Illuminated chloroplasts can mediate a photoxidation of diketogulonic acid (DKGA) with rates of oxygen uptake equivalent to rates of Hill reactions with ferricyanide or quinone. The photoxidation of DKGA is sensitive to dichlorophenyl dimethylurea (DCMU) and exhibits the drop in quantum yield at long wavelengths characteristic of photosystem 11. Still, the reaction is only partially inactivated by heating chloroplasts at 50" for 10 min (which destroys oxygen evolution). The photoxidation is inhibited by copper and detergents; and is stimulated by added flavin (or methyl viologen) and manganous ions.A model system containing Mn3+ (as manganipyrophosphate) and DKGA, mimics the chloroplast system. Pre-illuminated chloroplast suspensions can be substituted for Mn3+ in the model dark reaction.I t seems that a light-dependent oxidation of MnZ+ to Mn3+ by photosystem I1 is the essential contribution of the chloroplasts. Electrons from Mn2+ move through the electron transport system to femcyanide or to photosystem I where, via flavin (or methyl viologen), oxygen is reduced to H,O,.
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