Respiratory activity of isolated rat liver mitochondria following in vitro exposure to oxygen species: a threshold study

Respiratory activity of isolated rat liver mitochondria was assayed following in vitro exposure to oxygen radicals. Our results show that mitochondrial respiration is more sensitive to O₂ ^•- than to H₂O₂ However, ferrous ions drastically enhance the toxicity of the enzymatic system generating H₂O₂ because of the production of the hydroxyl radicals. A protection against those oxygen species could be given by SOD in the xanthine/xanthine oxidase system and by catalase with the glucose/glucose oxidase system. The most damaging system was the combination of Fe²⁺ with H₂O₂. In this case, OH^• is formed in a Fenton-like reaction. The fact that the OH²⁺ with the most damaging molecule accounts for the finding that catalase and desferrioxamine were efficient protectors in this system. Threshold levels of O₂ ^•- and H₂O₂ able to inhibit the mitochondrial respiration have been estimated. It is concluded that under normal respiration such thresholds are not reached in vivo and that the impairment of the mitochondrial respiratory activity does not seem to originate only from the natural free radical production in those organelles. However, if the production of free radicals is such to exceed the defense capability, like under oxidative stress, then the critical threshold can be surpassed and the respiration impaired leading to irreversible damages.