Induction of a Specific Isoenzyme of Glutamate Dehydrogenase by Ammonia in Oat Leaves

THE reaction catalysed by glutamate dehydrogenase (GDH) is one of the principal routes by which ammonia is incorporated into organic nitrogen. Substrate-controlled changes in the rate of GDH synthesis were detected in microorganisms1,2, but have not been demonstrated in higher plants. A report on induction of GDH by ammonia in Lemma minor3 was later found by the same author to result from differential sensitivity of the enzyme to EDTA inhibition, rather than a true induction4. An approximately 3.5-fold increase in GDH concentration was observed in cucumber leaves during ammonium toxicity5; the nature of the activation process has not, however, been elucidated. The study described here is conclusive indication of an ammonium-dependent induction of GDH synthesis in plant tissue.