Ammonium-induced increase in NADH-glutamate dehydrogenase activity is caused by de-novo synthesis of the α-subunit

When callus derived from shoot segments of Vitis vinifera L. was transferred to ammonium-containing medium the aminating activity of NAD(H)-glutamate dehydrogenase (GDH, EC 1.4.1.2) increased significantly. This increase in enzyme activity closely paralleled an increase in the protein of the GDH α-subunit (43.0 kDa), as detected by sodium dodecyl sulfate (SDS) gel electrophoresis and Western-blotting. A similar correlation was observed between the deaminating activity and the β-subunit (42.5 kDa) which both decreased during this treatment. Using [(35)S]methionine and immunochemical detection it was shown that the rate of synthesis of the α-subunit increased considerably in the ammonium-containing medium while there was no detectable synthesis of the β-subunit. At the isoenzyme level, ammonium caused an increase in the de-novo synthesis and hence the activity staining of the more anodic isoenzymes, which are hexameric and consist mainly of α-subunits. The results indicate that the increase in NADH-GDH specific activity was due to de-novo synthesis of the α-subunit of GDH and the assembly of only the more anodic isoenzymes.