[20] Peroxynitrite-dependent tyrosine nitration catalyzed by superoxide dismutase, myeloperoxidase, and horseradish peroxidase

This chapter discusses the peroxynitrite-dependent tyrosine nitration catalyzed by superoxide dismutase, myeloperoxidase, and horseradish peroxidase. Peroxynitrite is a potent nitrating agent formed by the near diffusion limited reaction of superoxide and nitric oxide (NO). Peroxynitrite is more cytotoxic to Escherichia coli and Trypanosoma cruzi than NO, superoxide, or hydrogen peroxide. Peroxynitrite is a strong oxidant capable of yielding the chemistry commonly attributed to hydroxyl radical damage. The peroxynitrite anion can exist in two stable conformers. The cis conformer, which is ring-like because of overlapping of the terminal oxygen orbitals, is slightly more stable and is the only conformer found in alkaline solution, cis-Peroxynitrite cannot directly isomerize to nitrate. The simplest means to observe tyrosine nitration is to mix rapidly a small volume of peroxynitrite with a solution containing the target phenol and measure the resulting yellow absorbance from the nitrophenolic. Because peroxynitrite rapidly decomposes at neutral pH, the secret to reproducibility involves rapid mixing and accurate pipetting.