Nitric Oxide Trapping of the Tyrosyl Radical of Prostaglandin H Synthase-2 Leads to Tyrosine Iminoxyl Radical and Nitrotyrosine Formation

The determination of protein nitrotyrosine content has become a frequently used technique for the detection of oxidative tissue damage. Protein nitration has been suggested to be a final product of the production of highly reactive nitrogen oxide intermediates (e.g. peroxynitrite) formed in reactions between nitric oxide (NO⋅) and oxygen-derived species such as superoxide. The enzyme prostaglandin H synthase-2 (PHS-2) forms one or more tyrosyl radicals during its enzymatic catalysis of prostaglandin formation. In the presence of the NO⋅-generator diethylamine nonoate, the electron spin resonance spectrum of the PHS-2-derived tyrosyl radical is replaced by the spectrum of another free radical containing a nitrogen atom. The magnitude of the nitrogen hyperfine coupling constant in the latter species unambiguously identifies it as an iminoxyl radical, which is likely formed by the oxidation of nitrosotyrosine, a stable product of the addition of NO⋅ to tyrosyl radical. Addition of superoxide dismutase did not alter the spectra, indicating that peroxynitrite was not involved. Western blot analysis of PHS-2 after exposure to the NO⋅-generator revealed nitrotyrosine formation. The results provide a mechanism for nitric oxide-dependent tyrosine nitration that does not require formation of more highly reactive nitrogen oxide intermediates such as peroxynitrite or nitrogen dioxide.