A study on the NO-myeloperoxidase complex by absorption and electron paramagnetic (spin) resonance spectroscopies

Abstract

The nitric oxide (NO) complexes of native myeloperoxidase and a modified myeloperoxidase (a cytochrome oxidase-like derivative) were investigated by absorption and electron paramagnetic (spin) resonance (ESR) spectrometries. The ferrous native myeloperoxidase formed two different complexes with NO, at low and high NO concentrations, while the ferrous modified myeloperoxidase formed only one complex regardless of the concentration of NO. The ESR spectrum of the NO complex of native myoloperoxidase (ferrous form) showed a characteristic triplet-triplet hyperfine structure, indicating a six-coordinate heme, and that of the modified myeloperoxidase (ferrous from) showed a single triplet hyperfine structure, indicating a five-coordinate heme. These results reveal that the native myeloperoxidase has two binding sites to NO, probably heme iron and the formyl group (-CHO) on the porphyrin ring of heme, and that the modified myeloperoxidase has only one such binding site, probably heme iron. Thus, the modification caused the loss of one binding site, probably that corresponding to the formyl group in the native myeloperoxidase. The ESR spectra of the native and modified myeloperoxidase complexes also indicated that the heme iron in native myeloperoxidase bonds with a fifth (proximal) ligand, probably the N nucleus of a histidine residue in the protein moiety of this enzyme, and that the heme iron in modified myeloperoxidase interacts weakly or does not bond with a fifth (proximal) ligand, probably the N nucleus of an amino residue in the protein moiety of this enzyme. The myeloperoxidase in vivo may scavenge NO, resulting in elimination of the NO molecule's paramagnetism.