Mutations in the human protein DJ-1 cause early onset of Parkinson's disease. A reactive cysteine residue) (Cys(106)) of DJ-1 is crucial for its protective function, although the underlying mechanisms are unclear. Here we show that a fraction of bacterially expressed polyhistidine-tagged human DJ-1 could not be eluted from a Ni-nitrilotriacetate (Ni-NTA) column with 150 mM imidazole. This unusually tight binding was accompanied by the appearance of blue violet color on the Ni-NTA column. We demonstrate by Xray crystallography that Cys(106) is carboxymethylated in a fraction of DJ-1 tightly bound to Ni-NTA and that the replacement of Cys(106) by serine abrogates the tight binding and the appearance of blue violet color. However, carboxymethylation of purified DJ-1 is insufficient to confer the tight binding to Ni-NTA. Moreover, when eluted protein was re-applied to the Ni-NTA column, no tight binding was observed, indicating that the formation of high affinity complex with Ni-NTA depends on a transient modification of Cys(106) that transforms into a Cys(106)-carboxymethyl adduct upon elution from Ni-NTA. We conclude that an unknown metabolite reacts with Cys(106) of DJ-1 to result in a transient post-translational modification. This modification is distinct from simple oxidation to sulfinic or sulfenic acids and confers altered binding properties to DJ-1 suggesting that it could serve as a signal for sensing oxidant stress. (C) 2018 Elsevier Inc. All rights reserved.