Inhibition of Wheat Nitrate Reductase Activity by Zinc

The effect of Zn^2- on nitrate reductase (NR, EC 1.6.6.1) activity was studied in botá wheat (Triticum aestivum cv. Oasis) leaves and in the NR enzyme partially purified from wheat leaves. Leaf segments were floated on 0 to 5 mM ZnSO₄ solutions (pH 6.0) for 24 h under continuous light. Zn^2- at 250 M decreased NR activity and increased membrane permeability. However, parameters of cellular oxidative damage were scarcely affected by Zn^2- treatments. Accordingly, the decrease of NR activity induced by Zn^2- was not prevented by benzoate (a scavenger of oxygen radicals). The effect of Zn^2- was dependent on leaf age: it decreased NR activity in mature but not in young leaves. Zn² inhibited the partially purified NR. This inhibition was not reversed by either co- or post-incubation with cysteine, and the amount of -SH groups of the purified NR was not affected by Zn²⁺ indicating that Zn^2- inhibition does not involve key -SH groups of the enzyme. However, o-phenantroline both prevented and reversed Zn²⁺-induced NR inhibition. We concluded that the effect of Zn²⁺ on NR activity in vivo is not associated with an increase in active oxygen generation and involves a direct and reversible inhibition of the enzyme.