Binding of nitric oxide and carbon monoxide to soluble guanylate cyclase as observed with Resonance raman spectroscopy

Interactions between substrates and the haem-bound nitric oxide of ferric and ferrous bacterial nitric oxide synthases

We report here the resonance Raman spectra of the FeIII-NO and FeII-NO complexes of the bacterial NOSs (nitric oxide synthases) from Staphylococcus aureus and Bacillus subtilis. The haem-NO complexes of these bacterial NOSs displayed Fe-N-O frequencies similar to those of the mammalian NOSs, in presence and absence of L-arginine, indicating that haem-bound NO and L-arginine had similar haem environments in bacterial and mammalian NOSs. The only notable difference between the two types of NOS was the lack of change in Fe-N-O frequencies of the FeIII-NO complexes upon (6R) 5,6,7,8-tetrahydro-L-biopterin binding to bacterial NOSs. We report, for the first time, the characterization of NO complexes with NOHA (N(omega)-hydroxy-L-arginine), the substrate used in the second half of the catalytic cycle of NOSs. In the FeIII-NO complexes, both L-arginine and NOHA induced the Fe-N-O bending mode at nearly the same frequency as a result of a steric interaction between the substrates and the haem-bound NO. However, in the FeII-NO complexes, the Fe-N-O bending mode was not observed and the nu(Fe-NO) mode displayed a 5 cm(-1) higher frequency in the complex with NOHA than in the complex with L-arginine as a result of direct interactions that probably involve hydrogen bonds. The different behaviour of the substrates in the FeII-NO complexes thus reveal that the interactions between haem-bound NO and the substrates are finely tuned by the geometry of the Fe-ligand structure and are relevant to the use of the FeII-NO complex as a model of the oxygenated complex of NOSs.

Dissociation of nitric oxide from soluble guanylate cyclase and heme-nitric oxide/oxygen binding domain constructs

Regulation of soluble guanylate cyclase (sGC), the primary NO receptor, is linked to NO binding to the prosthetic heme group. Recent studies have demonstrated that the degree and duration of sGC activation depend on the presence and ratio of purine nucleotides and on the presence of excess NO. We measured NO dissociation from full-length alpha1beta1 sGC, and the constructs beta1(1-194), beta1(1-385), and beta2(1-217), at 37 and 10 degrees C with and without the substrate analogue guanosine-5'-[(alpha,beta-methylene]triphosphate (GMPCPP) or the activator 3-(5'-hydroxymethyl-3'-furyl)-1-benzylindazole (YC-1). NO dissociation from each construct was complex, requiring two exponentials to fit the data. Decreasing the temperature decreased the contribution of the faster exponential for all constructs. Inclusion of YC-1 moderately accelerated NO dissociation from sGC and beta2(1-217) at 37 degrees C and dramatically accelerated NO dissociation from sGC at 10 degrees C. The presence of GMPCPP also dramatically accelerated NO dissociation from sGC at 10 degrees C. This acceleration is due to increases in the observed rate for each exponential and in the contribution of the faster exponential. Increases in the contribution of the faster exponential correlated with higher activation of sGC by NO. These data indicate that the sGC ferrous-nitrosyl complex adopts two 5-coordinate conformations, a lower activity "closed" form, which releases NO slowly, and a higher activity "open" form, which releases NO rapidly. The ratio of these two species affects the overall rate of NO dissociation. These results have implications for the function of sGC in vivo, where there is evidence for two NO-regulated activity states.

