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  • Title: Photoreactions of cytochrome b6 and cytochrome f in chloroplast photosystem I fragments.
    Author: Böhme H.
    Journal: Z Naturforsch C Biosci; 1976; 31(1-2):68-77. PubMed ID: 5829.
    Abstract:
    Photosystem I fragments were prepared by digitonin treatment of spinach chloroplasts. The midpoint potential of cytochrome b6 in the fragments is close to 0V, showing a one electron transition. No cytochrome b559 was detectable, neither in difference absorption spectra nor in light-induced absorbance changes. In the absence of added cofactors only cytochrome b6 photoreduction can be observed. This photoreduction is stimulated by ferredoxin. Ferredoxin-NADP+ reductase appears not to be involved in cytochrome b6 reduction. Photooxidation of cytochrome b6 is dependent on plastocyanin addition and inhibited by DBMIB, a plastoquinone antagonist. Addition of plastocyanin restores cytochrome f photooxidation as well, reacting quite specifically in about equimolar concentrations to bound cytochrome f. The stimulation of cytochrome f oxidation is abolished by an antibody prepared against plastocyanin, indicating a surface location of plastocyanin in digitonin treated membranes. Biphasic kinetics of dark-reduction of cytochrome f by ascorbate indicate that part of this cytochrome f is relatively inaccessible in the membrane. After preillumination a monophasic reduction is observed and the slowly oxidized component is absent. Illumination in the presence of plastocyanin causes a fast and complete reduction of cytochrome f, suggesting equilibration of cytochrome f with added plastocyanin, residing in the membrane surface. It appears that actinic light causes conformation and/or structural changes in the membrane of these digitonin fragments, influencing cytochrome f asseccibility.
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