Mechanism of heme degradation by heme oxygenase

Protein expressed by the ho2 gene of the cyanobacterium Synechocystis sp. PCC 6803 is a true heme oxygenase. Properties of the heme and enzyme complex

Two isoforms of a heme oxygenase gene, ho1 and ho2, with 51% identity in amino acid sequence have been identified in the cyanobacterium Synechocystis sp. PCC 6803. Isoform-1, Syn HO-1, has been characterized, while isoform-2, Syn HO-2, has not. In this study, a full-length ho2 gene was cloned using synthetic DNA and Syn HO-2 was demonstrated to be highly expressed in Escherichia coli as a soluble, catalytically active protein. Like Syn HO-1, the purified Syn HO-2 bound hemin stoichiometrically to form a heme-enzyme complex and degraded heme to biliverdin IXalpha, CO and iron in the presence of reducing systems such as NADPH/ferredoxin reductase/ferredoxin and sodium ascorbate. The activity of Syn HO-2 was found to be comparable to that of Syn HO-1 by measuring the amount of bilirubin formed. In the reaction with hydrogen peroxide, Syn HO-2 converted heme to verdoheme. This shows that during the conversion of hemin to alpha-meso-hydroxyhemin, hydroperoxo species is the activated oxygen species as in other heme oxygenase reactions. The absorption spectrum of the hemin-Syn HO-2 complex at neutral pH showed a Soret band at 412 nm and two peaks at 540 nm and 575 nm, features observed in the hemin-Syn HO-1 complex at alkaline pH, suggesting that the major species of iron(III) heme iron at neutral pH is a hexa-coordinate low spin species. Electron paramagnetic resonance (EPR) revealed that the iron(III) complex was in dynamic equilibrium between low spin and high spin states, which might be caused by the hydrogen bonding interaction between the distal water ligand and distal helix components. These observations suggest that the structure of the heme pocket of the Syn HO-2 is different from that of Syn HO-1.

Crystal structure of heme oxygenase-1 from cyanobacterium Synechocystis sp. PCC 6803 in complex with heme

Heme oxygenase (HO) catalyzes the oxidative degradation of heme utilizing molecular oxygen and reducing equivalents. In photosynthetic organisms, HO functions in the biosynthesis of such open-chain tetrapyrroles as phyto-chromobilin and phycobilins, which are involved in the signal transduction for light responses and light harvesting for photosynthesis, respectively. We have determined the first crystal structure of a HO-1 from a photosynthetic organism, Synechocystis sp. PCC 6803 (Syn HO-1), in complex with heme at 2.5 A resolution. Heme-Syn HO-1 shares a common folding with other heme-HOs. Although the heme pocket of heme-Syn HO-1 is, for the most part, similar to that of mammalian HO-1, they differ in such features as the flexibility of the distal helix and hydrophobicity. In addition, 2-propanol derived from the crystallization solution occupied the hydrophobic cavity, which is proposed to be a CO trapping site in rat HO-1 that suppresses product inhibition. Although Syn HO-1 and mammalian HO-1 are similar in overall structure and amino acid sequence (57% similarity vs. human HO-1), their molecular surfaces differ in charge distribution. The surfaces of the heme binding sides are both positively charged, but this patch of Syn HO-1 is narrow compared to that of mammalian HO-1. This feature is suited to the selective binding of ferredoxin, the physiological redox partner of Syn HO-1; the molecular size of ferredoxin is approximately 10 kDa whereas the size of NADPH-cytochrome P450 reductase, a reducing partner of mammalian HO-1, is approximately 77 kDa. A docking model of heme-Syn HO-1 and ferredoxin suggests indirect electron transfer from an iron-sulfur cluster in ferredoxin to the heme iron of heme-Syn HO-1.

Crystal structures of ferrous and ferrous–NO forms of verdoheme in a complex with human heme oxygenase-1: catalytic implications for heme cleavage

Heme oxygenase oxidatively degrades heme to biliverdin resulting in the release of iron and CO through a process in which the heme participates both as a cofactor and substrate. One of the least understood steps in the heme degradation pathway is the conversion of verdoheme to biliverdin. In order to obtain a better understanding of this step we report the crystal structures of ferrous-verdoheme and, as a mimic for the oxy-verdoheme complex, ferrous-NO verdoheme in a complex with human HO-1 at 2.20 and 2.10 A, respectively. In both structures the verdoheme occupies the same binding location as heme in heme-HO-1, but rather than being ruffled verdoheme in both sets of structures is flat. Both structures are similar to their heme counterparts except for the distal helix and heme pocket solvent structure. In the ferrous-verdoheme structure the distal helix moves closer to the verdoheme, thus tightening the active site. NO binds to verdoheme in a similar bent conformation to that found in heme-HO-1. The bend angle in the verodoheme-NO structure places the terminal NO oxygen 1 A closer to the alpha-meso oxygen of verdoheme compared to the alpha-meso carbon on the heme-NO structure. A network of water molecules, which provide the required protons to activate the iron-oxy complex of heme-HO-1, is absent in both ferrous-verdoheme and the verdoheme-NO structure.

