title: Reduction of oxaporphyrin ring of CO-bound α-verdoheme complexed with heme oxygenase-1 by NADPH-cytochrome P450 reductase.
abstract: Heme oxygenase (HO) catalyses the degradation of heme to biliverdin, carbon monoxide (CO) and ferrous iron via three successive monooxygenase reactions, using electrons provided by NADPH-cytochrome P450 reductase (CPR) and oxygen molecules. For cleavage of the oxaporphyrin ring of ferrous α-verdoheme, an intermediate in the HO reaction, involvement of a verdoheme π-neutral radical has been proposed. To explore this hypothetical mechanism, we performed electrochemical reduction of ferrous α-verdoheme-rat HO-1 complex under anaerobic conditions. Upon binding of CO, an O(2) surrogate, the midpoint potential for one-electron reduction of the oxaporphyrin ring of ferrous α-verdoheme was increased from -0.465 to -0.392 V vs the normal hydrogen electrode. Because the latter potential is close to that of the semiquinone/reduced redox couple of FAD in CPR, the one-electron reduction of the oxaporphyrin ring of CO-bound verdoheme complexed with HO-1 is considered to be a thermodynamically likely process. Indeed the one-electron reduced species, [Fe(II)(verdoheme•)], was observed spectroscopically in the presence of CO in both NADPH/wild-type and FMN-depleted CPR systems under anaerobic conditions. Under physiological conditions, therefore, it is possible that O(2) initially binds to the ferrous iron of α-verdoheme in its complex with HO-1 and an electron is subsequently transferred from CPR, probably via FAD, to the oxaporphyrin ring.
authors.affiliation: Department of Medical Biochemistry, Kurume University School of Medicine, 67 Asahi-machi, Kurume 830-0011, Japan. hsato@med.kurume-u.ac.jp
authors.forename: Hideaki
authors.initials: H
authors.lastname: Sato
authors.forename: Yuichiro
authors.initials: Y
authors.lastname: Higashimoto
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Sakamoto
authors.forename: Masakazu
authors.initials: M
authors.lastname: Sugishima
authors.forename: Chizu
authors.initials: C
authors.lastname: Shimokawa
authors.forename: Jiro
authors.initials: J
authors.lastname: Harada
authors.forename: Graham
authors.initials: G
authors.lastname: Palmer
authors.forename: Masato
authors.initials: M
authors.lastname: Noguchi
mesh_terms: D000693:Anaerobiosis; D000818:Animals; D002248:Carbon Monoxide / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy; D005296:Ferrous Compounds / Q000737:chemistry*; D006418:Heme / Q000031:analogs & derivatives* / Q000737:chemistry; D051547:Heme Oxygenase-1 / Q000737:chemistry*; D009251:NADPH-Ferrihemoprotein Reductase / Q000737:chemistry*; D010084:Oxidation-Reduction; D011166:Porphyrins / Q000737:chemistry*; D051381:Rats
chemical_list: D005296:Ferrous Compounds; D011166:Porphyrins; C099547:verdoheme; D006418:Heme; D002248:Carbon Monoxide; D051547:Heme Oxygenase-1; D009251:NADPH-Ferrihemoprotein Reductase