p- and o-Hydroquinones, which act as cofactors of aerobic cyclic photophosphorylation in isolated chloroplasts, are reoxidized by way of a photo-oxidation. This photo-oxidation is inhibited by dichlorphenyldimethylurea and o-phenanthrolin. The possible relationship of the photo-oxidation to other photosynthetic reactions is considered.
Very little, if any, phenoloxidase-activity can be detected in fresh chloroplasts. However, a polyphenoloxidase, with specifity towards o-hydroquinones, is liberated by breaking the lipid-structure of the chloroplast. This raises the question as to the role of a latent copper-enzyme in photosynthesis.
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