Non-specific inhibition of human cytochrome P450-catalyzed reactions by hemin

Hemin, a stable form of heme, is known to have an antimutagenic effect. Inhibitory effects of hemin on the cytochrome P450 (CYP)-catalyzed reactions of human liver microsomes and reconstituted systems containing purified CYP and NADPH-cytochrome P450 reductase (NPR) were seen. Hemin non-specifically inhibited all of the microsomal CYP activities examined. Hemin also inhibited 7-ethoxyresorufin O-deethylation, 3-[2-(N,N-diethyl-N-methylammonium)ethyl]-7-methoxy-4-methylcoumarin O-demethylation, and testosterone 6beta-hydroxylation catalyzed by purified CYPs 1A2, 2D6, and 3A4, with IC50 values of 27, 19, and 2.4 microM, respectively. Hemin also inhibited reduction of cytochrome c and ferricyanide by NPR, as much as 47%. Spectrally detectable CYP was destroyed in human liver microsomes and in a reconstituted system in the presence of hemin and an NADPH-generating system. We propose that the antimutagenic effect of hemin might be due to inhibition of CYP and NPR enzymes involved in the bioactivation of mutagens.

Models and mechanisms of O-O bond activation by cytochrome P450. A critical assessment of the potential role of multiple active intermediates in oxidative catalysis

Cytochrome P450 enzymes promote a number of oxidative biotransformations including the hydroxylation of unactivated hydrocarbons. Whereas the long-standing consensus view of the P450 mechanism implicates a high-valent iron-oxene species as the predominant oxidant in the radicalar hydrogen abstraction/oxygen rebound pathway, more recent studies on isotope partitioning, product rearrangements with 'radical clocks', and the impact of threonine mutagenesis in P450s on hydroxylation rates support the notion of the nucleophilic and/or electrophilic (hydro)peroxo-iron intermediate(s) to be operative in P450 catalysis in addition to the electrophilic oxenoid-iron entity; this may contribute to the remarkable versatility of P450s in substrate modification. Precedent to this mechanistic concept is given by studies with natural and synthetic P450 biomimics. While the concept of an alternative electrophilic oxidant necessitates C-H hydroxylation to be brought about by a cationic insertion process, recent calculations employing density functional theory favour a 'two-state reactivity' scenario, implicating the usual ferryl-dependent oxygen rebound pathway to proceed via two spin states (doublet and quartet); state crossing is thought to be associated with either an insertion or a radicalar mechanism. Hence, challenge to future strategies should be to fold the disparate and sometimes contradictory data into a harmonized overall picture.

Oxidative Verdoheme Formation and Stabilization by Axial Isocyanide Ligation

The effect of isocyanides as axial ligands on the formation and stability of verdoheme by oxidation has been examined. The reaction of [Fe(III)(OEPO)]2 with t-butyl isocyanide under dioxygen-free conditions results in the formation of (t-BuNC)2Fe(II)(OEPO*) with an electron paramagnetic resonance at g=2.009 with a peak-to-peak separation of 23.5 G at 4 K. (OEPO is the trianion of octaethyloxophlorin and OEPO* is the radical dianion obtained from OEPO by one-electron oxidation.) Exposure of chloroform solutions of either (2,6-xylylNC)2Fe(II)(OEPO*) or (t-BuNC)2Fe(II)(OEPO*) to dioxygen followed by the addition of ammonium hexafluorophosphate results in their transformation into the diamagnetic verdohemes, [(2,6-xylylNC)2Fe(II)(OEOP)](PF6) and [(t-BuNC)2Fe(II)(OEOP)](PF6), yields 68 and 70%, respectively. (OEOP is the anion of octaethyl-5-oxaporphyrin.) The oxidation reactions of (2,6-xylylNC)2Fe(II)(OEPO*) and (t-BuNC)2Fe(II)(OEPO*) have also been monitored by 1H NMR spectroscopy. No resonances due to paramagnetic products could be detected, the reactions appear to result only in the formation of the diamagnetic verdohemes, and the products are not susceptible to further oxidation.

Reactions ofmeso-Hydroxyhemes with Carbon Monoxide and Reducing Agents in Search of the Elusive Species Responsible for theg= 2.006 Resonance of Carbon Monoxide-Treated Heme Oxygenase. Isolation of Diamagnetic Iron(II) Complexes of Octaethyl-meso-hydroxyporphyrin

To examine possible models for the g = 2.006 resonance seen when the hydroxylated heme-heme oxygenase complex in the Fe(III) state is treated with CO, the reactivities of CO and reducing agents with (py)(2)Fe(III)(OEPO) and [Fe(III)(OEPO)](2) (OEPO is the trianion of octaethyl-meso-hydroxyporphyrin) have been examined. A pyridine solution of (py)(2)Fe(III)(OEPO) reacts in a matter of minutes with zinc amalgam (or with hydrazine) under an atmosphere of dioxygen-free dinitrogen to produce bright-red (py)(2)Fe(II)(OEPOH).2py.0.33H(2)O, which has been isolated in crystalline form. The (1)H NMR spectrum of (py)(2)Fe(II)(OEPOH) in a pyridine-d(5) solution is indicative of the presence of a diamagnetic compound, and no EPR resonance was observed for this compound. Treatment of a solution of (py)(2)Fe(II)(OEPOH) in pyridine-d(5) with carbon monoxide produces spectral changes after a 30 s exposure that are indicative of the formation of diamagnetic (OC)(py)Fe(II)(OEPOH). Treatment of a green pyridine solution of (py)(2)Fe(III)(OEPO) with carbon monoxide reveals a slow color change to deep red over a 16 h period. Although a resonance at g = 2.006 was observed in the EPR spectrum of the sample during the reaction, the isolated product is EPR silent. The spectroscopic features of the final solution are identical to those of a solution formed by treating (py)(2)Fe(II)(OEPOH) with carbon monoxide. Addition of hydrazine to solutions of (OC)(py)Fe(II)(OEPOH) produces red, diamagnetic (OC)(N(2)H(4))Fe(II)(OEPOH).py in crystalline form. The X-ray crystal structures of (py)(2)Fe(II)(OEPOH).2py.0.33H(2)O and (OC)(N(2)H(4))Fe(II)(OEPOH).py have been determined. Solutions of diamagnetic (OC)(N(2)H(4))Fe(II)(OEPOH).py and (OC)(py)Fe(II)(OEPOH) are extremely air sensitive and are immediately converted in a pyridine solution into paramagnetic (py)(2)Fe(III)(OEPO) in the presence of dioxygen.

