Liszkay Anja, Kenk Barbara, Schopfer Peter
Institut für Biologie II der Universität, Schänzlestrasse 1, 79104 Freiburg, Germany.
Planta. 2003 Aug;217(4):658-67. doi: 10.1007/s00425-003-1028-1. Epub 2003 May 9.
Hydroxyl radicals (*OH), produced in the cell wall, are capable of cleaving wall polymers and can thus mediate cell wall loosening and extension growth. It has recently been proposed that the biochemical mechanism responsible for *OH generation in the cell walls of growing plant organs represents an enzymatic reaction catalyzed by apoplastic peroxidase (POD). This hypothesis was investigated by supplying cell walls of maize ( Zea mays L.) coleoptiles and sunflower ( Helianthus annuus L.) hypocotyls with external NADH, an artificial substrate known to cause *OH generation by POD in vitro. The effects of NADH on wall loosening, growth, and *OH production in vivo were determined. NADH mediates cell wall extension in vitro and in vivo in an H2O2-dependent reaction that shows the characteristic features of POD. NADH-mediated production of *OH in vivo was demonstrated in maize coleoptiles using electron paramagnetic resonance spectroscopy in combination with a specific spin-trapping reaction. Kinetic properties and inhibitor/activator sensitivities of the *OH-producing reaction in the cell walls of coleoptiles resembled the properties of horseradish POD. Apoplastic consumption of external NADH by living coleoptiles can be traced back to the superimposed action of two enzymatic reactions, a KCN-sensitive reaction mediated by POD operating in the OH-forming mode, and a KCN-insensitive reaction with the kinetic properties of a superoxide-producing plasma-membrane NADH oxidase the activity of which can be promoted by auxin. Under natural conditions, i.e. in the absence of external NADH, this enzyme may provide superoxide (O2-) (and H2O2 utilized by POD for) *OH production in the cell wall.
在细胞壁中产生的羟基自由基（OH）能够裂解细胞壁聚合物，从而介导细胞壁松弛和伸长生长。最近有人提出，负责在生长中的植物器官细胞壁中产生OH的生化机制是一种由质外体过氧化物酶（POD）催化的酶促反应。通过向玉米（Zea mays L.）胚芽鞘和向日葵（Helianthus annuus L.）下胚轴的细胞壁提供外部NADH来研究这一假设，NADH是一种已知在体外可通过POD导致OH产生的人工底物。测定了NADH对体内细胞壁松弛、生长和OH产生的影响。NADH在体外和体内介导细胞壁伸长，这是一个依赖H2O2的反应，显示出POD的特征。使用电子顺磁共振光谱结合特定的自旋捕获反应，在玉米胚芽鞘中证明了NADH在体内介导的OH产生。胚芽鞘细胞壁中OH产生反应的动力学特性和抑制剂/激活剂敏感性类似于辣根POD的特性。活的胚芽鞘对外部NADH的质外体消耗可追溯到两种酶促反应的叠加作用，一种是由以OH形成模式运行的POD介导的对KCN敏感的反应，另一种是具有产生超氧化物的质膜NADH氧化酶动力学特性的对KCN不敏感的反应，生长素可促进其活性。在自然条件下，即在没有外部NADH的情况下，这种酶可能在细胞壁中提供超氧化物（O2-）（以及POD用于产生*OH的H2O2）。
