chiniofon and 2-iodo-6-isopropyl-3-methyl-2--4-4--trinitrodiphenyl-ether

We show that CCCP, known as an uncoupler of photophosphorylation and an ADRY agent, inhibits FeCy photoreduction and coupled O2 evolution by isolated chloroplasts equally (I50 approximately 2 microM), but is practically without effect on the O2 evolution coupled with SiMo reduction within the 0.2-10 microM concentration range. CCCP has no effect on the nanosecond chlorophyll fluorescence in chloroplasts incubated at low light intensity, but decreases it at high light intensity. The electron transfer from reduced TMPD or duroquinol to methylviologen is resistant to CCCP. The efficiency of the CCCP inhibitory action on the FeCy photoreduction depends on the rate of electron flow, which is controlled by the light intensity. The data obtained show that CCCP is oxidized by the photosystem II donor side and is reduced by QP, competing for electrons with FeCy and the cytochrome blf complex.

Topics: Carbonyl Cyanide m-Chlorophenyl Hydrazone; Chloroplasts; Cytochrome b Group; Dibromothymoquinone; Diuron; Electron Transport; Fabaceae; Ferricyanides; Hydroxyquinolines; Light-Harvesting Protein Complexes; Molybdenum; Oxidation-Reduction; Photosynthetic Reaction Center Complex Proteins; Photosystem II Protein Complex; Plants, Medicinal; Silicon; Silicon Compounds; Tetramethylphenylenediamine; Trinitrobenzenes

The reconstituted system containing Photosystem I, plastocyanin and the cytochrome b6-f complex is used to study the effects of various quinone analogues on the redox behavior of cytochrome b6. The effects of DBMIB, DNP-INT and HQNO are compared in an attempt to discern the modes of action of these quinone analogues. Both DBMIB and DNP-INT are potent inhibitors of the plastocyanin reductase activity of the isolated cytochrome complex. However, while DBMIB abolished the oxidant-induced reduction of cytochrome b6, DNP-INT only inhibited about 25% of the net reduction. On the other hand, HQNO does not show any significant inhibition of plastocyanin reductase activity of the isolated cytochrome complex at concentrations up to 20 microM. An enhancement of the net amount of cytochrome b6 reduced is observed in the presence of HQNO. Both DNP-INT and HQNO inhibited the dark oxidation rate of cytochrome b6. The possible identity of the oxidant for cytochrome b6 is discussed. Plastoquinone is concluded to be the most likely candidate. DNP-INT is concluded to have at least two sites of inhibition in the cytochrome complex. The implications of these findings on quinone functions in the cytochrome b6-f complex are discussed.

Topics: Cytochrome b Group; Cytochrome b6f Complex; Cytochromes; Cytochromes f; Dibromothymoquinone; Hydroxyquinolines; Kinetics; Oxidation-Reduction; Oxygen; Photosynthesis; Plants; Quinones; Trinitrobenzenes