1H NMR characterization of the solution active site structure of substrate-bound, cyanide-inhibited heme oxygenase from Neisseria meningitidis: comparison to crystal structures

Heme oxygenase, HO, from the pathogenic bacterium Neisseria meningitidis catabolizes heme for the iron necessary for infection. The enzyme, labeled HemO, exhibits less sequence homology to mammalian HO than another studied HO from Corynebacterium diphtheriae. Solution 1H NMR has been utilized to define the active site molecular and electronic structure of the cyanide-inhibited, substrate-bound complex for comparison with those provided by several crystal structures. Extensive assignments by solely 1H NMR 2D methods reveal a structure that is very strongly conserved with respect to the crystal structure, although 1H/2H exchange indicates dynamically much more stable distal and proximal helices than those for other HOs. Several residues found with alternate orientations in crystal structures of water- and NO-ligated complexes were shown to occupy positions found solely in the NO complex, confirming that there are structural accommodations in response to ligating the substrate complex with a diatomic, H-bond acceptor ligand. The observed dipolar shifts allow the determination of the magnetic axes that show that the Fe-CN unit is tilted approximately 10 degrees toward the alpha-meso position, thereby facilitating the alpha-stereoselectivity of the enzyme. Numerous labile protons with larger than usual low-field bias are identified and, in common with the other HO complexes, shown to participate in an extended, distal side H-bond network. This H-bond network orders several water molecules, most, but not all, of which have been detected crystallographically. A series of three C-terminal residues, His207-Arg208-His209, are not detected in crystal structures. However, 1H NMR finds two residues, His207 and likely Arg208 in contact with pyrrole D, which in crystal structures is exposed to solvent. The nature of the NOEs leads us to propose a H-bond between the proximally oriented His207 ring and the carboxylate of Asp27 and a salt-bridge between the terminus of Arg208 and the reoriented 7-propionyl carboxylate. While numerous ordered water molecules are found near both propionates in the crystal structure, we find much larger water NOEs to the 6- than 7-propionate, suggesting that water molecules near the 7-propionate have been expelled from the cavity by the insertion of Arg208 into the distal pocket. The conversion of the 7-propionate link from the N-terminal region (Lys16) to the C-terminal region (Arg208) in the ligated substrate complex both closes the heme cavity more tightly and may facilitate product exit, the rate-limiting step in the enzyme activity.

Heme oxygenase in experimental intracerebral hemorrhage: the benefit of tin-mesoporphyrin

The prognosis of intracerebral hemorrhage (ICH) is unfavorable. Beyond immediate mass effect and tissue destruction, ICHs cause additional neuronal loss in a "perifocal reactive zone." Heme in ICH induces heme oxygenase-1 (HO-1), and the action of this enzyme on heme yields ferrous iron, biliverdin, and carbon monoxide. Iron is ultimately converted to ferritin and hemosiderin. Free iron is tissue-toxic, and inhibition of HO-1 should provide protection against additional damage. Experimental ICHs were made in adult rabbits by the stereotaxic injection of autologous blood, and the induction of HO-1 and increase in ferritin were followed by confocal immunofluorescence microscopy. Heme diffused rapidly through perivascular spaces, and HO-1 reaction product first occurred in perivascular cells and microglia. At this stage, HO-1 and ferritin showed extensive colocalization. As ICH resolution progressed, HO-1 immunoreactivity faded while ferritin and hemosiderin continued to accumulate. This process was accompanied by a gradient of destruction of neuronal cell bodies and dendrites in the perifocal reactive zone. In an effort to inhibit HO-1, repeated intravenous injections of tin-mesoporphyrin IX (SnMP) were given to ICH-bearing rabbits. The ICH disrupted the blood-brain barrier sufficiently to allow SnMP to enter the brain in pharmacological amounts, and the metalloporphyrin provided significant protection against neuronal loss.

