Analytical methods for 3-nitrotyrosine quantification in biological samples: the unique role of tandem mass spectrometry

Reactive-nitrogen species, such as peroxynitrite (ONOO⁻) and nitryl chloride (NO₂Cl), react with the aromatic ring of tyrosine in soluble amino acids and in proteins to form 3-nitrotyrosine. The extent of nitration can be quantified by measuring 3-nitrotyrosine in biological matrices, such as blood, urine, and tissue. This article reviews and discusses current analytical methodologies for the quantitative determination of 3-nitrotyrosine in their soluble and protein-associated forms, with the special focus being on free 3-nitrotyrosine. Special emphasis is given to analytical approaches based on the tandem mass spectrometry methodology. Pitfalls and solutions to overcome current methodological problems are emphasized and requirements for quantitative analytical approaches are recommended. The reliability of current analytical methods and the suitability of 3-nitrotyrosine as a biomarker of nitrative stress are thoroughly examined.