REGULATION OF GLUTAMATE-DEHYDROGENASE ACTIVITY BY MANIPULATION OF NUCLEOTIDE SUPPLY IN DAUCUS-CAROTA SUSPENSION-CULTURES

The enzyme glutamate dehydrogenase (GDH, EC 1.4.1.2) is ubiquitous in plant species. It is now generally accepted that the primary role of t his enzyme is not assimilation of ammonium and it has been suggested t hat GDH may be important in provision of carbon skeletons under condit ions of carbon Limitation. In carrot (Daucus carota L. Chantenay) cell suspension cultures carbon starvation results in de-repression of GDH activity. The regulation of this de-repression has not been investiga ted. This paper examines the possibility that the availability of aden osine nucleotides is instrumental in the regulation of GDH activity. I n repressed cultures the adenosine nucleotides cAMP (0.2 mM), AMP (0.2 mM) and ADP (0.4 mM) caused an increase in GDH activity of 61, 33 and 7%, respectively. ATP (0.2 mM) had the opposite effect in maintaining repression of GDH. Under de-repressed conditions only cAMP (0.2 mM) e nhanced GDH activity (14%). Inhibition of oxidative phosphorylation us ing a range of inhibitors resulted in de-repression of GDH and stimula tion of respiration. The results from this work indicate that exogenou sly applied adenosine nucleotides and electron transport inhibitors al ter the GDH repression/de-repression status. Addition of these compoun ds alters or disrupts ATP levels, mimicking carbon depletion. This cau ses an increase in GDH activity, supporting the idea that GDH may prov ide carbon skeletons for carbon metabolism and suggesting that ATP sta tus is important in regulation of the enzyme activity.