The heme oxygenase dilemma in cellular homeostasis: new insights for the feedback regulation of heme catabolism

Heme must be synthesized and degraded within an individual nucleated cell. Heme degradation is catalyzed by the two isozymes of heme oxygenase, heme oxygenase-1 (HO-1) and HO-2, eventually yielding biliverdin/bilirubin, CO, and iron. These products possess important physiological roles but are potentially toxic to cells. Characteristically, human HO-1 contains no Cys residues, whereas HO-2 contains the potential heme-binding motifs of the Cys-Pro dipeptide. Expression of HO-1 is inducible or repressible, depending on cell types or cellular microenvironments, but expression levels of HO-2 are fairly constant. Thus, the main regulation of heme catabolism is a problem of the balance between induction and repression of HO-1. Notably, HO-1 expression is induced by heme in all mammalian cells examined, but is repressed by hypoxia in certain types of cultured human cells. The recent discovery of Bach1 as a heme-regulated and hypoxia-inducible repressor for transcription of the HO-1 gene has provided a missing link in the feedback control of heme catabolism. On the other hand, the human HO-1 gene promoter contains the (GT)n repeat polymorphism and a single nucleotide polymorphism (-427A --> T), both of which may contribute to fine-tuning of the transcription. Importantly, long (GT)n alleles are associated with susceptibility to smoking-induced emphysema or coronary artery disease, but may provide with resistance to cerebral malaria. The latter finding suggests a novel therapeutic strategy with inhibitors of HO-1 for the treatment of cerebral malaria. We discuss the potential regulatory role of Bach1 and HO-2 in heme catabolism and update the understanding of the regulation of HO-1 expression.

Multigenic control of hepatic iron loading in a murine model of hemochromatosis

BACKGROUND & AIMS

Hereditary hemochromatosis is a common disorder of iron homeostasis characterized by increased dietary iron absorption and progressive iron accumulation, mainly in the liver. Most patients are homozygous for the C282Y mutation in the HFE gene. However, not all individuals carrying the hemochromatosis-predisposing genotype in the general population become iron loaded. Genetic modifiers have been shown to influence disease penetrance, but their number and chromosomal locations remain unknown, and their identification is hampered by complex interactions with environmental factors. To circumvent these difficulties, we used 2 strains of mice made deficient for the Hfe gene that strongly differ in their propensity to develop hepatic iron loading.

METHODS

To localize the loci controlling hepatic iron loading in this murine model of hemochromatosis, we produced 1028 mice by an F2 intercross between the C57BL/6 and DBA/2 Hfe-deficient strains. We selected the 276 mice that contributed the most to the total linkage information for genotyping with 145 microsatellite markers.

RESULTS

We mapped 4 modifier loci on chromosomes 7, 8, 11, and 12, with logarithm of odds scores of 14.47, 12.96, 6.04, and 6.72, respectively, in regions containing several genes recently shown to exert important roles in the regulation of iron metabolism.

CONCLUSIONS

Our data provide a clear demonstration of the polygenic pattern of hepatic iron loading inheritance in Hfe-deficient mice. Examination of candidate genes residing at the loci identified in this study and genetic analysis of the syntenic chromosomal regions in humans may provide important insight into the heterogeneous disease presentation observed among HFE C282Y homozygotes.

Unique features of recombinant heme oxygenase of Drosophila melanogaster compared with those of other heme oxygenases studied

We cloned a cDNA for a Drosophila melanogaster homologue of mammalian heme oxygenase (HO) and constructed a bacterial expression system of a truncated, soluble form of D. melanogaster HO (DmDeltaHO). The purified DmDeltaHO degraded hemin to biliverdin, CO and iron in the presence of reducing systems such as NADPH/cytochrome P450 reductase and sodium ascorbate, although the reaction rate was slower than that of mammalian HOs. Some properties of DmHO, however, are quite different from other known HOs. Thus DmDeltaHO bound hemin stoichiometrically to form a hemin-enzyme complex like other HOs, but this complex did not show an absorption spectrum of hexa-coordinated heme protein. The absorption spectrum of the ferric complex was not influenced by changing the pH of the solution. Interestingly, an EPR study revealed that the iron of heme was not involved in binding heme to the enzyme. Hydrogen peroxide failed to convert it into verdoheme. A spectrum of the ferrous-CO form of verdoheme was not detected during the reaction from hemin under oxygen and CO. Degradation of hemin catalyzed by DmDeltaHO yielded three isomers of biliverdin, of which biliverdin IXalpha and two other isomers (IXbeta and IXdelta) accounted for 75% and 25%, respectively. Taken together, we conclude that, although DmHO acts as a real HO in D. melanogaster, its active-site structure is quite different from those of other known HOs.

How Heme Metabolism Occurs in Heme Oxygenase: Computational Study of Oxygen-Donation Ability of the Oxo and Hydroperoxo Species

We report a density functional theory study on the heme metabolism in heme oxygenase using iron-hydroperoxo and -oxo models. The activation energies for heme oxidation at the alpha-carbon by the iron-hydroperoxo and -oxo species are calculated to be 42.9 and 39.9 kcal/mol, respectively. These high activation barriers lead us to reconsider the catalytic mechanism of heme oxygenase

Solution 1H, 15N NMR spectroscopic characterization of substrate-bound, cyanide-inhibited human heme oxygenase: water occupation of the distal cavity

