Participation of Peroxidase in the Metabolism of 3,4-Dihydroxyphenylalanine and Hydrogen Peroxide In Vacuoles of Vicia faba L. Mesophyll Cells :

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When leaves of Vicia faba were treated with H_2O_2 Or visible light in the presence of methyl viologen (MV), the orange-red compound dopachrome was formed transiently and melanin was accumulated. With the darkening of leaves, the level of 3,4-dihydroxyphenylalanine (DOPA) decreased and then recovered to the original level upon addition of 1 mM H_2O_2. However, if leaves were incubated in the presence of 10 mM H_2O_2, the level of DOPA decreased again after the increase. The time course of the changes in levels of DOPA observed during the accumulation of melanin as a result of illumination in the presence of MV was very similar to that observed after the addition of 10 mM H_2O_2. Illumination of leaves in the absence of MV did not result in any accumulation of melanin, but the level of DOPA changed slightly. When isolated mesophyll cells were incubated in the dark, the level of DOPA decreased. Illumination of the cells stimulated this decrease. Tropolone, an inhibitor of phenol oxidase, did not inhibit and actually stimulated the H_2O_2- and light-induced oxidation of DOPA and accumulation of melanin in leaves. Tropolone also stimulated the decrease in the levels of DOPA both in the dark and in the light in isolated mesophyll cells. These data suggest that a peroxidase-H_2O_2 system, and not phenol oxidase, participates in the oxidation of DOPA. When DOPA was oxidized by a basic peroxidase isolated from V. faba leaves, an intermediate, which was perhaps dopaquinone and which was reducible by ascorbate, was formed. Based on the data, a discussion is presented of the physiological significance of the oxidation of DOPA by peroxidase in vacuoles.

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