Ferric α-Hydroxyheme Bound to Heme Oxygenase Can Be Converted to Verdoheme by Dioxygen in the Absence of Added Reducing Equivalents

• Published in: The Journal of biological chemistry 1999-06, Vol.274 (26), p.18196
• Main Authors: Hiroshi Sakamoto, Yoshiaki Omata, Graham Palmer, Masato Noguchi
• Format: Article
• Language: English
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.274.26.18196

Whether or not reducing equivalents are indispensable for the conversion of ferric α-hydroxyheme bound to heme oxygenase-1 to verdoheme remains controversial (Matera, K. M., Takahashi, S., Fujii, H., Zhou, H., Ishikawa, K., Yoshimura, T., Rousseau, D. L., Yoshida, T., and Ikeda-Saito, M. (1996) J. Biol. Chem. 271, 6618–6624; Liu, Y., Moënne-Loccoz, P., Loehr, T. M., and Ortiz de Montellano, P. R. (1997) J. Biol. Chem. 272, 6906–6917). To resolve this controversy, we have prepared a ferric α-hydroxyheme-heme oxygenase-1 complex and titrated the complex with O 2 under strictly anaerobic conditions. The formation of verdoheme was monitored by optical and electron spin resonance spectroscopies. Electron spin resonance spectra of the complex showed that α-hydroxyheme exists as a mixture of resonance structures composed of the iron(III) porphyrin and the iron(II) porphyrin π neutral radical. Upon addition of CO the latter species becomes dominant. The results obtained from these titration experiments indicate that α-hydroxyheme can be converted to verdoheme by an approximately equimolar amount of O 2 without any requirement for exogenous electrons. The verdoheme formed from α-hydroxyheme was shown to be in the ferrous oxidation state by the addition of CO or potassium ferricyanide to the resultant verdoheme-heme oxygenase-1 complex.