Nitrogen Metabolism of Lemna minor. II. Enzymes of Nitrate Assimilation and Some Aspects of Their Regulation

In L. minor grown in sterile culture, the primary enzymes of nitrate assimilation, nitrate reductase (NR), nitrite reductase (NiR) and glutamate dehydrogenase (GDH) change in response to nitrogen source. NR and NiR levels are low when grown on amino acids (hydrolyzed casein) or ammonia; both enzymes are rapidly induced on addition of nitrate, while addition of nitrite induces NiR only. Ammonia represses the nitrate induced synthesis of both NR and NiR. NADH dependent GDH activity is low when grown on amino acids and high when grown on nitrate or ammonia, but the activities of NADPH dependent GDH and Alanine dehydro-genase (AIDH) are much less affected by nitrogen source. NADH-GDH and AIDH are induced by ammonia, and it is suggested that these enzymes are involved in primary nitrogen assimilation.