Oxidative Modifications of Parkin Underlie its Selective Neuroprotection in Adult Human Brain

SUMMARY The mechanisms by which Parkinson disease-linked parkin confers neuroprotection of human dopamine cells remain elusive. We hypothesized that its cysteines mediate multiple anti-oxidant effects in the midbrain. By studying >60 control specimens, we found that in adult human brain - but not in skeletal muscle- parkin is mostly aggregated and insoluble due to oxidative modifications, such as at C253. In vitro , parkin’s oxidation directly reduces hydrogen peroxide (H 2 O 2 ) to water. In parkin-deficient human brain, H 2 O 2 concentrations are elevated. In dopamine toxicity studies, wild-type parkin -but not disease-associated mutants-prevents neural death by lowering H 2 O 2 and sequestering radicals within insoluble aggregates. Parkin conjugates dopamine metabolites at the human-specific residue C95 and augments melanin formation in vitro . Using epitope-mapped antibodies, we found that in adult Substantia nigra neurons parkin localizes to neuromelanin within LAMP-3/CD63-positive lysosomes. We conclude that parkin’s own oxidation, previously considered a loss-of-function event, underlies three neuroprotective effects in adult midbrain: its cysteines participate in H 2 O 2 reduction, dopamine radical conjugation and the formation of neuromelanin.