Participation of plastoquinone,cytochrome c 553 and ferrodoxin-NADP+ oxido-reductase in both photosynthesis and respiration in Spirulina maxima

Dark and light oxidation of NADPH was measured in Spirulina maxima thylakoid membranes. The dark reaction was more cyanide sensitive than the light reaction. In light, 83% of the electrons from NADPH produced H₂O₂ on reducing oxygen, whereas in the dark this number was only 36%. These results are explained by assuming the presence of an electron transport segment common to the photosynthetic and the respiratory chains, so that electrons flowing through the cyanide sensitive oxidase in the dark are diverted to the photosytem (PS) I reaction center (P700). In addition, cytochrome (cyt) c ₅₅₃ was found to be an electron donor for both cyt oxidase and P700. Half maximum reduction rates were obtained with 7 M cyt c ₅₅₃. The intrathylakoidal concentration of cyt c ₅₅₃ was determined to be 83 M. About 60% of the respiratory NADPH oxidation activity was lost by extracting the membranes with pentane and was restored by adding plastoquinone (the main photosythetic quinone). NADPH oxidation activity was also inhibited upon washing the membranes with a low salt buffer. This activity was restored by adding partially purified ferredoxin-NADP⁺ oxido-reductase (FNR). A model for the electron transport in thylakoids, in which cyt c ₅₅₃, plastoquinone and FNR participate in both photosynthesis and respiration is proposed.