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Photophosphorylation Associated with Photosystem II

II. Effects of Electron Donors, Catalyst Oxidation, and Electron Transport Inhibitors on Photosystem II Cyclic Photophosphorylation

¹ Department of Cell and Molecular Biology, Division of Biological Sciences, The University of Michigan, Ann Arbor, Michigan 48109

Incubation of KCN-Hg-NH₂OH-inhibited spinach (Spinacia oleracea L.) chloroplasts with p-phenylenediamine for 10 minutes in the dark prior to illumination produced rates of photosystem II cyclic photophosphorylation up to 2-fold greater than the rates obtained without incubation. Partial oxidation of p-phenylenediaine with ferricyanide produced a similar stimulation of ATP synthesis; addition of dithiothreitol suppressed the stimulation observed with incubation. Addition of ferricyanide in amounts sufficient to oxidize completely p-phenylenediamine failed to inhibit completely photosystem II cyclic activity. This is due at least in part to the fact that the ferrocyanide produced by oxidation of p-phenylenediamine is itself a catalyst of photosystem II cyclic photophosphorylation. N,N,N'N'-Tetramethyl-p-phenylenediamine catalyzes photosystem II cyclic photophosphorylation at rates approaching those observed with p-phenylenediamine. The activities of both proton/electron and electron donor catalysts of the photosystem II cycle are inhibited by dibromothyoquinone and antimycin A. These findings are interpreted to indicate that photosystem II cyclic photophosphorylation requires the operation of endogenous membrane-bound electron carriers for optimal coupling of ATP synthesis to electron transport.