Covalent modification of cellular proteins by electrophiles affects electrophilic signal transduction and the dysfunction of enzymes that is involved in cytotoxicity. We have recently found a unique reaction which restores glyceraldehyde-3-phosphate dehydrogenase (GAPDH) that has been modified by 1,2-naphthoquinone (1,2-NQ) through a glutathione (GSH)-dependent S-transarylation reaction. We report here that ubiquitin carboxyl-terminal hydrolase L1 (UCH-L1) undergoes the same reaction. Exposure of human neuroblastoma SH-SY5Y cells to 1,2-NQ after pretreatment with buthionine sulfoximine (BSO) to deplete GSH resulted in an enhancement of covalent modification of UCH-L1 by 1,2-NQ. With recombinant human UCH-L1, we demonstrated that UCH-L1 underwent arylation by 1,2-NQ through Cys152 and Lys4, thereby decreasing its catalytic activity. Addition of GSH to an incubation mixture of 1,2-NQ-UCH-L1 adduct partially restored this decline in enzyme activity which was accompanied by decreased covalent attachment of 1,2-NQ, together with production of 1,2-NQ-GSH adduct. UCH-L1 in which Lys4 was mutated exhibited a lower level of covalent modification and enzyme inhibition, but completely recovered after addition of GSH. Taken together, these results suggest that Cys152 modification in UCH-L1 by 1,2-NQ is reversible via GSH-mediated S-transarylation reaction whereas Lys4 modification by 1,2-NQ is irreversible by GSH. Because UCH-L1 dysfunction has been associated with neurodegeneration, the electrophilic modification of Lys rather than Cys in UCH-L1 may be implicated in such neurodegenerative diseases.