Mechanism for Relaxation of Photooxidative Stress in Plants.

在植物暴露在无法利用的明亮阳光下的环境条件下，过量的光子会产生活性氧，从而对植物造成光氧化损伤。为了保护植物免受这种损害，植物有以下两种放松制度。抑制活性氧的产生循环电子流通过下调光系统II来抑制活性氧的产生，但光还原剂(铁还蛋白或NADPH)之间的中介尚未确定。在集胞藻6803中，NADPH专一性的吡啶核苷酸脱氢酶(NDH)介导了从NADPH到类囊体质酚(PQ)的电子传递，这是通过对突变体细胞P700的叶绿素荧光和氧化还原反应的分析而建立的。此外，在玉米类囊体中，LED阵列分光光度计可以检测到具有低氧化还原电位的细胞色素b-559及其参与铁氧还蛋白介导的光系统I周围的循环电子流。类囊体体中O_2^--产生的动力学分析也表明O_2^-介导的光系统I周围的循环电子流。2.叶绿体中活性氧的清除通过细胞器分级和免疫电子显微镜对参与酶的定位建立了叶绿体中活性氧清除酶的微观比较。研究发现，这种清除是由类囊体系统和基质系统进行的。进一步，利用纯化的酶和在大肠杆菌中高产的酶对抗坏血酸过氧化物酶和单脱氢抗坏血酸还原酶的分子性质进行了研究。

Under environmental conditions where plants are exposed to a bright sunlight that cannot be utilized, excess photons generate active oxygens, which cause photooxidative damages to plants. For the protection from such damages, plants have the following two relaxation systems.1. Suppression of the production of active oxygensCyclic electron flow suppresses the production of active oxygens through the down-requlation of photosystem II,but, the mediator between the photoreductants (either ferredoxin or NADPH) had not been identified. In the cyanobacterium Synechocystis 6803, a NADPH-specific pyridine nucleotide dehydrogenase (NDH) mediates an electron transfer from NADPH to the thylakoid-plasto-quinone (PQ), which was established by analysis of chlorophyll fluorescence and redox reaction of P700 using the NDH-less mutant cells. Further, in maize thylakoids, LED-array spectrophotometer allows to detect a cytochrome b-559 with a low redox potential and its participatation in the ferredoxin-mediated cyclic electron flow around photosystem I.Kinetic analysis of O_2^--production in thylakoids indicates also the O_2^--mediated cyclic electron flow around photosystem I.2.Scavenging of active oxygens in chloroplastsMicrocomparteantalization of the scavenging enzymes of active oxygens in chloroplasts was established by localization of the participating enzymes by fractionation of the organella and by immunoelectronmicrosccopy. It was found that the scavenging is proceeded by the thylakoidal and stromal systems. Further, molecular properties of ascorbate peroxidase and monodehydroascorbate reductase were characterized using the purifed enzymes and the overproducing enzymes in Escherichia coli.