Reduction of oxaporphyrin ring of CO-bound α-verdoheme complexed with heme oxygenase-1 by NADPH-cytochrome P450 reductase

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 alpha-verdoheme, an intermediate in the HO reaction, involvement of a verdoheme pi-neutral radical has been proposed. To explore this hypothetical mechanism, we performed electrochemical reduction of ferrous alpha-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 alpha-verdoheme was increased from -0.465 to 0392 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 center dot)], 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 alpha-verdoheme in its complex with HO-1 and an electron is subsequently transferred from CPR, probably via FAD, to the oxaporphyrin ring. (C) 2010 Elsevier Inc. All rights reserved.