Formation of superoxide anion and carbon-centered radicals by photosystem II under high light and heat stress-EPR spin-trapping study.

In this study, electron paramagnetic resonance spin-trapping spectroscopy was used to study the light-induced production of superoxide anion (O2 (•-)) and carbon-centered (R(•)) radicals by Photosystem II (PSII). It is evidenced here that exposure of PSII membranes to high light (2,000 μmol photons m(-2) s(-1)) or heat (47 °C) treatments prior to the illumination suppressed O2 (•-) production, while R(•) was formed. Formation of R(•) in the both high light- and heat-treated PSII membranes was enhanced by DCMU. Removal of molecular oxygen by glucose/glucose oxidase/catalase system and O2 (•-) scavenging by exogenous superoxide dismutase completely suppressed carbon-centered radical formation. It is proposed here that the oxidation of polyunsaturated fatty acids and amino acids by O2 (•-) on the electron acceptor side of PSII results in the formation of R(•), known to initiate a cascade reaction leading to the lipid peroxidation and protein degradation, respectively.