Nitric Oxide (NO) Production by Plant Nitrate Reductase

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Nitric oxide (NO) is an important molecule that is involved in diverse physiological functions of living organisms. It has been shown that NO can produced endogenously by NO synthase (NOS, EC 1.14.13.39) in vertebrate, invertebrates and bacteria. In contrast, the mechanism for enzymatic NO production in plants and algae is still in debate. Until recently, NOS had been presumed as the only enzyme that could produce NO in plant cells. Despite many efforts to identify a gene and protein similar to mammalian-type NOS, however, there has been no substantial evidence to conclude the presence of such NOS in plants. We have proposed an alternative NO production mechanism for plants. Nitrate reductase (NR) is a well-known protein for plant biologists because it is a key enzyme of nitrate assimilation metabolism. The enzyme normally catalyzes the reduction of nitrate to form nitrite using NAD(P)H. we have shown that the enzyme is capable of further reducing the product nitrite to produce NO as the result. Importantly, NO can be subsequently converted to peroxynitrite, the most toxic active nitrogen, under aerobic conditions. Although we cannot exclude a possibility for discovering a new type NOS that is unique to plants, it is now evident that plants do produce NO by the distinct mechanism from animals. These findings, the classical enzyme possessing unexplored important functions, offers us a new opportunity to re-consider and re-investigate plant biology with integrated knowledge in terms of "NO".