Nature of the inhibition of horseradish peroxidase and mitochondrial cytochrome c oxidase by cyanyl radical

Previous studies established that the cyanyl radical ((*)CN), detected as 5,5-dimethyl-1-pyrroline N-oxide (DMPO)/(*)CN by the electron spin resonance (ESR) spin-trapping technique, can be generated by horseradish peroxidase (HRP) in the presence of hydrogen peroxide (H(2)O(2)) and by mitochondrial cytochrome c oxidase (CcO) in the absence of H(2)O(2). To investigate the mechanism of inhibition by cyanyl radical, we isolated and characterized the iron protoporphyrin IX and heme a from the reactions of CN(-) with HRP and CcO, respectively. The purified heme from the reaction mixture of HRP/H(2)O(2)/KCN was unambiguously identified as cyanoheme by the observation of the protonated molecule, (M + H)(+), of m/z = 642.9 in the matrix-assisted laser desorption/ionization (MALDI) mass spectrum. The proton NMR spectrum of the bipyridyl ferrous cyanoheme complex revealed that one of the four meso protons was missing and had been replaced with a cyanyl group, indicating that the single, heme-derived product was meso-cyanoheme. The holoenzyme of HRP from the reconstitution of meso-cyanoheme with the apoenzyme of HRP (apoHRP) showed no detectable catalytic activity. The Soret peak of cyanoheme-reconstituted apoHRP was shifted to 411 nm from the 403 nm peak of native HRP. In contrast, the heme a isolated from partially or fully inhibited CcO did not show any change in the structure of the protoporphyrin IX as indicated by its MALDI mass spectrum, which showed an (M + H)(+) of m/z = 853.6, and by its pyridine hemochromogen spectrum. However, a protein-centered radical on the CcO can be detected in the reaction of CcO with cyanide and was identified as the thiyl radical(s) based on inhibition of its formation by N-ethylmaleimide pretreatment, suggesting that the protein matrix rather than protoporphyrin IX was attacked by the cyanyl radical. In addition to the difference in heme structures between HRP and CcO, the available crystallographic data also suggested that the distinct heme environments may contribute to the different inhibition mechanisms of HRP and CcO by cyanyl radical.