Effect of nitrogen sources upon the activity of l-glutamate dehydrogenase of Lemna gibba

When Lemna gibba cultures, grown on medium containing l-glutamate as the sole nitrogen source are transferred to medium in which ammonium is the only source of nitrogen, the activity of a NAD-dependent l-glutamate dehydrogenase (GDH) increases approximately 5-fold over 3 days. Upon re-transfer to glutamate medium the activity declines to its initial value after a further 6 days. The rise in activity is independent of the presence of EDTA and is not the result of an increase in the ease with which the enzyme can be extracted. p-Fluoro-dl-phenylalanine, azetidine-2-carboxylic acid and puromycin but not d-threo-chloramphenicol, erythromycin or lincomycin inhibit the increase when included in ammonium medium. These observations, together with those obtained from the use of a deuterium oxide-labelling technique, suggest that the increase in GDH activity is due to de novo synthesis on 80S ribosomes.