Production of superoxide in chloroplast thylakoid membranes: ESR study with cyclic hydroxylamines of different lipophilicity

Accumulation of nitroxide radicals, DCP or TMT, under illumination of a thylakoid suspension containing either hydrophilic, DCP-H, or lipophilic, TMT-H, cyclic hydroxylamines that have high rate constants of the reaction with superoxide radicals, was measured using ESR. A slower accumulation of TMT in contrast with DCP accumulation was explained by re-reduction of TMT by the carriers of the photosynthetic electron transport chain within the membrane. Superoxide dismutase suppressed TMT accumulation to a lesser extent than DCP accumulation. The data are interpreted as evidencing the production of intramembrane superoxide in thylakoids.