Slow Oxygen Release on the First Two Flashes in Chemically Stressed Photosystem II Membrane Fragments Results from Hydrogen Peroxide Oxidation

Flash-induced amperometric signals were measured with a Joliot-type O₂ rate electrode in spinach Photosystem II (PS II) membrane fragments exposed to very low concentrations of added hydroxylamine or hydrogen peroxide. In both cases 'anomalous O₂ signals' were observed on the first two flashes, and oscillating four-flash patterns were observed on subsequent flashes. The anomalous signals were eliminated in the presence of catalase but not EDTA. The rise times of the O₂-release kinetics associated with the anomalous signals were slow (ca. 20 ms with NH₂OH and ca. 120 ms with H₂O₂) compared to the kinetics of O₂ release on subsequent flashes and in control membranes (3-6 ms). It is proposed that when the intact PS II O₂-evolving complex is perturbed with small concentrations of added reductant, H₂O₂ can gain access and bind to the complex. Bound H₂O₂ can then reduce lower S states in some centers leading to anomalous O₂ signals on the first two flashes. A model is presented to explain both types of anomalous O₂ production. Oxygen observed on the third and subsequent flashes is due to the normal photosynthetic O₂-evolution process arising from the S₃-state. Anomalous O₂ production could be a protective mechanism in PS II centers subjected to stress conditions.