**5.6 Prevention of protein modification**

Besides lipid peroxidation and DNA damage, protein modification through nitration or chloration of amino acids also is caused by ROS. *In vivo*, peroxynitrite, O═N─O─O¯ , is a powerful oxidant and nitrating agent formed through the reaction of O2 •¯ with free radical nitric oxide via a diffusion-controlled reaction. In cells, ONOO¯ is a much stronger oxidizing agent than O2 •¯ and is able to damage a wide range of different molecules such as DNA and proteins. ONOO¯ and its protonated form peroxynitrous acids (ONOOH) are capable of exerting direct oxidative modifications during one or two electron oxidation processes [67]. *In vivo*, ONOO¯ reacts nucleophically with CO2 to produce nitrosoperoxy carbonate, which is the predominant pathway for ONOO¯. These modifications often cause the alteration of protein function or structure, in addition to enzyme activities inhibition. Proteins containing nitrotyrosine residues have been detected in various pathogenic conditions, such as diabetes, hypertension, and atherosclerosis, all linked with promoted oxidative stress, including increased formation of ONOO¯. Antioxidants and antioxidant enzyme are utilized to prevent the protein modification of ONOO¯. Antioxidants or enzyme such as CAT is able to remove H2O2 and also inhibit HOCl formation; similarly, SOD or antioxidants, like polyphenols, may scavenge O2 •¯ and inhibit ONOO¯ formation [3].
