The bicarbonate/carbon dioxide pair increases hydrogen peroxide-mediated hyperoxidation of human peroxiredoxin 1

2-Cys peroxiredoxins (Prxs) rapidly reduce H₂O₂, thereby acting as antioxidants and also as sensors and transmitters of H₂O₂ signals in cells. Interestingly, eukaryotic 2-Cys Prxs lose their peroxidase activity at high H₂O₂ levels. Under these conditions, H₂O₂ oxidizes the sulfenic acid derivative of the Prx peroxidatic Cys (C_PSOH) to the sulfinate (C_PSO₂^-) and sulfonated (C_PSO₃^-) forms, redirecting the C_PSOH intermediate from the catalytic cycle to the hyperoxidation/inactivation pathway. The susceptibility of 2-Cys Prxs to hyperoxidation varies greatly and depends on structural features that affect the lifetime of the C_PSOH intermediate. Among the human Prxs, Prx1 has an intermediate susceptibility to H₂O₂ and was selected here to investigate the effect of a physiological concentration of HCO₃^-/CO₂ (25 mm) on its hyperoxidation. Immunoblotting and kinetic and MS/MS experiments revealed that HCO₃^-/CO₂ increases Prx1 hyperoxidation and inactivation both in the presence of excess H₂O₂ and during enzymatic (NADPH/thioredoxin reductase/thioredoxin) and chemical (DTT) turnover. We hypothesized that the stimulating effect of HCO₃^-/CO₂ was due to HCO₄^-, a peroxide present in equilibrated solutions of H₂O₂ and HCO₃^-/CO₂ Indeed, additional experiments and calculations uncovered that HCO₄^- oxidizes C_PSOH to C_PSO₂^- with a second-order rate constant 2 orders of magnitude higher than that of H₂O₂ ((1.5 ± 0.1) × 10⁵ and (2.9 ± 0.2) × 10³ m^-1·s^-1, respectively) and that HCO₄^- is 250 times more efficient than H₂O₂ at inactivating 1% Prx1 per turnover. The fact that the biologically ubiquitous HCO₃^-/CO₂ pair stimulates Prx1 hyperoxidation and inactivation bears relevance to Prx1 functions beyond its antioxidant activity.