Cytochrome c is released from mitochondria in a reactive oxygen species (ROS)-dependent fashion and can operate as a ROS scavenger and as a respiratory substrate in cerebellar neurons undergoing excitotoxic death

In rat cerebellar granule cells both reactive oxygen species production and release of cytochrome c take place during glutamate toxicity. This investi gation was aimed (i) to ascertain whether and how these two processes are r elated and (ii) to gain insight into the role played by the released cytoch rome c in the onset of neurotoxicity. Cytochrome c release takes place owin g to the generation of reactive oxygen species both in glutamate-treated ce rebellar granule cells and in sister control cultures incubated in the pres ence of the reactive oxygen species generating system consisting of xanthin e plus xanthine oxidase. In the early phase of neurotoxicity (30-min glutam ate exposure) about 40% of the maximum las measured at 3 h of glutamate exp osure) cytochrome c release was found to occur in cerebellar granule cells from mitochondria that were essentially coupled and intact and that had a n egligible production of oxygen free radicals. Contrarily, mitochondria from cells treated with glutamate for 3 h were mostly uncoupled and produced re active oxygen species at a high rate. The cytosolic fraction containing the released cytochrome c was able to transfer electrons from superoxide anion to molecular oxygen via the respiratory chain and was found to partially p revent glutamate toxicity when added externally to cerebellar neurons under going necrosis. In the light of these findings, we propose that in the earl y phase of neurotoxicity, cytochrome c release can be part of a cellular an d mitochondrial defense mechanism against oxidative stress.