A solution NMR spectroscopic study of the cyanide-inhibited, substrate-bound complex of uniformly (15)N-labeled human heme oxygenase, hHO, has led to characterization of the active site with respect to the nature and identity of strong hydrogen bonds and the occupation of ordered water molecules within both the hydrogen bonding network and an aromatic cluster on the distal side. [(1)H-(15)N]-HSQC spectra confirm the functionalities of several key donors in particularly robust H-bonds, and [(1)H-(15)N]HSQC-NOESY spectra lead to the identification of three additional robust H-bonds, as well as the detection of two more relatively strong H-bonds whose identities could not be established. The 3D NMR experiments provided only a modest, but important, extension of assignments because of the loss of key TOCSY cross-peaks due to the line broadening from a dynamic heterogeneity in the active site. Steady-state NOEs upon saturating the water signal locate nine ordered water molecules in the immediate vicinity of the H-bond donors, six of which are readily identified in the crystal structure. The additional three are positioned in available spaces to account for the observed NOEs. (15)N-filtered steady-state NOEs upon saturating the water resonances and (15)N-filtered NOESY spectra demonstrate significant negative NOEs between water molecules and the protons of five aromatic rings. Many of the NOEs can be rationalized by water molecules located in the crystal structure, but strong water NOEs, particularly to the rings of Phe47 and Trp96, demand the presence of at least an additional two immobilized water molecules near these rings. The H-bond network appears to function to order water molecules to provide stabilization for the hydroperoxy intermediate and to serve as a conduit to the active site for the nine protons required per HO turnover.

Postnatal development of hepatic oxidative, hydrolytic and conjugative drug-metabolizing enzymes in female horses

Little is known about the effects of aging on the hepatic drug metabolizing capacity of horses despite the relatively long lifespan characterizing this species. A wide array of cytochrome P450 (CYP)-dependent monooxygenases, carboxylesterases and transferases were assayed in liver microsomes from 50 female horses in an age range between less than 1 year to over 12 years. Rather unexpectedly, both the CYP content and the activity of NADPH cytochrome c reductase rose as a function of age. Accordingly, a general increasing trend was recorded in the rate of the in vitro metabolism of the substrates reported to be related to CYP2B-, CYP2E- or CYP3A, although, as detected by Western immunoblotting, only the levels of proteins recognized by anti-rat CYP3A- and CYP2B antibodies appeared to increase consistently. Also the carboxylesterases and uridindiphosphoglucuronyl-transferase (UGT) activity toward 1-naphthol displayed a similar trend, glutathione S-transferase accepting 3,4-dichloronitrobenzene as a substrate being the only enzyme activity showing an age-related decline. A positive correlation was also found between liver cadmium content and CYP amount as well as the activities of most monooxygenases (except for those related to CYP1A), carboxylesterases, and UGT. While confirming that a number of enzyme activities are less expressed in foals, our results contradict the general view that the drug metabolizing capacity drops in elder individuals. Although several other factors can influence the kinetics of foreign compounds in aged animals, data from this study may provide insight in understanding possible age-related differences in drug efficacy and the response to toxic substances in horses.

1H NMR investigation of the solution structure of substrate-free human heme oxygenase: comparison to the cyanide-inhibited, substrate-bound complex

1H NMR was used to investigate the molecular structure, and dynamic properties of soluble, recombinant, substrate-free human heme oxygenase (apohHO) on a comparative basis with similar studies on the substrate complex. Limited but crucial sequence-specific assignments identify five conserved secondary structural elements, and the detection of highly characteristic dipolar or H-bond interactions among these elements together with insignificant chemical shift differences confirm a strongly conserved folding topology of helices C-H relative to that of substrate complexes in either solution or the crystal. The correction of the chemical shifts for paramagnetic and porphyrin ring current influences in the paramagnetic substrate complex reveals that the strength of all but one of the numerous relatively robust H-bonds are conserved in apohHO, and similar ordered water molecules are located near these H-bond donors as observed in the substrate complexes. The unique and significant weakening of the Tyr(58) OH hydrogen bond to the catalytically critical Asp(140) carboxylate in apohHO is suggested to arise from the removal of the axial H-bond acceptor ligand rather than the loss of substrate. The interhelical positions of the conserved strong H-bonds argue for a structural role in maintaining a conserved structure for helices C-H upon loss of substrate. While the structure and H-bond network are largely conserved upon loss of substrate, the variably increased rate of NH lability dictates a significant loss of dynamic stability in the conserved structure, particularly near the distal helix F.

Hemodextrin: a self-assembled cyclodextrin–porphyrin construct that binds dioxygen

Synthetic hemoprotein model compounds are of great interest due to the vital roles and complexities of hemoproteins. This study reports a novel, self-assembled hemoprotein model, hemodextrin. The synthesis and characterization of py-PPCD (2(A)-monopyridylmethyl-perPEGylated-beta-cyclodextrin) (2) is described. The molecular design is based on a pegylated cyclodextrin scaffold that bears both a heme-binding pocket and an axial ligand that binds an iron porphyrin. The binding constant for Fe(III)TPPS [iron(III) meso-tetra(4-sulphonatophenyl)porphyrin] by py-PPCD (2) was determined to be 2 x 10(6) M(-1) at pH 6.0 by observing characteristic changes in the UV-Vis spectrum of the porphyrin. The pyridyl nitrogen of py-PPCD (2) was shown to ligate to the iron center by observing signal changes in the Fe(II)-porphyrin 1H-NMR spectrum. This hemodextrin ensemble was shown to bind dioxygen reversibly and to form a stable ferryl species.

