A Caged Electrophilic Probe for Global Analysis of Cysteine Reactivity in Living Cells

Cysteine residues are subject to diverse modifications, such as oxidation, nitrosation, and lipidation. The resulting loss in cysteine reactivity can be measured using electrophilic chemical probes, which importantly provide the stoichiometry of modification. An iodoacetamide (IA)-based chemical probe has been used to concurrently quantify reactivity changes in hundreds of cysteines within cell lysates. However, the cytotoxicity of the IA group precludes efficient live-cell labeling, which is important for preserving transient cysteine modifications. To overcome this limitation, a caged bromomethyl ketone (BK) electrophile was developed, which shows minimal cytotoxicity and provides spatial and temporal control of electrophile activation through irradiation. The caged-BK probe was utilized to monitor cysteine reactivity changes in A431 cells upon epidermal growth factor (EGF)-stimulated release of cellular reactive oxygen species. Decreased reactivity was observed for cysteines known to form sulfenic acids and redox-active disulfides. Importantly, the caged-BK platform provided the first quantification of intracellular disulfide bond formation upon EGF stimulation. In summary, the caged-BK probe is a powerful tool to identify reactivity changes associated with diverse cysteine modifications, including oxidation, metal chelation, and inhibitor binding, within a physiologically relevant context.