Dopamine (DA) is an unstable neurotransmitter that readily oxidizes to the DA quinone and forms reactive oxygen species, such as superoxide and hydrogen peroxide. The oxidized dopamine also forms thiol conjugates with sulfhydryl groups on cysteine, glutathione, and proteins. In the present study, we determined the redox potential of the protein-bound DA and established a novel mechanism for the oxidative modification of the protein, in which the DA-cysteine adduct generated in the DA-modified protein causes oxidative modification of the DA-bound protein in the presence of Cu²⁺. Exposure of a sulfhydryl enzyme, glyceraldehyde-3-phosphate dehydrogenase, to DA resulted in a significant loss of sulfhydryl groups and the formation of the DA-cysteine adduct. When the DA-modified protein was incubated with Cu²⁺, we observed aggregation and degradation of the DA-bound protein and concomitant formation of a protein carbonyl, a marker of an oxidatively modified protein. Furthermore, we analyzed the carbonyl products generated during the Cu²⁺-catalyzed oxidation of the DA-modified protein and revealed the production of glutamic and aminoadipic semialdehydes, consisting of the protein carbonyls generated. The cysteinyl-DA residue generated in the DA-modified protein was suggested to represent a redox-active adduct, based on the observations that the cysteinyl-DA adduct, 5-S-cysteinyldopamine, produced by the reaction of cysteine with DA, gave rise to the oxidative modification of bovine serum albumin in the presence of Cu²⁺. These data suggest that the DA-modified protein may be involved in redox alteration under oxidative stress, whereby DA covalently binds to cysteine residues, generating the redox-active cysteinyl-DA adduct that causes the metal-catalyzed oxidation of protein.