Ligand Selectivity of Soluble Guanylyl Cyclase

Although soluble guanylyl cyclase (sGC) functions in an environment in which O(2), NO, and CO are potential ligands for its heme moiety, the enzyme displays a high affinity for only its physiological ligand, NO, but has a limited affinity for CO and no affinity for O(2). Recent studies of a truncated version of the sGC beta(1)-subunit containing the heme-binding domain (Boon, E. M., Huang, S H., and Marletta, M. A. (2005) Nat. Chem. Biol., 1, 53-59) showed that introduction of the hydrogen-bonding tyrosine into the distal heme pocket changes the ligand specificity of the heme moiety and results in an oxygen-binding sGC. The hypothesis that the absence of hydrogen-bonding residues in the distal heme pocket is sufficient to provide oxygen discrimination by sGC was put forward. We tested this hypothesis in a context of a complete sGC heterodimer containing both the intact alpha(1)- and beta(1)-subunits. We found that the I145Y substitution in the full-length beta-subunit of the sGC heterodimer did not produce an oxygen-binding enzyme. However, this substitution impeded the association of NO and destabilized the NO.heme complex. The tyrosine in the distal heme pocket also impeded both the binding and dissociation of the CO ligand. We propose that the mechanism of oxygen exclusion by sGC not only involves the lack of hydrogen bonding in the distal heme pocket, but also depends on structural elements from other domains of sGC.

Mechanism of NO transfer from NO-donors (SNAP and G-MNBS) to ferrous tetraphenylporphyrin in CH3OH

[reaction: see text] The mechanism of NO transfer from NO-donors (SNAP and G-MNBS) to ferrous tetraphenylporphyrin (TPPFe(II)) in CH(3)OH is discovered for the first time by using a laser flash technique. The results show that the NO transfer is completed by NO(+) transfer followed by electron transfer rather than direct NO transfer in one step.

Functional characterization of two nucleotide-binding sites in soluble guanylate cyclase

Soluble guanylate cyclase is a heterodimeric hemoprotein composed of alpha- and beta-subunits with a homologous motif to the nucleotide-binding sites of adenylate cyclases. Homology modeling of guanylate cyclase, based on the crystal structure of adenylate cyclase, reveals a single GTP-binding site and a putative second site pseudosymmetric to the GTP-binding site. However, the role of this pseudosymmetric site has remained unclear. Using equilibrium dialysis, we identified two nucleotide-binding sites with high and low affinity for alpha,beta-methylene guanosine 5'-triphosphate (GMP-CPP). In contrast, 2'-dADP occupied both sites with equivalent affinities. Adenosine-5'-beta,gamma-imido triphosphate (AMP-PNP), which competitively inhibited the cyclase reaction, bound solely to the high affinity site, indicating the role of this site as the catalytic site. The function of the low affinity site was examined using allosteric activators YC-1 and BAY 41-2272. YC-1 significantly reduced the affinity of 2'-dADP, probably by competing for the same site as 2'-dADP. BAY 41-2272 totally inhibited the specific binding of one molecule of 2'-dADP as well as GMP-CPP. This suggests that the activators compete with these nucleotides for the low affinity site. Infrared and EPR analyses of the enzymic CO- and NO-hemes also supported the suggested role of the low affinity site as a target for the activators. Our results imply that the low affinity site is the pseudosymmetric site, which binds YC-1 or BAY 41-2272.

Nitric oxide binding to prokaryotic homologs of the soluble guanylate cyclase beta1 H-NOX domain

The heme cofactor in soluble guanylate cyclase (sGC) is a selective receptor for NO, an important signaling molecule in eukaryotes. The sGC heme domain has been localized to the N-terminal 194 amino acids of the beta1 subunit of sGC and is a member of a family of conserved hemoproteins, called the H-NOX family (Heme-Nitric Oxide and/or OXygen-binding domain). Three new members of this family have now been cloned and characterized, two proteins from Legionella pneumophila (L1 H-NOX and L2 H-NOX) and one from Nostoc punctiforme (Np H-NOX). Like sGC, L1 H-NOX forms a 5-coordinate Fe(II)-NO complex. However, both L2 H-NOX and Np H-NOX form temperature-dependent mixtures of 5- and 6-coordinate Fe(II)-NO complexes; at low temperature, they are primarily 6-coordinate, and at high temperature, the equilibrium is shifted toward a 5-coordinate geometry. This equilibrium is fully reversible with temperature in the absence of free NO. This process is analyzed in terms of a thermally labile proximal Fe(II)-His bond and suggests that in both the 5- and 6-coordinate Fe(II)-NO complexes of L2 H-NOX and Np H-NOX, NO is bound in the distal heme pocket of the H-NOX fold. NO dissociation kinetics for L1 H-NOX and L2 H-NOX have been determined and support a model in which NO dissociates from the distal side of the heme in both 5- and 6-coordinate complexes.

