Commonly Used Alkylating Agents Limit Persulfide Detection by Converting Protein Persulfides into Thioethers

Protein persulfides (R-S-SH) have emerged as a common post-translational modification. Detection and quantitation of protein persulfides requires trapping with alkylating agents. Here we show that alkylating agents differ dramatically in their ability to conserve the persulfide's sulfur-sulfur bond for subsequent detection by mass spectrometry. The two alkylating agents most commonly used in cell biology and biochemistry, N-ethylmaleimide and iodoacetamide, are found to be unsuitable for the purpose of conserving persulfides under biologically relevant conditions. The resulting persulfide adducts (R-S-S-Alk) rapidly convert into the corresponding thioethers (R-S-Alk) by donating sulfur to ambient nucleophilic acceptors. In contrast, certain other alkylating agents, in particular monobromobimane and N-t-butyl-iodoacetamide, generate stable alkylated persulfides. We propose that the nature of the alkylating agent determines the ability of the disulfide bond (R-S-S-Alk) to tautomerize into the thiosulfoxide (R-(S=S)-Alk), and/or the ability of nucleophiles to remove the sulfane sulfur atom from the thiosulfoxide.