Polyphenol Oxidation by Vicia faba Chloroplast Membranes

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Abstract
The mechanism whereby light effects polyphenol oxidation was examined with V. faba chloroplast membranes known to contain a bound latent polyphenol oxidase. Results obtained with the inhibitors 3-(3'',4''-dichlorophenyl)-1,1-dimethylurea (DCMU) and 2,5-dibromo-3-methyl-6-idopropyl-p-benzoquinone (DBMIB) indicated an involvement of the noncyclic electron transport pathway in the light-dependent oxidation of polyphenols, such as dihydroxyphenylalanine (DOPA). DOPA replaced H2O as electron donor for the photoreduction of NADP. NADP replaced O2 as electron acceptor in the photochemical oxidation of DOPA. The variable fluorescence associated with photosystem II was increased by DOPA. The photochemical oxidation of DOPA by V. faba chloroplast membranes was insensitive to KCN and to antibodies against purified latent polyphenol oxidase. The light-dependent oxidation of polyphenols by V. faba chloroplast membranes is achieved independently of the latent membrane-bound polyphenol oxidase. Electrons derived from polyphenols seem to enter the noncyclic electron transport chain on the oxidizing side of photosystem II and to react with O2 at an unidentified site on the photosystem I side of the DCMU/DBMIB blocks. The physiological mechanism for the activation of latent polyphenol oxidase remains an unanswered question. Apparently activation could occur through either acidification or the release of free fatty acids.