Reversible Inactivation of Nitrate Reductase by NADH and the Occurrence of Partially Inactive Enzyme in the Wheat Leaf

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Abstract
Nitrate reductase from wheat (T. aestivum L. cv. Bindawarra) leaves is inactivated by pretreatment with NADH, in the absence of nitrate, a 50% loss of activity occurring in 30 min at 25.degree. C with 10 .mu.M NADH. Nitrate (50 .mu.M) prevented inactivation by 10 .mu.M NADH while cyanide (1 .mu.M) markedly enhanced the degree of inactivation. A rapid reactivation of NADH-inactivated nitrate reductase occurred after treatment with 0.3 mM ferricyanide or exposure to light (230 mW/cm2) plus 20 .mu.M FAD. When excess NADH was removed, the enzyme was also reactivated by autoxidation. Nitrate did not influence the rate of reactivation. Leaf nitrate reductase from plants grown for 12 days on 1 mM nitrate, isolated in the late photoperiod or dark period, was activated by ferricyanide or light treatment. Evidently, at these times of the day, the nitrate reductase in the leaves of the low nitrate plants is in a partially inactive state (NADH-inactivated). The nitrate reductase from moisture-stressed plants showed a greater degree of activation after light treatment and inactive enzyme in them was detected earlier in the photoperiod.