Nitric Oxide (NO) Production by Plant Nitrate Reductase

一氧化氮(NO)是一种重要的分子，参与生物体的多种生理功能。已有研究表明，在脊椎动物、无脊椎动物和细菌中，NO可由一氧化氮合酶(NOS，EC 1.14.13.39)内源性产生。相比之下，植物和藻类中酶促产生NO的机制仍存在争议。直到最近，一氧化氮合酶一直被认为是唯一能在植物细胞中产生一氧化氮的酶。然而，尽管人们做出了许多努力来鉴定与哺乳动物类型的一氧化氮合酶相似的基因和蛋白质，但还没有实质性的证据来证明植物中存在这种一氧化氮合酶。我们已经为植物提出了一种替代的NO产生机制。硝酸还原酶(NR)是植物体内硝酸盐同化代谢的关键酶，是植物生物学家所熟知的蛋白质。该酶通常利用NAD(P)H催化硝酸盐还原生成亚硝酸盐。我们已经证明，该酶能够进一步还原产物亚硝酸盐，从而产生NO。重要的是，在好氧条件下，NO随后可以转化为过氧亚硝酸盐，这是毒性最大的活性氮。虽然我们不能排除发现一种植物特有的新型一氧化氮合酶的可能性，但现在很明显，植物确实通过与动物不同的机制产生一氧化氮。这些发现为我们提供了一个新的机会，从“NO”的角度用综合知识重新考虑和重新研究植物生物学。

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".