GLUTAMINE-SYNTHETASE AND GLUTAMATE SYNTHASE ACTIVITIES IN HIGH AMMONIUM GROWN WHEAT CELLS

Heterotrophic suspension cultures of wheat (Triticum aestivum L. cv He ines Koga II) were grown on media containing various amounts of nitrat e and ammonium. Increasing the external ammonium concentration from 2 to 25 mM led to a 200% increase in the specific NADH-dependent glutama te synthase activity. In contrast, the specific glutamine synthetase a ctivity decreased by 80%. High ammonium grown cells exhibited a two-10 -fold elevation of glutamine, asparagine, alanine and ammonium, but up to an 80% decrease in malate, alpha-ketoglutarate, and nitrate pools. Cells exclusively supplied with ammonium nitrogen (nitrate starvation ) ceased soluble protein synthesis and showed a specific increase in g lutamate dehydrogenase activity. Regardless of changes in the nitrogen supply, the in vitro measured activity of NADH-dependent glutamate sy nthase was similar to the calculated in vivo rate of ammonium assimila tion. The in vitro measured activity of glutamine synthetase was negat ively related to the rate of ammonium assimilation, while the product of the in vitro measured activity of glutamine synthetase and the cell ular concentration of ammonium was positively related to it. The resul ts are discussed in terms of an in vivo regulation of glutamine synthe tase activity by glutamine, alpha-ketoglutarate and the cytosolic conc entration of ammonium.