Development of a sensitive assay to detect reversibly oxidized protein cysteine sulfhydryl groups

Protein sulfhydryl groups can undergo reversible oxidation reactions in response to reactive oxygen and reactive nitrogen species. Sensitive detection of sulfhydryl group oxidation in specific proteins is required to further our understanding of protein redox changes in biological systems. In general, to detect reversible oxidation reactions the oxidized sulfur atom is reduced to a sulfhydryl group followed by a reaction with a quantifiable agent. Our aim was to develop a sensitive method to detect reversibly oxidized protein sulfhydryl groups in a Western blot format. Conjugation of methoxypolyethylene glycol-maleimide (MAL-PEG) to protein sulfhydryl groups was optimized. Once MAL-PEG forms a covalent bond with the protein, the MAL-PEG-protein conjugate can be detected as a band shift by western analysis. The efficiency of MAL-PEG conjugation to protein was determined with creatine kinase. MAL-PEG conjugated to approximately 100% of the available sulfhydryl groups on creatine kinase within 30 min. Band shift detection sensitivity was measured using the redox-regulated protein p53. MAL-PEG conjugation coupled to western analysis detected a minimum of 0.23 pmol of oxidized p53. The MAL-PEG conjugation method described in this communication can be used to assess the reversible sulfhydryl oxidation status of proteins for which antibodies suitable for western analysis are available.