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

摘要

Light-induced changes of b-type cytochromes inEuglenachloroplasts were studied spectrophotometrically.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 0.15µMEuglenacytochrome 552, or 100µM methyl viologen.Euglena chloroplasts, incubated at 55°C for 5 min showed no significant absorbance changes for about 10 min after the onset of illumination. However, greater photooxidation of cytochrome 558 was observed after prolonged illumination, or in the presence of DCMU or ethylenediaminetetraacetic acid (EDTA). Similar results were obtained with chloroplasts pre-treated at pH 9.0–10.0 for 5 min.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 photooxidized, 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.At pH 6.5 cytochrome 558, on cessation of 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.At pH 9.5 ascorbate and manganese served as electron donors for die DCMU-sensitive photooxidation of cytochromes 558 and 563.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 d