL oxidation; AMINO acids; PLANT cells &; tissues
- Publication
Planta: An International Journal of Plant Biology, 2003, Vol 216, Issue 4, p639
- ISSN
0032-0935
- Publication type
Article
- DOI
10.1007/s00425-002-0892-4