Reversible Inactivation of Ferredoxin-Nitrate Reductase from the Cyanobacterium Plectonema boryanum. The Role of Superoxide Anion and Cyanide

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Assimilatory nitrate reductase (NR) from the cyanobacterium Plectonema boryanum exhibits both ferredoxin (Fd)- and methyl viologen (MV)-linked activities. Native (Fd-linked) activity was reversibly inactivated by the exposure of a dithionite solution of the enzyme to air, whereas MV-linked activity remained unaffected. Cyanate and azide, competitive inhibitors of NR, suppressed the dithionite-induced inactivation, and the inactivation was specifically prevented by superoxide dismutase (SOD) . Xanthine oxidase reaction inactivated Fd-linked activity but not MV-linked activity, and the native activity was protected by SOD and catalase. Photoactivated FAD/EDTA irreversibly inactivated both Fd- and MV-linked activities, and the former activity was protected by ascorbic acid. Dithionite-inactivated enzyme was restored to its Fd-linked activity when incubated with cyanate or azide in the presence of dithionite or reduced Fd. Cyanide inactivated both Fd- and MV-linked activities when the enzyme was incubated under reducing conditions. The cyanide-inactivated enzyme was also reactivated by cyanate and azide under reducing conditions. It is suggested that superoxide and cyanide act as ligands to the molybdenum center in the reversible inactivation of the native activity of cyanobacterial Fd-NR.
日本植物生理学会の論文