Sulfonation of the resolving cysteine in human peroxiredoxin 1: A comprehensive analysis by mass spectrometry.

Peroxiredoxin 1 (Prx1) is an essential peroxidase that reduces cellular peroxides. It holds 2 indispensable cysteines for its activity: a peroxidatic cysteine (CP) for peroxide reduction and a resolving cysteine (CR) for CP regeneration. CP can be readily sulfonated to CP-SO3H by protracted oxidative stress, which inactivates Prx1 as a peroxidase. By comparison, sulfonation of CR to CR-SO3H in mammalian cells has only been reported once. The rare report of CR sulfonation prompts the following questions: "can CR-SO3H be detected more readily with the current high sensitivity mass spectrometers (MS)?" and "do CP and CR have distinct propensities to sulfonation?" Answers to these questions could shed light on how differential sulfonation of CP and CR regulates Prx1 functions in cells. We used a sensitive Orbitrap MS to analyze both basal and H2O2-induced sulfonation of CR and CP in either recombinant human Prx1 (rPrx1) or HeLa cell Prx1 (cPrx1). In the Orbitrap MS, we optimized both collision-induced dissociation and higher-energy collisional dissociation methods to improve the analytical sensitivity of cysteine sulfonation. In the basal states without added H2O2, both CP and CR were partially sulfonated in either rPrx1 or cPrx1. Still, exogenous H2O2 heightened the sulfonation levels of both CP and CR by ~200-700%. Titration with H2O2 revealed that CP and CR possessed distinct propensities to sulfonation. This surprising discovery of prevalent Prx1 CR sulfonation affords a motivation for future investigation of its precise functions in cellular stress response.