Menadione (vitamin K3) was found to be completely reduced by illuminated spinach chloroplasts under highly anaerobic condition and in the presence of ethylenediamine tetraacetate (EDTA) in the reaction mixture. This photoreductive reaction is sensitive toward heat-treatment and inhibited by 2×10-3Mhydroxylamine.In the presence of oxygen, the reduced form of menadione is rapidly photooxidized by chloroplasts. This photooxidative activity also is suppressed by heat-treatment but not inhibited by hydroxylamine.Dyes which are inefficient as HILL oxidants such as thionine and methylene blue were found to be readily reduced by illuminated chloroplasts, if the experimental conditions were appropriate to prevent the reoxidation of the photoreduced dyes;i.e., exhaustive removal of oxygen and the addition of EDTA in the reaction mixture. Menadione was found to accelerate the HILL reaction with these dyes as oxidant under such experimental conditions.In the presence of molecular oxygen in the reaction mixture, menadione was found to inhibit the HILL reaction with 2,6–dichlorophenol indophenol as oxidant, while the reaction rate was little influenced in high anaerobiosis.These findings are explained by the intermediary oxidation and (photo-) reduction of menadione as an intermediary hydrogen carrier, and by the trends toward rapid photooxidation of reduced menadio
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