Differential expressions of heme oxygenase-1 gene in CD25- and CD25+ subsets of human CD4+ T cells

Growing evidence suggests that the immunomodulatory heme oxygenase-1 (HO-1) may have an important role in regulating T-cell responses. In this study, we investigated whether CD4(+)CD25(-) and CD4(+)CD25(+) T cells of human CD4(+) subpopulation could differentially express HO-1. Our results obtained from qualitative reverse transcriptase-polymerase chain reaction and quantitative flow cytometry analyses revealed that the CD4(+)CD25(+) T cells constitutively express HO-1 and that T cell stimulation with plate-bound anti-CD3 in combination with soluble anti-CD28 not only induced HO-1 gene expression in the CD4(+)CD25(-) T cells but also up-regulated HO-1 gene expression in the CD4(+)CD25(+) T cells. Our further studies showed that CD28 signal alone was enough to induce HO-1 expression and CD3 signal, of which signal alone did not induce HO-1 expression, was required at least for full HO-1 expression in both CD25(-) and CD25(+) subsets of human CD4(+) T cells. In addition, transfection of human Jurkat T cells with HO-1 suppressed the cellular proliferation, and this effect was reversed by zinc protoporphyrin, a specific HO competitive inhibitor. Taken together, we have first reported that human CD4(+)CD25(+) regulatory T cells constitutively express HO-1 and that HO-1 inhibits Jurkat T cell proliferation.

Regulation of CO production in cerebral microvessels of newborn pigs

Carbon monoxide (CO) is produced from heme by heme oxygenase-2 (HO-2) in cerebral blood vessels. Gas chromatography-mass spectrometry was used on piglet cerebral microvessels to address the hypothesis that CO production is regulated by heme delivery and HO-2 catalytic activity. CO production appears to be substrate limited because heme and its precursor aminolevulinate increase CO production. Ionomycin also increases CO production. However, CO production from exogenous heme was the same in Ca-replete medium, Ca-free medium with ionomycin, and Ca-replete medium with ionomycin. Phorbol myristate acetate increases CO production but does not change the catalytic activity of HO-2. Also, the protein kinase C inhibitor 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine has no effect on the HO-2 catalytic activity. Protein tyrosine kinase inhibition reduces HO-2 catalytic activity. Inhibition of protein tyrosine phosphatases increased HO-2 catalytic activity. Therefore, regulation of CO production by cerebral microvessels can include changing heme availability and HO-2 catalytic activity. HO-2 catalytic activity is stimulated by tyrosine phosphorylation.

Stereoselectivity of each of the three steps of the heme oxygenase reaction: hemin to meso-hydroxyhemin, meso-hydroxyhemin to verdoheme, and verdoheme to biliverdin

Heme oxygenase catalyzes the regiospecific oxidation of hemin to biliverdin IXalpha with concomitant liberation of CO and iron by three sequential monooxygenase reactions. The alpha-regioselectivity of heme oxygenase has been thought to result from the regioselective oxygenation of the heme alpha-meso position at the first step, which leads to the reaction pathway via meso-hydroxyheme IXalpha and verdoheme IXalpha intermediates. However, recent reports concerning heme oxygenase forming biliverdin isomers other than biliverdin IXalpha raise a question whether heme oxygenase can degrade meso-hydroxyhemin and isomers other than the alpha-isomers. In this paper, we investigated the stereoselectivity of each of the two reaction steps from meso-hydroxyhemin to verdoheme and verdoheme to biliverdin by using a truncated form of rat heme oxygenase-1 and the chemically synthesized four isomers of meso-hydroxyhemin and verdoheme. Heme oxygenase-1 converted all four isomers of meso-hydroxyhemin to the corresponding isomers of verdoheme. In contrast, only verdoheme IXalpha was converted to the corresponding biliverdin IXalpha. We conclude that the third step, but not the second, is stereoselective for the alpha-isomer substrate. The present findings on regioselectivities of the second and the third steps have been discussed on the basis of the oxygen activation mechanisms of these steps.

Regulatory defects in liver and intestine implicate abnormal hepcidin and Cybrd1 expression in mouse hemochromatosis

Individuals with hereditary hemochromatosis suffer from systemic iron overload due to duodenal hyperabsorption. Most cases arise from a founder mutation in HFE (845G-->A; ref. 2) that results in the amino-acid substitution C282Y and prevents the association of HFE with beta2-microglobulin. Mice homozygous with respect to a null allele of Hfe (Hfe-/-) or homozygous with respect to the orthologous 882G-->A mutation (Hfe(845A/845A)) develop iron overload that recapitulates hereditary hemochromatosis in humans, confirming that hereditary hemochromatosis arises from loss of HFE function. Much work has focused on an exclusive role for the intestine in hereditary hemochromatosis. HFE deficiency in intestinal crypt cells is thought to cause intestinal iron deficiency and greater expression of iron transporters such as SLC11A2 (also called DMT1, DCT1 and NRAMP2) and SLC11A3 (also called IREG1, ferroportin and MTP1; ref. 3). Published data on the expression of these transporters in the duodenum of HFE-deficient mice and humans are contradictory. In this report, we used a custom microarray to assay changes in duodenal and hepatic gene expression in Hfe-deficient mice. We found unexpected alterations in the expression of Slc39a1 (mouse ortholog of SLC11A3) and Cybrd1, which encode key iron transport proteins, and Hamp (hepcidin antimicrobial peptide), a hepatic regulator of iron transport. We propose that inappropriate regulatory cues from the liver underlie greater duodenal iron absorption, possibly involving the ferric reductase Cybrd1.

