Nitric oxide trapping of the tyrosyl radical-chemistry and biochemistry

The quenching of the Y(D) tyrosyl radical in photosystem II by nitric oxide was reported to result from the formation of a weak tyrosyl radical-nitric oxide complex. This radical/radical reaction is expected to generate an electron spin resonance (ESR)-silent nitrosocyclohexadienone species that can reversibly regenerate the tyrosyl radical and nitric oxide or undergo rearrangement to form 3-nitrosotyrosine. It has been proposed that 3-nitrosotyrosine can be oxidized by one electron to form the tyrosine iminoxyl radical (>C=N-O.). This proposal was put forth as a result of ESR detection of the iminoxyl radical intermediate when photosystem II was exposed to nitric oxide. Although the detection of the iminoxyl radical in photosystem II strongly suggested a mechanism involving 3-nitrosotyrosine, the iminoxyl radical ESR spectrum was not unequivocally identified as originating from tyrosine. Subsequently, non-protein L-tyrosine iminoxyl radical was generated by two methods: (1) peroxidase oxidation of synthetic 3-nitroso-N-acetyl-L-tyrosine; and (2) peroxidase oxidation of free L-tyrosine in the presence of nitric oxide. The determination of protein nitrotyrosine content has become a frequently used technique for the detection of nitrosative 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 also forms a tyrosyl radical during its enzymatic catalysis of prostaglandin formation. In the presence of the NO.-generator diethylamine nonoate, the tyrosyl radical of prostaglandin H synthase-2 also changes to that of an iminoxyl radical. Western blot analysis of prostaglandin H synthase-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.