Photooxidation of ferrocyanide and iodide ions and associated phosphorylation in NH₂OH-treated chloroplasts

NH₂OH-treated, non-water oxidizing chloroplasts are shown to be capable of oxidizing ferrocyanide and I^- via Photosystem II at appreciable rates (≥ 200 μequiv/h per mg chlorophyll). Using methylviologen as electron acceptor, ferrocyanide oxidation can be measured as O₂ uptake, as ferricyanide formation, or as H⁺ consumption (2 Fe²⁺ + 2H⁺ + O₂ → 2 Fe³⁺ + H₂O₂). I^- oxidation can be measured as methylviologen-mediated O₂ uptake, or spectrophotometrically, using ferricyanide as electron acceptor. The oxidation product I₂ is re-reduced, as it is formed, by unknown reducing substances in the reaction system. The rate-saturating concentrations of these donors are very high: 30 mM with ferricyanide and 15 mM with I^-. Relatively lipophilic Photosystem II donors such as catechol, benzidine and p-aminophenol saturate the photooxidation rate at much lower concentrations (< 0.5 mM). It thus seems that the oxidation of hydrophilic reductants such as ferricyanide and I^- is limited by permeability barriers. Very likely the site of Photosystem II oxidation is embedded in the thylakoid membrane or is situated on the inner surface of the membrane. The efficiency of phosphorylation (P/e₂) is 0.5 to 0.6 with ferrocyanide and about 0.5 with I^-. In contrast the P/e₂ ratio is 1.0 to 1.2 when water, catechol, p-aminophenol or benzidine serves as electron donor. These differences imply that only one of two phosphorylation sites operate when ferrocyanide and I^- are oxidized. Ferrocyanide and I^- are also chemically distinct from other Photosystem II donors in that their oxidation does not involve proton release. It is suggested that the mechanism of energy conservation associated with Photosystem II may be only operative when the removal of electrons from the donor results in release of protons (i.e. with water, hydroquinones, phenylamines, etc.).