Peroxiredoxins (Prx) are abundant thiol peroxidases that react rapidly with H2O2, constituting an important antioxidant defense and acting as sensors and transmitters of H2O2 signals in cells. Certain Prxs are readily inactivated by H2O2 at submillimolar concentrations, whereas others are less sensitive. This oxidative inactivation, or hyperoxidation, occurs when H2O2 further reacts with the Cp-SOH producing Cp-SO2- and/or Cp-SO3-. Since HCO3- accelerates several of the reactions of H2O2 with biothiols, we examined whether HCO3- could increase recombinant human peroxiredoxin 1 (Prx1) hyperoxidation by H2O2. Kinetic studies of NADPH consumption by Prx1 in presence of yeast Trx, yeast Trr and various H2O2 concentrations in the absence and presence of HCO3- (25 mM) provided values of Chyper1% of 335 µM and 110 µM, respectively. Western blot (with an antiPrx-SO3-/SO2- antibody) analyses and Nano-ESI-Q-TOF MS/MS analyses of incubations of Prx1 (2.5 µM) and H2O2 (25 µM) showed higher levels of hyperoxidized Prx1 in the presence of HCO3- (25 mM) than in its absence. Taken together, the results indicate that bicarbonate increases Prx1 susceptibility to hyperoxidation by H2O2. We discuss the possible mechanism of the process and its biological implications. (FAPESP13/07937-8)