Tamura N, Kuwahara M, Sasaki Y, Wakamatsu K, Oku T
Laboratory of Plant Physiology, Faculty of Human Environmental Science, Fukuoka Women's University, Japan.
Biochemistry. 1997 May 20;36(20):6171-7. doi: 10.1021/bi962659g.
Effects of reducing reagents and redox potentials on photoactivation were studied in Mn-depleted chloroplasts and PSII membranes. Exogenous reducing reagents abolished photoactivation in PSII membranes, while they stimulated photoactivation in chloroplasts. To determine how reducing reagents can have such opposing effects in these preparations, we studied how redox potentials affect photoactivation in the range from 0 mV to +500 mV. In chloroplasts, a modest yield of photoactivation was obtained in the redox potential range of +100 and +330 mV at pH 7.5. The yield of photoactivation decreased at redox potentials above +330 mV, and drastically increased below potentials of +100 mV. Nernst plots of the data show that an n = 1 redox component with an Em7.5 of +374 mV, as well as an n = 2 redox component with an Em7.5 of +61 mV, is involved in photoactivation of chloroplasts isolated from dark-grown spruce seedlings. In the case of PSII membranes, photoactivation decreased sharply on either side of +335 mV at pH 5.5. The n = 1 redox components with Em5.5 of +375 and +319 mV may be involved, both of which showed pH dependences of -60 mV/pH unit. DCMU abolished photoactivation in chloroplasts, but did not affect the dependence of photoactivation on oxidation-reduction potentials in PSII membranes. The component with an Em5.5 of +319 mV involved in photoactivation of PSII membranes was also observed in the dependence of Mn solubilization on oxidation-reduction potentials with PSII membranes lacking extrinsic proteins, suggesting that the reduction of Mn with higher valences to Mn(II) by exogenous reducing reagents reversibly occurs in the intermediates or an active center during photoactivation in PSII membranes. Involvement of such redox components in photoactivation in chloroplasts and PSII membranes is discussed.
在缺锰的叶绿体和光系统II（PSII）膜中研究了还原剂和氧化还原电位对光激活的影响。外源还原剂消除了PSII膜中的光激活，而它们在叶绿体中刺激了光激活。为了确定还原剂在这些制剂中如何产生如此相反的作用，我们研究了氧化还原电位在0 mV至+500 mV范围内如何影响光激活。在叶绿体中，在pH 7.5时，在+100和+330 mV的氧化还原电位范围内获得了适度的光激活产率。在高于+330 mV的氧化还原电位下，光激活产率下降，而在低于+100 mV的电位下急剧增加。数据的能斯特图表明，Em7.5为+374 mV的n = 1氧化还原组分以及Em7.5为+61 mV的n = 2氧化还原组分参与了从黑暗生长的云杉幼苗分离的叶绿体的光激活。在PSII膜的情况下，在pH 5.5时，在+335 mV两侧光激活急剧下降。Em5.5为+375和+319 mV的n = 1氧化还原组分可能参与其中，两者均显示出-60 mV/pH单位的pH依赖性。敌草隆（DCMU）消除了叶绿体中的光激活，但不影响PSII膜中光激活对氧化还原电位的依赖性。在缺乏外在蛋白的PSII膜中，Mn溶解对氧化还原电位的依赖性中也观察到了参与PSII膜光激活的Em5.5为+319 mV的组分，这表明在PSII膜光激活过程中，外源还原剂将高价态的Mn还原为Mn(II)在中间体或活性中心可逆地发生。讨论了这些氧化还原组分在叶绿体和PSII膜光激活中的作用。
