Inhibition of Nitrate Reductase Activity by Lead in Greening Bean Leaf Segments: A Mechanistic Approach

Substrate inducibility of in vivo nitrate reductase activity (NRA) in greening bean leaf segments was inhibited with the supply of 0.01 to 0.10 mM Pb acetate. However, endogenous NO₃ pool, measured indirectly as nitrite secreted into the medium was decreased at 0.1 mM Pb only and not at lower concentrations. Supply of Pb inhibited in vitro NADH-NRA as well as FADH₂-NRA but did not affect cytochrome c reductase activity. The percent inhibition of enzyme activity in the presence of Pb was more pronounced at higher concentration of KNO₃ than at lower concentration. Inhibitory effect of Pb on enzyme activity was observed during the supply of various organic and inorganic nitrogenous compounds and not in their absence. The experiments demostrate that lead has an inhibitory role on nitrate reductase activity apparently through an effect on intracellular mobilization of nitrate.