1,4‐Benzoquinone forms an unstable thioether bond with cysteine residues that is eliminated in basic conditions

Abstract

Hydroquinone (HQ) is an environmental toxicant found in skin creams, cigarette smoke, and it is also a byproduct of combustion. After exposure, HQ is oxidized to benzoquinone (BQ) and conjugated with GSH; HQGSHs target the outer stripe of the outer medulla of the kidney causing necrosis. The adverse effects of HQGSHs are caused by its redox cycling capabilities and the ability to adduct proteins. In vitro and in vivo studies have revealed that Lys, Glu and Arg residues are targets of BQ adduction. Cys residues, which are the more commonly reported sites of electrophilic chemical adduction, have been identified as targets of BQ, but only with in vitro experiments. Following reaction of a Cys containing peptide and apocytochrome c (2 Cys) with BQ and mass spec analysis we identified Cys‐BQ adducts. Then by mimicking a standard protein digestion protocol we observed a time‐dependent elimination of the thioether bond. Other electrophiles with affinity for Cys, such as acrolein and iodoacetamide, retained the Cys thioether bond on the peptide during exposure to identical conditions. In conclusion, we have identified BQ‐Cys adducts in peptide and protein models, where the thioether bond is eliminated at basic pH over time. As a result, quinone post‐adduction chemistry ex vivo plays a critical role in determining our ability to identify amino acid targets of these compounds. (GM007890, ES006694, ES007091).