Abstract
Superoxide is a reactive oxygen species (ROS) with complex roles in biological systems. It can contribute to the development of serious diseases, from aging to cancers and neurodegenerative disorders. However, it can also serve as a signaling molecule for important life processes. Monitoring superoxide levels and identifying proteins regulated by superoxide are crucial to enhancing our understanding of this growing field of redox biology and signaling. Given the high reactivity and very short lifetime of superoxide compared to other ROS in biological systems, proteins redox-modified by superoxide should be in close proximity to where superoxide is generated endogenously, i.e. superoxide hotspots. This inspires us to develop superoxide-specific quinone methide-based precursors, QMP-SOs, for proximity labeling of proteins within/near superoxide hotspots to image superoxide and profile proteins associated with superoxide biology by chemoproteomics. QMP-SOs specifically react with superoxide to generate an electrophilic quinone methide intermediate, which subsequently reacts with nucleophilic amino acids to induce a covalent tag on proteins, as revealed by liquid chromatography-mass spectrometry (LC-MS) and shotgun MS experiments. The alkyne handle on the covalent tag enables installation of fluorophores onto the tagged proteins for fluorescence imaging of superoxide in cells under oxidative stress. By establishing a chemoproteomics platform, QMP-SO-TMT, we identify DJ-1 and DLDH as proteins associated with superoxide biology in liver cancer cells treated with menadione. This work should provide insights into the crosstalk between essential cellular events and superoxide redox biology, as well as the design principles of quinone methide-based probes to study redox biology through proximity labeling and chemoproteomics.