Abstract
Spinach NADH-nitrate oxido-reductase (EC 1.6.6.1) is inactivated progressively during several min by NADH, or less rapidly, by ferrocytochrome c in a biphasic reaction. Inactivation is synergistically increased by the presence of both during the first phase of the reaction. The enzyme is immune to inactivation by either compound during turnover with nitrate. When the enzyme is incubated with ferrocytochrome c without NADH, inactivation does not affect binding of NADH or nitrate, but when incubated together with NADH, ferrocytochrome c appears to impede binding by nitrate in addition to causing inactivation. The ping-pong kinetics of NADH nitrate reductase are thought to protect the enzyme against inactivation by NADH or ferrocytochrome c in the presence of nitrate.
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