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Polyphenol Oxidation by Vicia faba Chloroplast Membranes

STUDIES ON THE LATENT MEMBRANE-BOUND POLYPHENOL OXIDASE AND ON THE MECHANISM ¹ OF PHOTOCHEMICAL POLYPHENOL OXIDATION

Paolo Montalbini

Department of Cell Physiology, University of California, Berkeley, California 94720, Institute of Plant Pathology, University of Perugia, Perugia, Italy

The mechanism whereby light effects polyphenol oxidation was examined with Vicia 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 non-cyclic electron transport pathway in the light-dependent oxidation of polyphenols, such as dihydroxyphenylalanine (DOPA). Further evidence was provided by experiments in which (a) DOPA replaced H₂O as electron donor for the photoreduction of NADP, (b) NADP replaced O₂ as electron acceptor in the photochemical oxidation of DOPA, and (c) 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 results are consistent with the conclusion that 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. Present results suggest that activation could occur through either acidification or the release of free fatty acids.

¹ This work was supported in part by the United States-Italy Cooperative Science Program through grants from the National Science Foundation (to B. B. B.) and from the Consiglio Nazionale delle Ricerche (to P. M.).