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. Nitrotyrosine AChE Tracer is a conjugate formed between nitrotyrosine and acetylcholinesterase (AChE). It is used in competitive ELISAs designed to quantify nitrotyrosine.