Increase in NADH-glutamate dehydrogenase activity by mercury in excised bean leaf segments

Application of Hg to excised bean leaf segments increased the glutamate dehydrogenase (NADH-GDH) activity substantially. However, specific activity of the enzyme decreased at lower concentration of Hg, and increased to lesser extent at higher concentration of Hg. Mercury supply increased the glutamate synthase (NADH-GOGAT) activity also. Mercury supply increased the NADH-GDH activity in the presence of NH4NO3, but to a lesser extent than in the absence of NH4NO3. The specific activity of the enzyme decreased considerably at lower concentration of Hg, but increased significantly at higher concentration of Hg. An increase in NADH-GOGAT activity was observed in the presence of NH4NO3, but specific activity of the enzyme decreased marginally. Increase in GDH activity due to Hg remained unaffected by the supply of sucrose, but was reduced by glutamine and glutathione and enhanced by Al. The glutamate dehydrogenase (+Hg enzyme) from mercury treated leaf segments had higher value of S0.5 for NADH than the enzyme (-Hg enzyme) from material not treated with mercury indicating that Hg binding to enzyme prevented NADH binding to the enzyme possibly at thiol groups. However, + Hg enzyme has more reactivity, as apparent Vmax value was higher for it. It has been suggested that Hg activates the NADH-GDH enzyme in the bean leaf segments by binding to thiol groups of protein and pronounced increase in activity by Hg suggests a possible role of enzyme under Hg-stress.