Role of Heme Iron Coordination and Protein Structure in the Dynamics and Geminate Rebinding of Nitric Oxide to the H93G Myoglobin Mutant

The influence of the heme iron coordination on nitric oxide binding dynamics was investigated for the myoglobin mutant H93G (H93G-Mb) by picosecond absorption and resonance Raman time-resolved spectroscopies. In the H93G-Mb, the glycine replacing the proximal histidine does not interact with the heme iron so that exogenous substituents like imidazole may coordinate to the iron at the proximal position. Nitrosylation of H93G-Mb leads to either 6- or 5-coordinate species depending on the imidazole concentration. At high concentrations, (imidazole)-(NO)-6-coordinate heme is formed, and the photoinduced rebinding kinetics reveal two exponential picosecond phases ( approximately 10 and approximately 100 ps) similar to those of wild type myoglobin. At low concentrations, imidazole is displaced by the trans effect leading to a (NO)-5-coordinate heme, becoming 4-coordinate immediately after photolysis as revealed from the transient Raman spectrum. In this case, NO rebinding kinetics remain bi-exponential with no change in time constant of the fast component whose amplitude increases with respect to the 6-coordinate species. Bi-exponential NO geminate rebinding in 5-coordinate H93G-Mb is in contrast with the single-exponential process reported for nitrosylated soluble guanylate cyclase (Negrerie, M., Bouzhir, L., Martin, J. L., and Liebl, U. (2001) J. Biol. Chem. 276, 46815-46821). Thus, our data show that the iron coordination state or the heme iron out-of-plane motion are not at the origin of the bi-exponential kinetics, which depends upon the protein structure, and that the 4-coordinate state favors the fast phase of NO geminate rebinding. Consequently, the heme coordination state together with the energy barriers provided by the protein structure control the dynamics and affinity for NO-binding enzymes.

Regulation of the monomer-dimer equilibrium in inducible nitric-oxide synthase by nitric oxide

The oxygenase domain of inducible nitric-oxide synthase exists as a functional tight homodimer in the presence of the substrate L-arginine and the cofactor tetrahydrobiopterin (H4B). In the absence of H4B, the enzyme is a mixture of monomer and loose dimer. We show that exposure of H4B-free enzyme to NO induces dissociation of the loose dimer into monomers in a reaction that follows single exponential decay kinetics with a lifetime of approximately 300 min. It is followed by a faster autoreduction reaction of the heme iron with a lifetime of approximately 30 min and the concurrent breakage of the proximal iron-thiolate bond, forming a five-coordinate NO-bound ferrous species. Mass spectrometry revealed that the NO-induced monomerization is associated with intramolecular disulfide bond formation between Cys104 and Cys109, located in the zinc-binding motif. The regulatory effect of NO as a dimer inhibitor is discussed in the context of the structure/function relationships of this enzyme.

