Nonhydrolytic chemical conversion of octaethylverdochemocrome to octaethylbiliverdin

O(2)- and H(2)O(2)-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 IXalpha complexed with rat HO-1. Our findings include: 1) HO degrades verdoheme through a dual pathway using either O(2) or H(2)O(2); 2) the verdoheme reactivity with O(2) is the lowest among the three O(2) reactions in the HO catalysis, and the newly found H(2)O(2) pathway is approximately 40-fold faster than the O(2)-dependent verdoheme degradation; 3) both reactions are initiated by the binding of O(2) or H(2)O(2) 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 H(2)O(2) on the verdoheme iron with the aid of a nearby water molecule linked with Asp(140). 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 H(2)O(2)-dependent verdoheme degradation in vivo, and potential roles of the dual pathway reaction of HO against oxidative stress are proposed.

Formation of a Highly Oxidized Iron Biliverdin Complex upon Treatment of a Five-Coordinate Verdoheme with Dioxygen

Treatment of a green solution of the five-coordinate octaethylverdoheme, XFeII(OEOP) 1 (X = Cl or Br), with dioxygen results in the formation of a new iron complex of octaethylbiliverdin, 2, within a matter of minutes. The reaction has been monitored by 1H NMR spectroscopy, and the product 2 (X = Cl) has been isolated and examined by X-ray crystallography. The structure of 2 (X = Cl) shows that the iron is five-coordinate with bonds to the four nitrogen atoms of the helical tetrapyrrole ligand and to an axial chloride. Treatment of 2 (X = Cl or Br) with zinc amalgam produces the known iron(III) complex of biliverdin, {FeIII(OEB)}2. The unusual pattern of resonances in the 1H NMR spectrum of 2 and its facile reduction to {FeIII(OEB)}2 indicate that 2 is an oxidized complex that can be formulated by resonance structures involving either an Fe(IV) ion bound to a bilindione trianion or an Fe(III) ion bound to an oxidized, dianionic, radical form of the ligand.

Verdoheme reactivity. Remarkable paramagnetically shifted (1)H NMR spectra of intermediates from the addition of hydroxide or methoxide with Fe(II) and Fe(III) verdohemes

Studies of the reaction of 5-oxaporphyrin iron complexes (verdohemes) with methoxide ion or hydroxide ion have been undertaken to understand the initial step of ring opening of verdohemes. High-spin [ClFe(III)(OEOP)] undergoes a complex series of reactions upon treatment with hydroxide ion in chloroform, and similar species are also detected in dichloromethane, acetonitrile, and dimethyl sulfoxide. Three distinct paramagnetic intermediates have been identified by (1)H NMR spectroscopy. These reactive species are formed by addition of hydroxide to the macrocycle and to the iron as an axial ligand. Treatment of low-spin [(py)(2)Fe(II)(OEOP)]Cl (OEOP is the monoanion of octaethyl-5-oxaporphyrin) with excess methoxide ion in pyridine solution produces [(py)(n)()Fe(II)(OEBOMe)] (n = 1 or 2) ((OEBOMe), dianion of octaethylmethoxybiliverdin), whose (1)H NMR spectrum undergoes marked alteration upon addition of further amounts of methoxide ion. An identical (1)H NMR spectrum, which is characterized by methylene resonances with both upfield and downfield paramagnetic shifts, is formed upon treatment of [Fe(II)(OEBOMe)](2) with methoxide in pyridine solution and results from the formation of [(MeO)Fe(II)(OEBOMe)](-).

In Situ Monitoring of the Degradation of Iron Porphyrins by Dioxygen with Hydrazine as Sacrificial Reductant. Detection of Paramagnetic Intermediates in the Coupled Oxidation Process by (1)H NMR Spectroscopy

The effects of using hydrazine rather than ascorbic acid on the coupled oxidation of (OEP)Fe(II)(py)(2) (OEP is the dianion of octaethylporphyrin, py is pyridine) have been investigated with the goal of directly detecting reactive intermediates during the process of heme degradation by dioxygen. The reaction products, [(OEOP)Fe(II)(py)(2)]Cl and (OEB)Fe(III)(py)(2) (OEOP is the monoanion of octaethyl-5-oxaporphyrin and OEB is the trianion of octaethylbilindione), and their yields are similar to those of the standard coupled oxidation process. The reaction has been monitored in situ in pyridine/dichloromethane mixtures by (1)H NMR spectroscopy. The recently isolated and crystallographically characterized complex, (OEPO)Fe(py)(2) (OEOP is the trianion of octaethyloxaphlorin), has been identified as a key intermediate. Addition of dioxygen to (OEP)Fe(II)(py)(2) in pyridine with hydrazine present also produces two new transient species: (OEPO)Fe(py)(N(2)D(4)) and (OEPO)Fe(N(2)D(4))(2). These complexes have also be produced independently by low-temperature titration of hydrazine into a solution of {(OEPO)Fe}(2). Thus, hydrazine acts as an axial ligand during the early stages of the coupled oxidation process. However, the two hydrazine-containing complexes eventually are converted into (OEPO)Fe(py)(2) before [(OEOP)Fe(II)(py)(2)]Cl and (OEB)Fe(III)(py)(2) are formed. The observations reported here suggest that the coupled oxidation process can be divided into two stages. The first stage involves the meso C-H bond and results in introduction of oxygen at that site with the formation of the three intermediates: (OEPO)Fe(N(2)H(4))(2), (OEPO)Fe(N(2)H(2))(py), and (OEPO)Fe(py)(2). The second stage of the coupled oxidation process involves C-C bond breaking and the conversion of the hydroxylated heme, (OEPO)Fe(py)(2), into the final products, [(OEOP)Fe(II)(py)(2)](+) and (OEB)Fe(III)(py)(2).