The nitrosyl compounds of ferrous animal haloperoxidases

Human myeloperoxidase, human eosinophil peroxidase and bovine lactoperoxidase (donor: hydrogen-peroxide oxidoreductase, EC 1.11.1.7) reduced with ascorbic acid form nitrosyl compounds which show rhombic EPR signals centered at g = 2. Using 14NO (IN = 1), the central resonance signal exhibited a hyperfine structure of nine lines originating from a triplet with a small hyperfine splitting (AII(zeta) = 0.69 mT for myeloperoxidase and 0.73 mT for eosinophil peroxidase and lactoperoxidase) superimposed upon a triplet with a larger hyperfine splitting (AI(zeta) = 2.34, 2.32 and 2.09 mT for myeloperoxidase, eosinophil peroxidase and lactoperoxidase, respectively). Using 15NO (IN = 1/2), the nitrosyl compound of ferrous myeloperoxidase and ferrous lactoperoxidase showed a doublet of triplets superimposed upon the central resonance signal. These results demonstrate that a nitrogen nucleus is present at the fifth ligand position of the haem iron in these peroxidases.