Quantitative identification of protein nitration sites

Several labelling strategies have been developed targeting specific amino acid residues and/or PTMs. Methods specifically tailored for the qualitative and sometimes quantitative determination of PTMs have emerged. Many research groups have focused their attention towards o‐nitrotyrosine residues, developing various methodologies for their identification, while direct quantification has remained elusive. So far the iTRAQ chemistry has been limited to primary amines. Here, we report a new strategy based on the use of iTRAQ reagents coupled to MS analysis for the selective labelling of o‐nitrotyrosine residues. This method was proved to lead to the simultaneous localisation and quantification of nitration sites both in model proteins and in biological systems.