Quantitative display of the redox status of proteins with maleimide-polyethylene glycol tagging

Cysteine oxidation, either biologically reversible or irreversible, is the main posttranslational modification associated with redox signaling and oxidative stress. Maleimide-polyethylene glycol (m-PEG) has been used to detect reversibly oxidized proteins by reacting to the reduced cysteine residues leading to mobility shift in immunoblots; a method called PEG-switch. With PEG-switch, both reduced and oxidized proteins can be observed on the same immunoblot simultaneously, providing a simple quantitative measurement for protein thiol modifications. In this report, we optimized the assay conditions and exploited the applications of PEG-switch in quantitation of the extent of protein thiol oxidation in cells in response to H₂ O₂ and insulin. In addition, we have proposed a redox scoring system for measuring the redox status of any given protein from the m-PEG immunoblot. Our results provided quantitative data showing that two cysteine residues of protein tyrosine phosphatase 1B are prone to oxidation following insulin treatment in cultured HeLa cells.