Carbon Dioxide Enhancement of Peroxynitrite-Mediated Protein Tyrosine Nitration

Production of reactive species has been associated with tissue injury in diverse human disorders and experimental models of disease. Peroxynitrite is a strong oxidant with multiple pathways of reactivity. One protein modification reaction that may be specific to peroxynitrite is the nitration of the ortho position of tyrosine residues and nitrotyrosine has been used as a marker for peroxynitrite-mediated oxidative stress. Nitrotyrosine was formed when peroxynitrite was reacted at physiological pH with fatty acid-free bovine serum albumin or with human plasma proteins. Nitrotyrosine was not formed when proteins were incubated with nitric oxide, nitrogen dioxide, or nitric oxide plus hydrogen peroxide in the presence of ferrous iron or ferrihorseradish peroxidase. Low-molecular-weight molecules such as uric acid, ascorbate, and sulfhydryls inhibited protein tyrosine nitration in the absence of bicarbonate. Addition of bicarbonate catalytically enhanced the yield of nitration and overcame the inhibition of these antioxidants. Bicarbonate/CO2enhanced the yield of protein nitrotyrosine in a concentration-dependent manner. Catalysis of nitration is achieved by the interaction of CO2with the peroxynitrite anion. A mechanism is proposed involving an ONOO(O)CO−intermediate, which readily nitrates tyrosine residues in a non-radical-dependent manner. Thus, peroxynitrite nitrates tyrosine residues by a mechanism that is catalyzed by CO2under normal physiological conditions.