Superoxide reduction as a mechanism of ascorbate-stimulated oxygen uptake by isolated chloroplasts |
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Authors: | J F Allen D O Hall |
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Institution: | University of London, King''s College, Botany Department, 68 Half Moon Lane, London SE24 9JF UK |
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Abstract: | Addition of 1m ascorbate to isolated chloroplasts with methyl viologen (MV) as electron acceptor trebled the rate of oxygen uptake and decreased the ratio to a third of that with no ascorbate present. These effects of ascorbate were reversed by superoxide dismutase (SOD), which in the absence of ascorbate had little effect on O2 uptake or ratio. A chloroplast-associated SOD activity equivalent to 500 units/mg chlorophyll was detected. The effects of ascorbate and SOD on O2 uptake were similar in both coupled and uncoupled chloroplasts. The results are consistent with the hypothesis that ascorbate stimulates O2 uptake by reduction of superoxide, which is formed by autoxidation of the added electron acceptor (MV), and which dismutates in the absence of ascorbate. Ascorbate does not seem to stimulate O2 uptake by replacing water as the photosystem II donor. |
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