Generation of hydrogen peroxide by brain mitochondria: The effect of reoxygenation following postdecapitative ischemia

Abstract

The hypothesis that mitochondria damaged during complete cerebral ischemia generate increased amounts of superoxide anion radical and hydrogen peroxide (H 2O 2) upon postischemic reoxygenation has been tested. In rat brain mitochondria, succinate supported H 2O 2 generation, whereas NADH-linked substrates, malate plus glutamate, did so only in the presence of respiratory chain inhibitors. Succinate-supported H 2O 2 generation was diminished by rotenone and the uncoupler carbonyl cyanide m-chlorphenylhydrazone and enhanced by antimycin A and increased oxygen tensions. When maximally reduced, the NADH dehydrogenase and the ubiquinone-cytochrome b regions of the electron transport chain are sources of H 2O 2. These studies suggest that a significant portion of H 2O 2 generation in brain mitochondria proceeds via the transfer of reducing equivalents from ubiquinone to the NADH dehydrogenase portion of the electron transport chain. Succinate-supported H 2O 2 generation by mitochondria isolated from rat brain exposed to 15 min of postdecapitative ischemia was 90% lower than that of control preparations. The effect of varying oxygen tensions on H 2O 2 generation by postischemic mitochondrial preparations was negligible compared with the increased H 2O 2 generation measured in control preparations. Comparison of the effects of respiratory chain inhibitors and oxygen tension on succinate-supported H 2O 2 generation suggests that the ability for reversed electron transfer is impaired during ischemia. These data do not support the hypothesis that mitochondrial free radical generation increases during postischemic reoxygenation.