title: Nitration of the tyrosyl radical in ribonucleotide reductase by nitrogen dioxide: a gamma radiolysis study.
abstract: Nitrogen dioxide is a product of peroxynitrite homolysis and peroxidase-catalyzed oxidation of nitrite. It is of great importance in protein tyrosine nitration because most nitration pathways end with the addition of *NO2 to a one-electron-oxidized tyrosine. The rate constant of this radical addition reaction is high with free tyrosine-derived radicals. However, little is known of tyrosine radicals in proteins. In this paper, we have used *NO2 generated by gamma radiolysis to study the nitration of the R2 subunit of ribonucleotide reductase, which contains a long-lived tyrosyl radical on Tyr122. Most of the nitration occurred on Tyr122, but nonradical tyrosines were also modified. In addition, peptidic bonds close to nitrated Tyr122 could be broken. Nitration at Tyr122 was not observed with a radical-free metR2 protein. The estimated rate constant of the Tyr122 radical reaction with *NO2 was of 3 x 10(4) M(-1) s(-1), thus several orders of magnitude lower than that of a radical on free tyrosine. Nitration rate of other tyrosine residues in R2 was even lower, with an estimated value of 900 M(-1) s(-1). This study shows that protein environment can significantly reduce the reactivity of a tyrosyl radical. In ribonucleotide reductase, the catalytically active radical residue is very efficiently protected against nitrogen oxide attack and subsequent nitration.
authors.affiliation: UMR CNRS 8619, Institut de Biochimie et Biophysique Moléculaires et Cellulaires, Building 430, University Paris XI, Orsay, F-91405 Orsay cedex, France. michel.lepoivre@bbmpc.u-psud.fr
authors.forename: Michel
authors.initials: M
authors.lastname: Lepoivre
authors.forename: Chantal
authors.initials: C
authors.lastname: Houée-Levin
authors.forename: Karen
authors.initials: K
authors.lastname: Coeytaux
authors.forename: Paulette
authors.initials: P
authors.lastname: Decottignies
authors.forename: Geneviève
authors.initials: G
authors.lastname: Auger
authors.forename: Geneviève
authors.initials: G
authors.lastname: Lemaire
mesh_terms: D002384:Catalysis; D004307:Dose-Response Relationship, Radiation; D004583:Electrons; D004926:Escherichia coli / Q000201:enzymology; D005609:Free Radicals / Q000737:chemistry*; D005720:Gamma Rays; D008956:Models, Chemical; D015233:Models, Statistical; D009569:Nitric Oxide / Q000737:chemistry; D009584:Nitrogen / Q000737:chemistry*; D009585:Nitrogen Dioxide / Q000737:chemistry*; D010100:Oxygen / Q000378:metabolism; D009195:Peroxidase / Q000378:metabolism; D010957:Plasmids / Q000378:metabolism; D012264:Ribonucleotide Reductases / Q000737:chemistry; D013997:Time Factors; D014443:Tyrosine / Q000737:chemistry*
chemical_list: D005609:Free Radicals; C408694:tyrosine radical; D009569:Nitric Oxide; D014443:Tyrosine; D009195:Peroxidase; D012264:Ribonucleotide Reductases; D009584:Nitrogen; D009585:Nitrogen Dioxide; D010100:Oxygen