Tobacco isoenzyme 1 of NAD(H)-dependent glutamate dehydrogenase catabolizes glutamate in vivo.
Glutamate (Glu) dehydrogenase (GDH, EC 1.4.1.2-1.4.1.4) catalyzes in vitro the reversible amination of 2-oxoglutarate to Glu. The in vivo direction(s) of the GDH reaction in higher plants and hence the role(s) of this enzyme is unclear, a situation confounded by the existence of isoenzymes comprised totally of either GDH beta- (isoenzyme 1) or alpha- (isoenzyme 7) subunits, as well as another five alpha-beta isoenzyme permutations. To clarify the in vivo direction of the reaction catalyzed by GDH isoenzyme 1, [(15)N]Glu was supplied to roots of two independent transgenic tobacco (Nicotiana tabacum) lines with increased isoenzyme 1 levels (S4-H and S49-H). The [(15)N]ammonium (NH(4)(+)) accumulation rate in these lines was elevated approximately 65% compared with a null segregant control line, indicating that isoenzyme 1 catabolizes Glu in roots. Leaf glutamine synthetase (GS) was inhibited with a GS-specific herbicide to quantify any contribution by GDH toward photorespiratory NH(4)(+) reassimilation. Transgenic line S49-H did not show enhanced resistance to the herbicide, indicating that the large pool of isoenzyme 1 in S49-H leaves was unable to compensate for GS and suggesting that isoenzyme 1 does not assimilate NH(4)(+) in vivo.
Purnell
Matthew Peter
MP
Department of Botany, School of Integrative Biology, University of Queensland, Queensland 4072, Australia.
Botella
José Ramon
JR
D018086:Glutamate Dehydrogenase (NADP+) / Q000502:physiology*; D005974:Glutamate-Ammonia Ligase / Q000037:antagonists & inhibitors; D018698:Glutamic Acid / Q000378:metabolism*; D006540:Herbicides / Q000494:pharmacology; D007527:Isoenzymes / Q000502:physiology; D018515:Plant Leaves / Q000201:enzymology / Q000235:genetics / Q000378:metabolism; D018517:Plant Roots / Q000201:enzymology / Q000235:genetics / Q000378:metabolism; D030821:Plants, Genetically Modified / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism; D000644:Quaternary Ammonium Compounds / Q000378:metabolism; D014026:Nicotiana / Q000187:drug effects / Q000201:enzymology* / Q000235:genetics
D006540:Herbicides; D007527:Isoenzymes; D000644:Quaternary Ammonium Compounds; D018698:Glutamic Acid; D018086:Glutamate Dehydrogenase (NADP+); D005974:Glutamate-Ammonia Ligase