Protein nitration at tyrosine residues leading to 3-nitrotyrosine (NO 2 -Tyr) is one of the nonenzymatic post-translational modifications that can occur in cells under conditions of nitrosative stress.1) There are two main pathways of protein nitration. One is the generation of peroxynitrite (ONOO Ϫ ) by the reaction of nitric oxide ( · NO) with superoxide (O 2 · Ϫ ) and the subsequent reaction of ONOO Ϫ with CO 2 , which leads to the generation of nitrogen dioxide ( · NO 2 ) and the carbonate radical (CO 3
· Ϫ).2) The other is the generation of · NO 2 by the reaction of nitrite (NO 2 Ϫ ) with hydrogen peroxide (H 2 O 2 ) in the presence of heme proteins, such as myeloperoxidase.3) Tyrosine nitration can alter protein structure and function by forming NO 2 -Tyr, partly because of the shift in the pK a value of the tyrosine hydroxyl group from 10.1 to 7.2. 4) Tyrosine nitration may also prevent tyrosine phosphorylation, which is important in signal transduction.
5)Increased levels of protein nitration have been observed in various diseases, [6][7][8][9][10][11][12] such as cardiovascular disease , 9) Alzheimer's disease, 10,12) and Parkinson's disease. 11) Because protein nitration is associated with both physiological and pathological conditions, it is necessary to develop efficient methods to identify the nitration sites and quantify the nitration levels in proteins.NO 2 -Tyr is a sufficiently stable modification to be used as a biomarker of · NO/ONOO Ϫ -related damage. Several methods have been developed to detect protein nitration.12-16) Immunodetection with antibodies directed against nitrotyrosine is one of the most commonly used methods.12) Many highperformance liquid chromatography (HPLC)-based methods with ultraviolet, 13) electrochemical, 14) and fluorescent detection 15) have also been reported. However, these methods are based on the detection and/or quantification of free nitrotyrosine, either in plasma or generated after the cleavage of the peptide bond by enzymatic or acidic hydrolysis.16) These methods yield quantitative data only regarding the changes in nitration levels in whole tissues or whole proteins, and do not identify the sites of NO 2 -Tyr in proteins.Recently, mass spectrometry (MS)-based methods have been developed as an important technique for the identification of nitrated proteins and nitrated sites in proteins. Mass spectrometers with matrix-assisted laser desorption/ionization (MALDI) [17][18][19] or electrospray ionization (ESI) [20][21][22][23][24][25] have been widely used in these studies. The application of tandem mass spectrometry (MS/MS) makes it possible to identify nitrated sites in proteins. [20][21][22][23][24][25] In the MALDI analysis of nitrotyrosine-containing peptides, the characteristic mass pattern derived from nitrotyrosine has mass values lower than that of [MϩH] ϩ by 16 Da and 32 Da. These ions correspond to the loss of one and two oxygen atoms, respectively, from the nitro group (-NO 2 ), and are generated during the ionization process with laser irradiat...