Bicarbonate-catalyzed hydrogen peroxide oxidation of cysteine and related thiols

The effect of bicarbonate on the rates of the H₂O₂ oxidation of cysteine, gluthathione, and N-acetylcysteine to the corresponding disulfides was investigated. The relative oxidation rates at pH 8 for the different thiols are inversely related to the pK_a values of the thiol groups, and the reactive nucleophiles are identified as the thiolate anions or their kinetic equivalents. The second-order rate constants at 25 °C for the reaction of the thiolate anions with hydrogen peroxide are 17 ± 2 M⁻¹ s⁻¹ for all three substrates. In the presence of bicarbonate (>25 mM), the observed rate of thiolate oxidation is increased by a factor of two or more, and the catalysis is proposed to be associated with the formation of peroxymonocarbonate from the equilibrium reaction of hydrogen peroxide with bicarbonate (via CO₂). The calculated second-order rate constants for the direct reaction of the three thiolate anions with peroxymonocarbonate fall within the range of 900-2000 M⁻¹ s⁻¹. Further oxidation of disulfides by peroxymonocarbonate results in the formation of thiosulfonate and sulfonate products. These results strongly suggest that peroxymonocarbonate should be considered as a reactive oxygen species in aerobic metabolism with relevance in thiol oxidations.