Methionine oxidation by peroxymonocarbonate, a reactive oxygen species formed from CO2/bicarbonate and hydrogen peroxide

Kinetic and thermodynamic evidence is reported for the role of the peroxymonocarbonate ion, HCO4−, as a reactive oxygen species in biology. Peroxymonocarbonate results from the equilibrium reaction of hydrogen peroxide with bicarbonate via the perhydration of CO2. The kinetic parameters for HCO4− oxidation of free methionine have been obtained (k1 = 0.48 ± 0.08 M−1s−1 by a spectrophotometric initial rate method). At the physiological concentration of bicarbonate in blood ( 25 mM), it is estimated that peroxymonocarbonate formed in equilibrium with hydrogen peroxide will oxidize methionine approximately 2-fold more rapidly than plasma H2O2 itself. As an example of methionine oxidation in proteins, the bicarbonate-catalyzed hydrogen peroxide oxidation of α1-proteinase inhibitor (α1-PI) has been investigated via its inhibitory effect on porcine pancreatic elastase activity. The second-order rate constant for HCO4− oxidation of α1-PI (0.36 ± 0.06 M−1s−1) is comparable to that of free methionine, suggesting that methionine oxidation is occurring. Further evidence for methionine oxidation, specifically involving Met358 and Met351 of the α1-PI reactive center loop, has been obtained through amino acid analyses and mass spectroscopic analyses of proteolytic digests of the oxidized α1-PI. These results strongly suggest that HCO4− should be considered a reactive oxygen species in aerobic metabolism.