O2- and H2O2-dependent verdoheme degradation by heme oxygenase -: Reaction mechanisms and potential physiological roles of the dual pathway degradation

Heme oxygenase (HO) catalyzes the catabolism of heme to biliverdin, CO, and a free iron through three successive oxygenation steps. The third oxygenation, oxidative degradation of verdoheme to biliverdin, has been the least understood step despite its importance in regulating HO activity. We have examined in detail the degradation of a synthetic verdoheme IX alpha complexed with rat HO-1. Our findings include: 1) HO degrades verdoheme through a dual pathway using either O-2 or H2O2; 2) the verdoheme reactivity with O-2 is the lowest among the three O-2 reactions in the HO catalysis, and the newly found H2O2 pathway is similar to 40-fold faster than the O-2-dependent verdoheme degradation; 3) both reactions are initiated by the binding of O-2 or H2O2 to allow the first direct observation of degradation intermediates of verdoheme; and 4) Asp(140) in HO-1 is critical for the verdoheme degradation regardless of the oxygen source. On the basis of these findings, we propose that the HO enzyme activates O-2 and H2O2 on the verdoheme iron with the aid of a nearby water molecule linked with Asp140. These mechanisms are similar to the well established mechanism of the first oxygenation, meso-hydroxylation of heme, and thus, HO can utilize a common architecture to promote the first and third oxygenation steps of the heme catabolism. In addition, our results infer the possible involvement of the H2O2-dependent verdoheme degradation in vivo, and potential roles of the dual pathway reaction of HO against oxidative stress are proposed.