Understanding Polysulfide-Mediated Papain Inhibition and Differentiating between Disulfide vs Persulfide Formation

Protein cysteine residues are sensitive to redox-regulating molecules, including reactive sulfur species (RSS). As an important member of the RSS family, polysulfides are known to react with protein cysteines to form persulfides and disulfides, both affecting protein functions. In this work, we studied how polysulfides could impact cysteine proteases through careful mechanistic and kinetic studies. The model protein papain was treated with different polysulfides to elucidate the efficacy of polysulfides as inhibitors for this protein. We also explored the effects of different reductants that could regenerate papain activity after polysulfide-mediated inhibition. A triarylphosphine reagent, TXPTS, was found to be efficient in differentiating between papain persulfidation and disulfide formation.