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

<jats:title>SUMMARY</jats:title><jats:p>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.<jats:italic>In vitro</jats:italic>, parkin’s oxidation directly reduces hydrogen peroxide (H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub>) to water. In parkin-deficient human brain, H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub>concentrations are elevated. In dopamine toxicity studies, wild-type parkin -but not disease-associated mutants-prevents neural death by lowering H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub>and sequestering radicals within insoluble aggregates. Parkin conjugates dopamine metabolites at the human-specific residue C95 and augments melanin formation<jats:italic>in vitro</jats:italic>. Using epitope-mapped antibodies, we found that in adult<jats:italic>Substantia nigra</jats:italic>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<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub>reduction, dopamine radical conjugation and the formation of neuromelanin.</jats:p>