The FAD-Enzyme Monodehydroascorbate Radical Reductase Mediates Photoproduction of Superoxide Radicals in Spinach Thylakoid Membranes |
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Authors: | Miyake, Chikahiro Schreiber, Ulrich Hormann, Henning Sano, Satoshi Asada, Kozi |
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Affiliation: | 1 Graduate School of Biological Science, Nara Institute of Science and Technology 8916-5 Takayama, Ikoma, Nara, 630-0101 Japan 2 JuliuS-von-Sachs-Institut für Biowissenschaften mit Botanischen Garten der Universität Würzburg Lehrstuhl für Botanik I, Mittlerer Dallenbergweg 64, D-97082 Würzburg, Germany 3 Research Institute of Innovative Technology for the Earth (RITE) Kizu, Kyoto, 619-0225 Japan 4 Department of Biotechnology, Faculty of Engineering, Fukuyama University Gakuencho-1, Fukuyama, 729-0292 Japan |
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Abstract: | The photoreduction of dioxygen in spinach thylakoid membraneswas enhanced about 10-fold by the FAD-enzyme monodehydroascorbateradical (MDA) reductase at 1µM. The primary photoreducedproduct of dioxygen catalyzed by MDA reductase was the superoxideradical, as evidenced by the inhibition of photoreduction ofCytc by superoxide dismutase. The apparent Km for dioxygen ofthe MDA reductase-dependent photoreduction of dioxygen was 100µM,higher by one order of magnitude than that observed with thylakoidmembranes only. Glutathione reductase, ferredoxin-NADP+ reductase,and glycolate oxi-dase also mediated the photoproduction ofsuperoxide radicals in thylakoid membranes at rates similarto those with MDA reductase. Among these flavoenzymes, MDA reductaseis the most likely mediator stimulating the photoreduction ofdioxygen in chloroplasts; its function in the protection fromphotoinhibition under excess light is discussed. (Received February 24, 1998; Accepted May 19, 1998) |
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