Über eine p-Hydroxylierung in isolierten Chloroplasten

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Abstract
Salicylaldoxime at a concentration of 10-2[image] is an inhibitor of the Hill reaction and therefore also of aerobic photophosphorylation in isolated chloroplasts. This might indicate a functional role for Cu in the electron transport system of photosynthesis. At a concentration of 10-3[image] and lower, salicylaldixime is not an inhibitor, but a cofactor of aerobic photophosphorylation. This is due to its hydroxylation to the p-hydroquinone, which is the actual cofactor. This p-hydroxylation, which is probably catalyzed by a peroxidase, takes place only with salicylaldehyde and its oxime, but not with other phenols, whether they have a carbonyl function attached to the phenol ring or not. A number of o- and p-hydroquinones with a carbonyl function at the phenol ring and 2 naphthohydroquinones, of which the corresponding quinone cannot be prepared by chemical means, are reversibly oxidized and reduced in photosynthetic phosphorylation in chloroplasts. A possible explanation is that these hydroquinones are oxidized only to the semiquinone level and rapidly reduced again.