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

Light-induced changes of b-type cytochromes in Euglena chloroplastswere studied spectrophotometrically.

1. In the dark and at pH 6.5, most of the cytochrome 558 in chloroplastswas in the reduced state, and most of the cytochrome 563, inthe oxidized state. Illumination of chloroplasts at pH 6.5 induceda rapid, but slight oxidation of cytochrome 552 and cytochrome558. The magnitude of photooxidation of cytochrome 558 was greatlyenhanced by the addition of 3-(3',4'-dichlorophenyl)-1,1-dimethylurea(DCMU). The rate of photooxidation in the presence of DCMU wasstimulated by the addition of 0.15 µM Euglena cytochrome552, or 100 µM methyl viologen.
2. Euglena chloroplasts,incubated at 55°C for 5 min showedno significant absorbancechanges for about 10 min after theonset of illumination. However,greater photooxidation of cytochrome558 was observed afterprolonged illumination, or in the presenceof DCMU or ethylenediaminetetraaceticacid (EDTA). Similar resultswere obtained with chloroplastspre-treated at pH 9.0–10.0for 5 min.
3. At pH 9.5, andin the dark, both cytochrome 563 and cytochrome558 were inan almost reduced state. On illumination at thispH, both cytochromeswere photooxidized, with a complicatedkinetics, showing aninitial rapid and small absorbance decrease,followed by a stagnantphase of temporary retarded reaction.In the presence of DCMUor EDTA, photooxidation proceeded rapidlywithout a stagnantphase.
4. At pH 6.5 cytochrome 558, on cessation of illumination,wasquickly reduced to the initial level. At pH 9.5, there wasalsoappreciable re-reduction of cytochrome 558 and 563 whenthelight was turned off at an early stage of illumination.Theamounts of re-reduction of the cytochromes in the subsequentdark period, however, decreased as photooxidation of cytochromesproceeded. This decrease was accelerated by the presence ofDCMU.
5. At pH 9.5 ascorbate and manganese served as electrondonorsfor die DCMU-sensitive photooxidation of cytochromes558 and563.
6. Experimental results are discussed with specialreference tothe occurrence of two pools of electron carriers,one at thereducing side and the other at the oxidizing sideof photosystem2. The role of manganese in the latter pool ofelectron carriersis also discussed.

(Received March 11, 1970; )