Mitochondrial redox systems regulating ROS levels via thiol-disulfide exchange/coupling reactions. The mitochondrial form of Thioredoxin (Trx2) is likely to play the major role in reducing disulfides formed by vicinal thiols in both the mitochondrial Peroxiredoxin III (Prx III) and other proteins. Prx III is one of the main ways by which cancer cells can reduce their levels of H₂O₂ built up during active respiration. The glutathione redox system comprising GSH/GSSG, glutathione reductase, glutaredoxin, and glutathione peroxidase, although present in the mitochondria, is more likely to only become of major importance during the more extreme conditions of oxidative stress. Both of these systems are targets for inhibition by arsenic-containing compounds. See text for further detail.