Representative pathways for biological formation of 3-nitrotyrosine. This product has been widely assumed to arise from the interaction of secondary oxidants derived from peroxynitrite (ONOO^–) with tyrosine-containing proteins. However, several alternative pathways exist, including one mediated by the enzyme myeloperoxidase (MPO), that may be important in respiring tissues. tyr, tyrosine; tyr^•, tyrosyl radical.