Photosynthetic oxidation of sulphide by green plants. II

The photosynthetic oxidation of sulphide by green plants is inhibited by DCMU, o-phenanthroline and hydroxylamine. This means that reaction system II of photosynthesis is necessary for the oxidation reaction. — The powerful inhibitors of photophosphorylation, antimycin A and carbonyl-cyanide-phenylhydrazone, also depress the oxidation rate. This effect may be a secondary one, due to a diminished supply of necessary ATP. Mn-deficiency in the organisms results in an increase of S^---oxidation, indicating that S^-- is oxidized by the (OH)-radicals involved in the O₂-evolving system of photosynthesis according to the following equation: $$2(OH) + 2H^ + + S^--- \to 2H^ + + 2OH^--- + S^0 .$$ Salicylaldoxime in rather high concentrations likewise stimulates the photosynthetic oxidation of sulphide. Concentrations that completely block photosynthetic electron flow also stop sulphide oxidation. On the other hand, concentrations of this inhibitor which only lower photosynthetic electron flow bring about a marked increase of sulphide oxidation. We interpret this effect according to the concept of two photochemical systems that interact in series in the following way: salicylaldoxime decreases electron flow between the two reaction systems; reaction system I now makes available electrons from the sulphide ion which can enter the remaining lowered photosynthetic electron flow. Ferrous ions are necessary for this oxidation reaction and may take part in enzymatic reactions involved in this process. Die das Reaktionssystem II der Photosynthese hemmenden Substanzen DCMU, o-Phenanthrolin und Hydroxylamin unterbinden auch die photosynthetische S^---Oxydation. Die Phosphorylierungsgifte Antimycin A und Carbonylcyanidphenylhydrazon hemmen Photosynthese und Sulfidoxydation über Stunden hinaus. Mn-Mangel führt zu einer Steigerung der S^---Oxydation, was den Schluß zuläßt, daß die Reaktion im O₂-entwickelnden System abläuft. Ein Zusatz von Salicylaldoxim hat ebenfalls eine Beschleunigung der S^---Oxydation zur Folge. Dies deutet — zumindest für den Fall der Salicylaldoxim-Behandlung — auf eine Oxydation des Sulfids im Elektronentransport nach der Hemmstelle des Cu-Komplexbildners. Eisen ist für den Vorgang notwendig und läßt an eine Beteiligung Fe-haltiger Enzyme denken. Im diskutierten Photosyntheseschema ist S^-- allein schon wegen seiner Redox-Potentiale ein möglicher Elektronendonator für die CO₂-Reduktion.