Cloning and expression of a heme binding protein from the genome of Saccharomyces cerevisiae

The YLR205c gene of Saccharomyces cerevisiae does not show significant sequence identity to any known gene, except for heme oxygenase (22% to human HO-1). The YLR205 ORF was cloned and overexpressed in both Escherichia coli and S. cerevisiae. Both expression systems yielded proteins that bound heme tightly. The isolated YLR205c protein underwent reduction in the presence of either NADPH-cytochrome P450 reductase or NADH-putidaredoxin-putidaredoxin reductase but did not exhibit heme oxygenase activity. The protein exhibited modest H(2)O(2)-dependent peroxidase activities with guaiacol, potassium iodide, and 2,2(')-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid (ABTS). Thus, YLR205c codes for a hemoprotein of unknown physiological function that exhibits peroxidase activity.

Expression and characterization of cyanobacterium heme oxygenase, a key enzyme in the phycobilin synthesis. Properties of the heme complex of recombinant active enzyme

An efficient bacterial expression system of cyanobacterium Synechocystis sp. PCC 6803 heme oxygenase gene, ho-1, has been constructed, using a synthetic gene. A soluble protein was expressed at high levels and was highly purified, for the first time. The protein binds equimolar free hemin to catabolize the bound hemin to ferric-biliverdin IX alpha in the presence of oxygen and reducing equivalents, showing the heme oxygenase activity. During the reaction, verdoheme intermediate is formed with the evolution of carbon monoxide. Though both ascorbate and NADPH-cytochrome P450 reductase serve as an electron donor, the heme catabolism assisted by ascorbate is considerably slow and the reaction with NADPH-cytochrome P450 reductase is greatly retarded after the oxy-heme complex formation. The optical absorption spectra of the heme-enzyme complexes are similar to those of the known heme oxygenase complexes but have some distinct features, exhibiting the Soret band slightly blue-shifted and relatively strong CT bands of the high-spin component in the ferric form spectrum. The heme-enzyme complex shows the acid-base transition, where two alkaline species are generated. EPR of the nitrosyl heme complex has established the nitrogenous proximal ligand, presumably histidine 17 and the obtained EPR parameters are discriminated from those of the rat heme oxygenase-1 complex. The spectroscopic characters as well as the catabolic activities strongly suggest that, in spite of very high conservation of the primary structure, the heme pocket structure of Synechocystis heme oxygenase isoform-1 is different from that of rat heme oxygenase isoform-1, rather resembling that of bacterial heme oxygenase, H mu O.

Cryogenic absorption spectra of hydroperoxo-ferric heme oxygenase, the active intermediate of enzymatic heme oxygenation

Using radiolysis with (32)P enriched phosphate as an internal source of ionizing radiation, the formation of hydroperoxo-ferric complex from oxy-ferrous precursor with a high yield was monitored at 77 K in heme oxygenase (HO) by means of optical absorption spectroscopy. Well-resolved absorption spectra (maxima at 421 nm, 530 nm, 557 nm) of hydroperoxo-ferric intermediate of this heme enzyme were measured in 70% glycerol/buffer frozen glasses. After annealing at 210-215 K this complex converts to the product complex, alpha-meso hydroxyheme-HO. No heme degradation products were formed in control experiments with ferric HO or other heme proteins.

Heme degradation and human disease: diversity is the soul of life

We all depend on molecular oxygen and heme for our life, as evident from the pigments in blood and daily wastes. About 80% of serum bilirubin is derived from hemoglobin of senescent erythrocytes, which have finished their mission of 120 days and have been phagocytized by macrophages in the reticuloendothelial system. Here we present an overview of the heme degradation processes and relevant disorders by focusing on heme oxygenase-1 (HO-1), a key enzyme in heme catabolism. HO-1 cleaves the porphyrin macrocycle of heme at the expense of molecular oxygen to release a linear tetrapyrrole biliverdin, carbon monoxide, and ferrous iron; biliverdin is rapidly reduced to bilirubin. Bilirubin is transported to the liver (hepatocytes), conjugated with glucuronic acid by bilirubin UDP-glucuronosyltransferase, and excreted into bile. Genetic diversity, a strategy in the host defense, is seen in the human ho-1 and UDP-glucuronosyltransferase genes. Moreover, striking interspecies variations are noted in the regulation of HO-1 expression by hypoxia, heat shock, or interferon-gamma, each of which mainly represses HO-1 expression in human cells. Implications of such a variety are discussed in relevance to the pathogenesis of severe malaria caused by Plasmodium falciparum, the most ancient foe of humans.

Searching for the second oxidant in the catalytic cycle of cytochrome P450: a theoretical investigation of the iron(III)-hydroperoxo species and its epoxidation pathways

