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Oxidative stress and mitochondrial dysfunction signify two important biochemical events associated with the loss of dopaminergic neurons in Parkinson's disease (PD). Studies using in vitro and in vivo PD models and in affected tissues from the disease itself have demonstrated a selective inhibition of mitochondrial complex I activity that appears to affect normal mitochondrial physiology leading to neuronal cell death. Earlier experiments from our laboratory have demonstrated that induced depletion of glutathione (GSH + GSSG) in cultured dopaminergic cells resulted in increased oxidative stress and a decrease in mitochondrial function. Furthermore, this dysfunction was linked to a selective decrease in mitochondrial complex I activity that appears to be due to oxidation of this complex. Glutathione depletion is the earliest detectable biochemical event during PD progression and occurs prior to complex I inhibition. Recent observations have also indicated that oxidative damage to complex I via naturally occurring free radicals such as peroxynitrite leads to modification of tyrosine and/or cysteine residues resulting in complex I inhibition. Using the sucrose gradient method, we detected in complex I-enriched fractions from a glutathione-depleted dopaminergic cell line two bands corresponding to 25-kDa and 30-kDa polypeptides that demonstrate anti-nitrotyrosine immunoreactivity, suggesting the possible involvement of protein nitration by peroxynitrite in glutathione depletion-mediated complex I inhibition. Antioxid. Redox Signal. 7, 900–910.