Abstract Body

Loss of parkin causes early onset recessive Parkinson disease. We hypothesized that parkin confers thiol-dependent antioxidant activity as a redox molecule, thus protecting against radicals such as hydrogen peroxide (H₂O₂) and electrophilic dopamine metabolites. We found that in human brain, including normal midbrain, parkin is oxidized and transitions into insolubility between the ages of 28 and 42 years. This correlates with rising H₂O₂ levels and is specific to human brain; this transition does not occur in human spinal cord, skeletal muscle, or rodent brain. Using human and mouse tissues, cellular models and recombinant proteins, we found that parkin fulfils criteria for a redox molecule: One, it interacts with redox stressors via its thiol groups. Two, parkin’s structure and solubility change upon oxidation at select residues, including at Cys95 and Cys253 in human brain. Three, through its oxidation parkin effects redox change by lowering ROS levels and augmenting melanin formation in a Cys95-dependent manner. Further, in human midbrain parkin is physically associated with neuromelanin. We also demonstrate that parkin expression alters glutathione (GSH) metabolism. In human and murine parkin-deficient brain, we detected increased H₂O₂ and carbonyl content as evidence of chronically elevated oxidative stress; we also found compensatory elevation in reduced GSH and the GSH:GSSG ratio. Taken together, we present an expanded concept for parkin functions in that its oxidation is linked to redox balance and sequestration of dopamine radicals in human brain. We propose that these non-E3-mediated redox effects contribute to dopamine neuron health in ageing human midbrain.