Iron(III)-hydroperoxo, [Por(CysS)Fe(III)-OOH](-), a key species in the catalytic cycle of cytochrome P450, was recently identified by EPR/ENDOR spectroscopies (Davydov, R.; Makris, T. M.; Kofman, V.; Werst, D. E.; Sligar, S. G.; Hoffman, B. M. J. Am. Chem. Soc. 2001, 123, 1403-1415). It constitutes the last station of the preparative steps of the enzyme before oxidation of an organic compound and is implicated as the second oxidant capable of olefin epoxidation (Vaz, A. D. N.; McGinnity, D. F.; Coon, M. J. Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 3555-3560), in addition to the penultimate active species, Compound I (Groves, J. T.; Han, Y.-Z. In Cytochrome P450: Structure, Mechanism and Biochemistry, 2nd ed.; Ortiz de Montellano, P. R., Ed.; Plenum Press: New York, 1995; pp 3-48). In response, we present a density functional study of a model species and its ethylene epoxidation pathways. The study characterizes a variety of properties of iron(III)-hydroperoxo, such as the O-O bonding, the Fe-S bonding, Fe-O and Fe-S stretching frequencies, its electron attachment, and ionization energies. Wherever possible these properties are compared with those of Compound I. The proton affinities for protonation on the proximal and distal oxygen atoms of iron(III)-hydroperoxo, and the effect of the thiolate ligand thereof, are determined. In accordance with previous results (Harris, D. L.; Loew, G. H. J. Am. Chem. Soc. 1998, 120, 8941-8948), iron(III)-hydroperoxo is a strong base (as compared with water), and its distal protonation leads to a barrier-free formation of Compound I. The origins of this barrier-free process are discussed using a valence bond approach. It is shown that the presence of the thiolate is essential for this process, in line with the "push effect" deduced by experimentalists (Sono, M.; Roach, M. P.; Coulter, E. D.; Dawson, J. H. Chem. Rev. 1996, 96, 2841-2887). Finally, four epoxidation pathways of iron(III)-hydroxperoxo are located, in which the species transfers oxygen to ethylene either from the proximal or from the distal sites, in both concerted and stepwise manners. The barriers for the four mechanisms are 37-53 kcal/mol, in comparison with 14 kcal/mol for epoxidation by Compound I. It is therefore concluded that iron(III)-hydroperoxo, as such, cannot be a second oxidant, in line with its significant basicity and poor electron-accepting capability. Possible versions of a second oxidant are discussed.

Processing of the lipocalin alpha(1)-microglobulin by hemoglobin induces heme-binding and heme-degradation properties

Alpha(1)-microglobulin is a 26-kd protein, widespread in plasma and tissues and well-conserved among vertebrates. Alpha(1)-microglobulin belongs to the lipocalins, a protein superfamily with highly conserved 3-dimensional structures, forming an internal ligand binding pocket. The protein, isolated from urine, has a heterogeneous yellow-brown chromophore bound covalently to amino acid side groups around the entrance of the lipocalin pocket. Alpha(1)-microglobulin is found in blood both in free form and complex-bound to immunoglobulin A (IgA) via a half-cystine residue at position 34. It is shown here that an alpha(1)-microglobulin species, which we name t-alpha(1)-microglobulin (t = truncated), with a free Cys34 thiol group, lacking its C-terminal tetrapeptide, LIPR, and with a more polar environment around the entrance of the lipocalin pocket, is released from IgA-alpha(1)-microglobulin as well as from free alpha(1)-microglobulin when exposed to the cytosolic side of erythrocyte membranes or to purified oxyhemoglobin. The processed t-alpha(1)-microglobulin binds heme and the alpha(1)-microglobulin-heme complex shows a time-dependent spectral rearrangement, suggestive of degradation of heme concomitantly with formation of a heterogeneous chromophore associated with the protein. The processed t-alpha(1)-microglobulin is found in normal and pathologic human urine, indicating that the cleavage process occurs in vivo. The results suggest that alpha(1)-microglobulin is involved in extracellular heme catabolism.

Reactivity of mono-meso-substituted iron(II) octaethylporphyrin complexes with hydrogen peroxide in the absence of dioxygen. Evidence for nucleophilic attack on the heme

Treatment of the mono-meso-substituted iron(II) octaethylporphyrin complexes, (py)2Fe(II)(meso-NO2-OEP), (py)2Fe(II)(meso-CN-OEP), (py)2Fe(II)(meso-HC(O)-OEP), (py)2Fe(II)(meso-Cl-OEP), (py)2Fe(II)(meso-OMe-OEP), (py)2Fe(II)(meso-Ph-OEP), and (py)2Fe(II)(meso-n-Bu-OEP), with hydrogen peroxide in pyridine-d5 at -30 degrees C in the strict absence of dioxygen has been monitored by 1H NMR spectroscopy. The product oxophlorin complexes are stable as long as the samples are protected from exposure to dioxygen. Hydrogen peroxide reacts cleanly with mono-meso-substituted iron(II) porphyrins in pyridine solution under an inert atmosphere to form mixtures of three possible oxygenation products, (py)2Fe(cis-meso-R-OEPO), (py)2Fe(trans-meso-R-OEPO), and (py)2Fe(OEPO). The yields of (py)2Fe(OEPO), which results from replacement of the unique meso substituent, as a function of the identity of the meso substituent decrease in the order NO2 > HC(O) approximately equal to CN approximately equal to Cl > OMe > Ph, Bu, which suggests that the species responsible for attack on the porphyrin periphery is nucleophilic in nature. A mechanism involving isoporphyrin formation through attack of hydroxide ion on a cationic iron porphyrin with an oxidized porphyrin ring is suggested. The identity of the unique meso functionality also affects the regiospecificity of substitution when the unique meso group is retained. Although random attack at the two different meso sites is expected to yield a cis/trans product ratio of 2, the observed ratios vary in the following order: cyano, 5.0; n-butyl, 4.9; chloro, 3.2; formyl, 2.6; methoxy, 1.9; phenyl 1.4.

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)](-).