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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; Kozi Asada |
| Institution: | 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 |
| Abstract: | The photoreduction of dioxygen in spinach thylakoid membraneswas enhanced about 10-fold by the FAD-enzyme monodehydroascorbateradical (MDA) reductase at 1 µM. The primary photoreducedproduct of dioxygen catalyzed by MDA reductase was the superoxideradical, as evidenced by the inhibition of photoreduction ofCyt c by superoxide dismutase. The apparent Km for dioxygenof the MDA reductase-dependent photoreduction of dioxygen was100 µM, higher by one order of magnitude than that observedwith thylakoid membranes only. Glutathione reductase, ferredoxin-NADP+reductase, and glycolate oxidase also mediated the photoproductionof superoxide 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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