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 (O-2 (aEuro cent-)) and carbon-centered (R-aEuro cent) radicals by Photosystem II (PSII). It is evidenced here that exposure of PSII membranes to high light (2,000 mu mol photons m(-2) s(-1)) or heat (47 A degrees C) treatments prior to the illumination suppressed O-2 (aEuro cent-) production, while R-aEuro cent was formed. Formation of R-aEuro cent 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 O-2 (aEuro cent-) 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 O-2 (aEuro cent-) on the electron acceptor side of PSII results in the formation of R-aEuro cent, known to initiate a cascade reaction leading to the lipid peroxidation and protein degradation, respectively.