Electronic reorganization: Origin of sigma trans promotion effect

Binding of two ligands trans to each other by some transition metal complexes may be cooperative [Khoroshun et al., Mol Phys 2002, 100, 523]. Several interesting consequent effects include (i) inverse relationship between bond strength and binding affinity; (ii) smaller coordination barriers to formation of weaker bonds; (iii) enhancement of Lewis acidity with increased number of ligands. We describe a simple model, sigma trans promotion effect (TPE), which considers electronic reorganization between two Lewis structures, and predicts the above-mentioned effects. The applied result of present study is the unified perspective on several facts of heme chemistry. Particularly, we reiterate an important but often overlooked notion, developed previously within the spin pairing model [Drago and Corden, Acc Chem Res 1980, 13, 353], that, in hemoproteins, the proximal histidine and the distal ligand such as O2 or CO cooperate in promoting electronic reorganization. As a result, depopulation of dz2 orbital upon ligand binding contributes to the phenomenon of hemoglobin cooperativity. The presented density functional (B3LYP) calculations on realistic models, the processes of carbon monoxide binding by Fe(II) porphyrins and dinitrogen binding by triamido/triamidoamine Mo(III) complexes, particularly the evaluation of the coordination barriers due to spin-state change by location of the minima on seams of crossing, support the TPE model predictions. From a broader theoretical perspective, the present study would hopefully stimulate the development of much needed frameworks and tools for facile comparisons of wave functions and their properties between different geometries, species, and electronic states. Advancement of practical wave function comparisons may yield fresh qualitative perspectives on chemical reactivity, and promote better understanding of related concepts such as electronic reorganization.

Characterization of functional heme domains from soluble guanylate cyclase

Soluble guanylate cyclase (sGC) is a heterodimeric, nitric oxide (NO)-sensing hemoprotein composed of two subunits, alpha1 and beta1. NO binds to the heme cofactor in the beta1 subunit, forming a five-coordinate NO complex that activates the enzyme several hundred-fold. In this paper, the heme domain has been localized to the N-terminal 194 residues of the beta1 subunit. This fragment represents the smallest construct of the beta1 subunit that retains the ligand-binding characteristics of the native enzyme, namely, tight affinity for NO and no observable binding of O(2). A functional heme domain from the rat beta2 subunit has been localized to the first 217 amino acids beta2(1-217). These proteins are approximately 40% identical to the rat beta1 heme domain and form five-coordinate, low-spin NO complexes and six-coordinate, low-spin CO complexes. Similar to sGC, these constructs have a weak Fe-His stretch [208 and 207 cm(-)(1) for beta1(1-194) and beta2(1-217), respectively]. beta2(1-217) forms a CO complex that is very similar to sGC and has a high nu(CO) stretching frequency at 1994 cm(-)(1). The autoxidation rate of beta1(1-194) was 0.073/min, while the beta2(1-217) was substantially more stable in the ferrous form with an autoxidation rate of 0.003/min at 37 degrees C. This paper has identified and characterized the minimum functional ligand-binding heme domain derived from sGC, providing key details toward a comprehensive characterization.

Interdomain interactions regulate the activation of the heme-regulated eIF2α kinase

The heme-regulated inhibitor of protein synthesis (HRI) regulates translation through the phosphorylation of the alpha-subunit of eukaryotic initiation factor-2 (eIF 2). While HRI is best known for its activation in response to heme-deficiency, we recently showed that the binding of NO and CO to the N-terminal heme-binding domain (NT-HBD) of HRI activated and suppressed its activity, respectively. Here, we examined the effect of hemin, NO, and CO on the interaction between the NT-HBD and the catalytic domain of HRI (HRI/Delta HBD). Hemin stabilized the interaction of NT-HBD with HRI/Delta HBD, and NO and CO disrupted and stabilized this interaction, respectively. Mutant HRI (Delta H-HRI), lacking amino acids 116-158 from the NT-HBD, was less sensitive to heme-induced inhibition, and mutant NT-HBD lacking these residues did not bind to HRI/Delta HBD. HRI/Delta HBD and Delta H-HRI also activated more readily than HRI in response to heme-deficiency. Thus, HRI's activity is regulated through the modulation of the interaction between its NT-HBD and catalytic domain.

