Nitrotyrosine is formed by peroxynitrite-mediated nitration of protein tyrosine residues. Its presence on proteins can be used as a marker for peroxynitrite formation in vivo.{5439,1269,4663,5357} Both free and protein-bound nitrotyrosine are commonly found in mammalian tissues and are increased in pathological conditions.{1269,7112,11380} The basal levels of free nitrotyrosine in human plasma is approximately 3 nM as determined by GC/MS.{10970,7925}