Peroxynitrite-induced nitration of tyrosine-34 does not inhibit Escherichia coli iron superoxide dismutase

The peroxynitrite anion is a potent oxidizing agent, formed by the diffusion-limited combination of nitric oxide and superoxide, and its production under physiological conditions is associated with the pathologies of a number of inflammatory and neurodegenerative diseases. Nitration of Escherichia coli iron superoxide dismutase (Fe-SOD) by peroxynitrite was investigated, and demonstrated by spectral changes and electrospray mass spectroscopic analysis. HPLC and mass studies of the tryptic digests of the mono-nitrated Fe-SOD indicated that tyrosine-34 was the residue most susceptible to nitration by peroxynitrite. Exclusive nitration of this residue occurred when Fe-SOD was exposed to a cumulative dose of 0.4mM peroxynitrite. Unlike with human Mn-SOD, this single modification did not inactivate E. coli Fe-SOD at pH7.4. When Fe-SOD was exposed to higher concentrations of peroxynitrite (7mM), eight tyrosine residues per subunit of the protein, of the nine available, were nitrated without loss of catalytic activity of the enzyme. The pK_a of nitrated tyrosine-34 was determined to be 7.95±0.15, indicating that the peroxynitrite-modified enzyme appreciably maintains its protonation state under physiological conditions.