SIGNIFICANCE: Reactive oxygen species are produced during normal metabolism in cells, and their excesses have been implicated in protein damage and toxicity, as well as in the activation of signaling events. In particular, hydrogen peroxide participates in the regulation of different physiological processes as well as in the induction of antioxidant cascades, and often the redox molecular events triggering these pathways are based on reversible cysteine (Cys) oxidation. Recent Advances: Increases in peroxides can cause the accumulation of reversible Cys oxidations in proteomes, which may be either protecting thiols from irreversible oxidations or may just be reporters of future toxicity. It is also becoming clear, however, that only a few proteins, such as the bacterial OxyR or peroxidases, can suffer direct oxidation of their Cys residues by hydrogen peroxide and, therefore, may be the only true sensors initiating signaling events. CRITICAL ISSUES: We will in this study describe some of the methodologies used to characterize at the proteome level reversible thiol oxidations, specifically those combining gel-free approaches with mass spectrometry. In the second part of this review, we will summarize some of the electrophoretic and proteomic techniques used to monitor Cys oxidation at the protein level, needed to confirm that a protein contains redox Cys involved in signaling relays, using as examples some of the best characterized redox sensors such as bacterial OxyR or yeast Tpx1/Pap1. FUTURE DIRECTIONS: While Cys oxidations are often detected in proteomes and in specific proteins, major efforts have to be made to establish that they are physiologically relevant. Antioxid. Redox Signal. 26, 329-344

This work was supported by the Spanish Ministry of Science and Innovation (BFU2015-68350-P, MINECO/FEDER, UE), and by 2014-SGR-154 from Generalitat de Catalunya (Spain) to E.H. A.D. is recipient of a predoctoral fellowship from Generalitat de Catalunya (Spain). E.H. is recipient of an ICREA Academia Award (Generalitat de Catalunya, Spain)