May Cytochrome b-559 function to protect photosystem II from ligh stress?

用NH_2 OH处理的光系统II膜研究了光抑制对细胞色素B-559氧化还原性质的影响。该膜含有细胞色素B-559的三种氧化还原形式（HP-、IP-和LP-），Em值分别为+435、+237和+45mV。在光系统II供体侧的光抑制过程中产生了一种新的细胞色素B-559的中间电位形式：光抑制照明（7，000 μ Em-^2s^1）作用1分钟，诱导HP-型水平下降28%，并伴随产生中间电位（IP ′-）形式，其Em值约为+350mV。导致HP-型的完全丧失和IP-型水平的明显增加。在进一步的光抑制处理（60分钟）后，观察到IP '-形式的完全损失，IP-和LP-形式的水平各自增加至细胞色素b-559总量的约50%。动力学分析表明，HP型通过两种中间势形式转化为LP型HP-和IP '-的衰变动力学分析表明，HP-向IP'-的转化与Tyr_Z的光失活平行发生，IP ′-型的消失反映了Tyr_D的命运。根据这些发现，我们认为HP-和IP '-型细胞色素B-559分别向Tyr_Z提供电子，以抑制其过氧化，从而防止光系统II的光抑制。

Effects of photoinhibition on the redox properties of cytochrome b-559 were studied with NH_2OH-treated photosystem II membranes, which are depleted of the water-oxidizing complex. The membranes contained three redox forms (HP-, IP-and LP-forms) of cytochrome b-559, with Em valuse of +435, +237 and +45mV,respectively. A novel intermediate-potential form of cytochrome b-559 was generated during photoinhibition on the donor side of photosystem II : photoinhibitory illumination (7,000 muEm-^2s^1) for 1 min induced a 28% decrease in the level of the HP-form, with concomitant generation of an intermediate-potential (IP'-) form whose Em value was about+350mV.Prolonged illumination (10min) resulted in complete loss of the HP-form and an apparent increase in the level of the IP-form. After further photoinhibitory treatment (60min), complete loss of the IP'-form was observed and levels of the IP- and LP-forms each increased to about 50% of the total amount of cytochromeb-559. Kinetic analysis of these data led to the conclusion that the HP-form is converted to the LP-form via two intermediate-potential forms (IP' and IP), and that IP'-form appears only at the early phase of photoinhibition.Kinetical analysis of the decays of the HP- and IP'-forms shows that the conversion of the HP- to the IP'-form occures in parallel with the photoinactivation of Tyr_Z, and that the disapperance of the IP'-form reflects the fate of Tyr_D. On these findings, we propose that the HP- and IP'-forms of cyotochrome b-559 donate electrons to Tyr_Z, respectively, to suppress their overoxidation, and so prevent the photoinhibition of the phtosystem II.