Jp. Kamat et al., Differential modification by caffeine of oxygen-dependent and independent effects of gamma-irradiation on rat liver mitochondria, INT J RAD B, 76(9), 2000, pp. 1281-1288
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.