Mutation in DJ-1 gene is the cause of autosomal recessive Parkinson's disease, however, its physiological function remains unclear. The isoelectric point of DJ-1 shows an acidic shift after cells are treated with hydrogen peroxide. This suggests that DJ-1 is modified in response to oxidative stress. Here we report the structural characterization of an acidic isoform of DJ-1 using a proteomic approach with nanospray interface liquid chromotography-electrospray ionization/linear ion trap mass spectrometer. When human umbilical vein endothelial cells were exposed to hydrogen peroxide, all three cysteines in DJ-1 were oxidized to cysteine sulphonic acid. Although a small part of the Cys-46 and Cys-53 were oxidized, Cys-106 was oxidized completely at Lilly hydrogen peroxide concentration used here. These results suggest that Cys-106 is the most sensitive among three cysteine residues to oxidative stress, and that DJ-1 function is regulated in terms of the intracellular redox state, by oxidation of Cys-106. (C) 2004 Elsevier Inc. All rights reserved.