A role for peroxymonocarbonate in the stimulation of biothiol peroxidation by the bicarbonate/carbon dioxide pair

Abstract

Peroxymonocarbonate (HCO4-) is an oxidant whose existence in equilibrium with hydrogen peroxide and bicarbonate has been known since the 1980s. More recently, peroxymonocarbonate has been proposed to mediate oxidative processes stimulated by the bicarbonate/carbon dioxide pair. To better understand this emerging biological oxidant, we re-examined the kinetics of its formation from hydrogen peroxide and bicarbonate/carbon dioxide by 13C NMR. Also, we studied its role in the accelerating effects of bicarbonate on biothiol (GSH and BSA-cysSH) peroxidation by kinetics and product analysis. The rate constants for peroxymonocarbonate formation and decay were estimated and Keq values determined (pH 7.2, at 25 and 37 degrees C; in the absence and presence of BSA and liposomes of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine phosphatidylcholine). Noteworthy is the fact the rate constant for peroxymonocarbonate formation estimated here (k1 approximately 10-2 M-1 s-1) was more than 1 order of magnitude higher than a previously reported value. Also, peroxymonocarbonate equilibrium was shown to be affected by BSA, liposomes, and a carbonic anhydrase mimetic. The Keq values determined in the absence and presence of BSA (0.35 and 0.48 M-1, respectively, at 37 degrees C) were employed to analyze the kinetics of BSA-cysSH and GSH peroxidation in the presence of bicarbonate (2-25 mM). A good fit of experimental data with simulations indicated that peroxymonocarbonate is the main species responsible for biothiol peroxidation in the presence of bicarbonate. The results indicate that peroxymonocarbonate is a feasible biological oxidant, in addition to supporting emerging data that the main physiological buffer is redox active.