Spin-trap study of the reactions of ferredoxin with reduced oxygen species in pea chloroplasts

Abstract

Illumination of pea chloroplasts in the presence of the spin trap 5,5-dimethyl-1-pyrroline- N-oxide results in the formation of the superoxide spin adduct, an effect enhanced by addition of artificial Photosystem I electron acceptors such as paraquat. Addition of spinach ferredoxin alone slightly enhances superoxide formation, but causes inhibition of superoxide formation when the rate has been increased by added paraquat. This is not accompanied by an inhibition of O 2 uptake, and provides evidence that ferredoxin reduces superoxide to hydrogen peroxide. Decay of the superoxide adduct signal under illumination is not accompanied by the appearance of hydroxyl adduct. This signal, however, appears on cessation of illumination, and disappears on reillumination. This is probably due to conversion of the hydroxyl adduct into a non-radical species by a redox reaction linked to photosynthetic electron transport, since the light-induced decrease in this signal is inhibited by the photosynthetic electron-transport inhibitor 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea. Both Fe 3+ chelated by diethylenetriaminepentaacetic acid, and spinach ferredoxin, reduced by the photosynthetic electron-transport chain, react with hydrogen peroxide to produce hydroxyl radicals in a Fenton-type process.