RESUMEN
The modifying effect of sucrose on glutamate dehydrogenase (GDH) activity and isoenzyme pattern was investigated in isolated embryos of lupine (Lupinus luteus L.), cultured in vitro in a medium with sucrose (+S) or without sucrose (-S) and exposed to cadmium (Cd) and lead (Pb) stress. Sucrose starvation of lupine embryos led to a rapid increase in the specific activity of GDH, immunoreactive beta-polypeptide and it was accompanied by appearance of new cathodal isoforms of enzyme. This suggests that isoenzymes induced in lupine embryos by sucrose starvation combine into GDH hexamers with the predominance of beta-GDH subunits synthetized under GDH1 gene control. The addition of sucrose to the medium caused an opposite effect. Along with upregulation of catabolic activity of GDH by sucrose starvation, activity of proteolytic enzymes was also induced. These data can point to regulatory mechanism implying a sucrose dependent repression of the GDH1 gene according to the mechanism of catabolic repression. Treatment of embryos with Cd(2+) or Pb(2+) resulted in ammonium accumulation in the tissues, accompanied by an increase in anabolic activity of GDH and activity of anodal isoenzymes, in both (+S) and (-S) embryos without new de novo synthesis of alpha subunit proteins. Thus, GDH isoenzyme profiles may reflect the physiological function of GDH, which appears to be an important link of metabolic adaptation in cells, aimed at using carbon sources other than sugar during carbohydrate starvation (catabolic activity of GDH) and protecting plant tissues against ammonium accumulated because of heavy metal stress (anabolic activity of GDH).