Role of enzymatic and non-enzymatic processes in H2O2 removal by rat liver and heart mitochondria

We compared the capacity of rat liver and heart mitochondria to remove exogenously produced H₂O₂, determining their ability to decrease fluorescence generated by H₂O₂ detector system. In the absence of substrates, liver and heart mitochondria removed H₂O₂ at similar rates. Respiratory substrate addition increased removal rates, indicating a respiration-dependent process. Moreover, the rates were higher with pyruvate/malate than with succinate and in heart than in liver mitochondria. Generally, the changes in H₂O₂ removal rates mirrored those of H₂O₂ release rates excluding the possibility that endogenous and exogenous H₂O₂ competed for the removing system. This idea was supported by the observation that the heaviest of three liver mitochondrial fractions exhibited the highest rates of both H₂O₂ release and removal. Pharmacological inhibition showed tissue-linked differences in antioxidant enzyme contribution to H₂O₂ removal which were consistent with the differences in antioxidant system activities. The enzymatic processes accounted only in part for net H₂O₂ removal and the non-enzymatic ones participated to H₂O₂ scavenging to a degree that was higher for heart than for liver mitochondria. The idea that non-enzymatic scavenging was due in great part to hemoproteins action was consistent with observation that the concentration of cytochromes, in particular cytochrome c, was higher in heart mitochondria. Indirect support was also obtained by a technique of enhanced luminescence, utilizing the capacity of cytochrome c/H₂O₂ to catalyze the luminol oxidation, which showed that luminescence response to an oxidative challenge was higher in heart mitochondria.