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Published on October 11, 2007; 10.1104/pp.107.107813

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Received August 21, 2007
Accepted September 24, 2007

The Isoenzyme Seven of Tobacco NAD(H)-Dependent Glutamate Dehydrogenase Exhibits High Deaminating and Low Aminating Activities in vivo

Damianos S. Skopelitis , Nikolaos V. Paranychianakis , Antonios Kouvarakis , Apostolis Spyros , Euripides G. Stephanou , and Kalliopi A. Roubelakis-Angelakis *

Dept of Biology, University of Crete, P.O. Box 2208, 72209 Heraklion, Greece; Dept of Chemistry, University of Crete, P.O. Box 2208, 71409 Heraklion, Greece

* Corresponding author; email: poproube{at}biology.uoc.gr.

Following the discovery of Glutamine synthetase/Glutamate synthase in 1974 (Lea and Miflin, 1974), the physiological roles of Glutamate dehydrogenase (GDH) in nitrogen metabolism in plants remains obscure and the subject of considerable controversy. Recently, Purnell and Botella (2007) used transgenics over-expressing the gene encoding for the {beta}-subunit polypeptide of GDH and reported that GDH-isoenzyme 1 deaminates in vivo Glu. In this work, we present transgenic tobacco plants over-expressing the plant gdh gene encoding for the {alpha}-subunit polypeptide of GDH. The levels of transcript correlated well with the levels of total GDH-protein, the {alpha}-subunit polypeptide, and the abundance of GDH-anionic isoenzymes. Assays of transgenic plant extracts revealed high in vitro aminating and low deaminating activities. However, GC/MS analysis of the metabolic fate of 15NH4 or [15N]Glu revealed that GDH-isoenzyme 7 mostly deaminates Glu, and also exhibits low ammonium assimilating activity. These results along with the already published (Purnell and Botella, 2007) firmly establish the direction of the reactions catalyzed by the anionic and cationic isoenzymes of Glutamate dehydrogenase in vivo, under normal growth conditions, and reveal a paradox between the in vitro and in vivo enzyme activities.




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S. Labboun, T. Terce-Laforgue, A. Roscher, M. Bedu, F. M. Restivo, C. N. Velanis, D. S. Skopelitis, P. N. Moshou, K. A. Roubelakis-Angelakis, A. Suzuki, et al.
Resolving the Role of Plant Glutamate Dehydrogenase. I. in vivo Real Time Nuclear Magnetic Resonance Spectroscopy Experiments
Plant Cell Physiol., October 1, 2009; 50(10): 1761 - 1773.
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