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

研究了光抑制对细胞色素b-559氧化还原性能的影响，采用nh_2oh处理光系统II膜，去除水氧化配合物。膜中含有三种氧化还原形式的细胞色素b-559 (HP-、ip -和lp -)， Em值分别为+435、+237和+45mV。在光抑制过程中，在光系统II的供体侧产生了一种新的中间电位形式的细胞色素b-559：光抑制光照（7,000 μ m-^2s^1） 1 min，导致hp形式水平下降28%，同时产生一种中间电位形式（IP'-），其Em值约为+350mV。长时间光照（10min）导致hp形态完全丧失，ip形态水平明显升高。在进一步的光抑制处理（60min）后，观察到IP'-形态完全消失，IP-形态和lp -形态的水平分别增加到细胞色素-559总量的50%左右。对这些数据的动力学分析得出结论，hp形式通过两种中间电位形式（IP‘和IP）转化为lp形式，IP’形式仅在光抑制的早期阶段出现。HP-和IP'-衰变的动力学分析表明，HP-向IP'-的转变与Tyr_Z的光失活同时发生，IP'-的消失反映了Tyr_D的命运。基于这些发现，我们提出细胞色素b-559的HP-和IP'-形式分别给Tyr_Z电子来抑制它们的过度氧化，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.