Differential modification by caffeine of oxygen-dependent and independent effects of gamma-irradiation on rat liver mitochondria

Purpose: Following the demonstration that caffeine effectively competes wit h oxygen for electrons and also scavenges hydroxyl radicals and singlet oxy gen, the differential modification of oxygen-dependent and independent effe cts of gamma-radiation by caffeine in membranes was examined, using rat liv er mitochondria as a model system. Materials and methods: Mitochondria were isolated from the livers of Wistar rats and exposed to gamma-radiation in the dose range of 45-600 Gy (dose r ate 15 Gy/min) in the presence or absence of caffeine. To examine the 'oxyg en effect', post-irradiation incubation was carried out in the presence of oxygen or nitrogen in buffers saturated wit the respective gases. Membrane damage was examined as lipid peroxidation (assessed as formation of thiobar bituric acid-reactive substances (TBARS), lipid hydroperoxides (LOOH) and c onjugated dienes (CD), protein oxidation, depletion of protein thiols, supe roxide dismutase or glutathione. Results: Lipid 45 to 600 Gy. Post-irradiation incubation of mitochondria un der nitrogen decreased the response, while incubation under oxygen saturati on enhanced it significantly. The presence of caffeine during radiation exp osure inhibited lipid peroxidation significantly as a function of concentra tion, in the range of 5 mu m to 4 mM. The inhibition was highest with 4 mM of caffeine. Under oxic conditions, inhibition at 1 mM was significantly mo re than under anoxia. Anoxia was either ineffective ro marginally increased peroxidation in the presence of caffeine. A similar observation was obtain ed when membrane damage was assessed as protein oxidation. Radiation-induce d depletion of protein thiols was greatly enhanced by oxygen saturation and this was completely prevented by caffeine. This compound also protected ag ainst the radiation-induced loss of the antioxidant glutathione and the enz yme superoxide dismutase. Conclusions: The results suggest that caffeine effectively protected membra nes against the oxic component of damage but may not do so for the anoxic c omponent.