Alkyl peroxides reveal the ring opening mechanism of verdoheme catalyzed by heme oxygenase

Heme oxygenase (HO) catalyzes heme catabolism through three successive oxygenation steps where the substrate heme itself activates O₂. Although a rate-determining step of the HO catalysis is considered as third oxygenation, the verdoheme degradation mechanism has been the least understood in the HO catalysis. In order to discriminate three possible pathways proposed for the verdoheme ring-opening, we have examined reactions of the verdoheme-HO-1 complex with alkyl peroxides, namely MeOOH. Under reducing conditions, the MeOOH reaction afforded two novel products whose absorption spectra are similar to but slightly different from that of biliverdin. HPLC, ESI-MS, and NMR analysis show that these products are 1- and 19-methoxy-deoxy-biliverdins. The addition of a methoxy group at one end of the linear tetrapyrrole unambiguously indicates transient formation of the Fe-OOMe intermediate and rearrangement of its terminal methoxy group to the α-pyrrole carbon. The corresponding OH transfer of the Fe-OOH species is highly probable in the H₂O₂-dependent verdoheme degradation and is likely to be the case in the O₂-dependent reaction catalyzed by HO as well.