Glycolate-, Xanthine-and Paraquat-Mediated Inhibition of Nitrate Reductase in Detached Oat Leaves

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Leaf segments of 8-day-old oat plants were incubated with 100 μM glycolate or xanthine in the dark or with 100 μM paraquat in the light. These treatments were designed to enhance the in- . ternal production of activated oxygen species and to allow us to study the possible role of such species in the regulation of the activity of NO_3^-induced nitrate reductase (NR) in illuminated green tissues. Each of the treatments inhibited NO_3-induced NR activity. HPMS (an inhibitor of glycolate oxidase) and allopurinol (an inhibitor of xanthine oxidase) protected NR from inactivation by glycolate and by xanthine. Free radical scavengers also protected NR from inactivation by xanthine and paraquat but they had no effect on the inhibition by glycolate. The activities of catalase and superoxide dismutase did not increase in response to the various treatments. Thus, while the production of active oxygen species appears to be stimulated under such conditions, the rates of the enzymatic destruction and "scavenging" of these species seem to remain unchanged. Hence, the endogenous levels of these active oxygen species can be expected to increase in the treated leaves. We interpret our results as evidence that inhibition by glycolate of NR is caused by the H_2O_2 generated during the oxidation of glycolate while the effects of xanthine and paraquat depend on the enhanced production of free radicals. It is concluded that activated oxygen species that are formed endogenously may play a role in the regulation of NR in green tissues.
日本植物生理学会の論文

Glycolate-, Xanthine-and Paraquat-Mediated Inhibition of Nitrate Reductase in Detached Oat Leaves

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