Light-induced changes of b-type cytochromes in the electron transport chain of Euglena chloroplasts

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Light-induced changes of b-type cytochromes in Euglena chloroplasts were studied spectrophotometrically. 1. In the dark and at pH 6.5, most of the cytochrome 558 in chloroplasts was in the reduced state, and most of the cytochrome 563, in the oxidized state. Illumination of chloroplasts at pH 6.5 induced a rapid, but slight oxidation of cytochrome 552 and cytochrome 558. The magnitude of photooxidation of cytochrome 558 was greatly enhanced by the addition of 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU). The rate of photooxidation in the presence of DCMU was stimulated by the addition of 0.15μ_M Euglena cytochrome 552, or 100μ_M methyl viologen. 2. Euglena chloroplasts, incubated at 55℃ for 5 min showed no significant absorbance changes for about 10 min after the onset of illumination. However, greater photo-oxidation of cytochrome 558 was observed after prolonged illumination* or in the presence of DCMU or ethylenediaminetetraacetic acid (EDTA). Similar results were obtained with chloroplasts protracted at pH 9.0-10.0 For 5min. 3. At pH 9.5, and in the dark, both cytochrome 563 and cytochrome 558 were in an almost reduced state. On illumination at this pH, both cytochromes were photo-oxidized, with a complicated kinetics, showing an initial rapid and small absorbance decrease, followed by a stagnant. phase of temporary retarded reaction. In the presence of DCMU or EDTA, photooxidation proceeded rapidly without a stagnant phase. 4. At pH 6.5 cytochrome 558, on cessation on illumination, was quickly reduced to the initial level. At pH 9.5, there was also appreciable re-reduction of cytochrome 558 and 563 when the light was turned off at an early stage of illumination. The amounts of re-reduction of the cytochromes in the subsequent dark period, however, decreased as photooxidation of cytochromes proceeded. This decrease was accelerated by the presence of DCMU. 5. At pH 9.5 ascorbate and manganese served as electron donors for the DCMU-sensitive photooxidation of cytochromes 558 and 563. 6. Experimental results are discussed with special reference to the occurrence of two pools of electron carriers, one at the reducing side and the other at the oxidizing side of photosystem 2. The role of manganese in the latter pool of electron carriers is also discussed.
日本植物生理学会の論文

Light-induced changes of b-type cytochromes in the electron transport chain of Euglena chloroplasts

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