We recently hypothesized that parkin plays a role in redox homeostasis and provided evidence that it directly reduces hydrogen peroxide (H 2 O 2 ) in vitro. Here, we examined this anti-oxidant activity in vivo. Informed by findings in human brain, we demonstrate that elevated oxidative stress promotes parkin insolubility in mice. In normal mouse brain parkin was partially oxidized, e.g., at cysteines 195 and 252, which was augmented by oxidative stress. Although under basal conditions H 2 O 2 levels were unchanged in adult prkn -/brain, a parkin-dependent reduction of cytosolic H 2 O 2 was observed when mitochondria were impaired, either due to neurotoxicant exposure (MPTP) or Sod2 haploinsufficiency. In accordance, markers of oxidative stress, e.g., protein carbonylation and nitrotyrosination, were elevated in the cytosol but not in mitochondria from prkn -/mice. This rise in oxidative stress was associated with altered glutathione homeostasis. In parkin's absence reduced glutathione concentrations were increased in cells, murine brain and human cortex. This compensation was not due to new glutathione synthesis but attributed to elevated oxidized glutathione (GSSG)-reductase activity. Moreover, we discovered that parkin also recycled GSSG to its reduced form. With this reaction, parkin became S-glutathionylated, e.g., at cysteines 59 and human-specific 95. This oxidative modification was reversed by glutaredoxin. Our results demonstrate that cytosolic parkin mediates anti-oxidant reactions including H 2 O 2 reduction and glutathione regeneration.These reducing activities lead to a range of oxidative modifications in parkin itself. In parkin-deficient brain oxidative stress rises despite changes to maintain redox balance. wordsIn oxidative stress-exposed, human embryonic kidney (HEK293) cells, which transiently over-expressed PRKN, we also observed HMW smear formation. Parkin became progressively gel excluded (aggregated) and insoluble following exposure to higher levels of H 2 O 2 (Fig. 1b). Since these HMW forms of parkin were reversed with dithiothreitol (DTT; Extended Data Fig. 1a, c), we postulated that these modifications occurred due to increasing oxidation of parkin's cysteines 17 . We tested this using Nethylmaleimide (NEM) and iodoacetamide (IAA), which irreversibly alkylate free thiols.There, we found that pre-incubation of cells with NEM or IAA blocked HMW smear formation of parkin and preserved its solubility during H 2 O 2 stress (Fig. 1b; Extended Data Fig. 1b). Complementing these findings, we mapped several cysteines in recombinant (r-) parkin, which became oxidized when the protein was subjected to rising H 2 O 2 levels in vitro, thereby promoting its aggregation and insolubility 11 .Intriguingly, exposure to DTT also lowered the solubility of cellular parkin (Extended Data Fig. 1c), which suggested that parkin's structure is altered under excessive oxidizing and excessive reducing conditions. These results demonstrated that human parkin acts as a redox-sensitive molecule, which undergoes thio...