Fig. 1.

Generation of NO· by NOS in liver cells. NO· is relatively unstable in the presence of O₂ and will rapidly and spontaneoulsy auto-oxidize to yield a variety of nitrogen oxides.NO· also reacts with O₂^{· −} to generate ONOO⁻. Although ONOO⁻ is relatively stable, it has a pKa of 6.8, which implies that substantial amounts of ONOO⁻ will be protonated at physiologic pH to yield peroxynitrous acid. This conjugate acid rapidly decomposes to yield NO₃⁻. Nitration of tyrosine residues by ONOO⁻ forms the stable product, 3-nitrotyrosine (3-NT, the footprint for ONOO⁻) by addition of a nitro group to the 3-position adjacent to the hydroxyl group of tyrosine.