CO-dependent Activity-controlling Mechanism of Heme-containing CO-sensor Protein, Neuronal PAS Domain Protein 2

Neuronal PAS domain protein 2, which was recently established to be a heme protein, acts as a CO-dependent transcription factor. The protein consists of the basic helix-loop-helix domain and two heme-containing PAS domains (PAS-A and PAS-B). In this study, we prepared wild type and mutants of the isolated PAS-A domain and measured resonance Raman spectra of these proteins. Upon excitation of the Raman spectrum at 363.8 nm, a band assignable to Fe3+-S stretching was observed at 334 cm(-1) for the ferric wild type protein; in contrast, this band was drastically weaker in the spectrum of C170A, suggesting that Cys170 is an axial ligand of the ferric heme. The Raman spectrum of the reduced form of wild type was mainly of six-coordinate low spin, and the nu11 band, which is sensitive to the donor strength of the axial ligand, was lower than that of reduced cytochrome c3, suggesting coordination of a strong ligand and thus a deprotonated His. In the reduced forms of H119A and H171A, the five-coordinate species became more prevalent, whereas no such changes were observed for C170A, indicating that His119 and His171, but not Cys170, are axial ligands in the ferrous heme. This means that ligand replacement from Cys to His occurs upon heme reduction. The nu(Fe-CO) versus nu(C-O) correlation indicates that a neutral His is a trans ligand of CO. Our results support a mechanism in which CO binding disrupts the hydrogen bonding of His171 with surrounding amino acids, which induces conformational changes in the His171-Cys170 moiety, leading to physiological signaling.

A molecular basis for NO selectivity in soluble guanylate cyclase

Soluble guanylate cyclases (sGCs) function as heme sensors that selectively bind nitric oxide (NO), triggering reactions essential to animal physiology. Recent discoveries place sGCs in the H-NOX family (heme nitric oxide/oxygen-binding domain), which includes bacterial proteins from aerobic and anaerobic organisms. Some H-NOX proteins tightly bind oxygen (O2), whereas others show no measurable affinity for O2, providing the basis for selective NO signaling in aerobic cells. Using a series of wild-type and mutant H-NOXs, we established a molecular basis for ligand discrimination. A distal pocket tyrosine is requisite for O2 binding in the H-NOX family. These data suggest that sGC uses a kinetic selection against O2; we propose that the O2 dissociation rate in the absence of this tyrosine is fast and that a stable O2 complex does not form.

Ligand specificity of H-NOX domains: from sGC to bacterial NO sensors

Soluble guanylate cyclase (sGC) is a nitric oxide (NO) sensing hemoprotein that has been found in eukaryotes from Drosophila to humans. Prokaryotic proteins with significant homology to the heme domain of sGC have recently been identified through genomic analysis. This family of heme proteins has been named the H-NOX domain, for Heme-Nitric oxide/OXygen binding domain. The key observation from initial studies in this family is that some members, those proteins from most eukaryotes and facultative aerobic prokaryotes, bind NO in a five-coordinate heme complex, but do not bind oxygen (O(2)), the same ligand binding characteristics as sGC. H-NOX family members from obligate aerobic prokaryotes bind O(2) and NO in six-coordinate complexes, similar to the globins and other O(2)-sensing heme proteins. The molecular factors that contribute to these differences in ligand specificity, within a family of sequence related proteins, are the subject of this review.

Heme oxygenase-1 (HO-1), a protective gene that prevents chronic graft dysfunction

Heme oxygenase-1 (HO-1) is a stress-responsive enzyme that acts during inflammatory reactions as the rate-limiting step in the catabolism of heme, yielding equimolar amounts of iron (Fe), biliverdin, and the gas carbon monoxide (CO). Expression of HO-1 regulates inflammatory and immune responses, such as those involved in the rejection of transplanted organs. We will discuss here accumulating evidence supporting the notion that expression of HO-1 in a transplanted organ can prevent its rejection. We will argue that the protective effects exerted by HO-1 are mediated to a large extent by the end products that it generates via the catabolism of heme. Better knowledge of how to enhance these protective effects is likely to help create new therapeutic strategies to improve the outcome of transplanted organs.

Interactions of soluble guanylate cyclase with diatomics as probed by resonance Raman spectroscopy

Soluble guanylate cyclase (sGC, EC 4.6.1.2) acts as a sensor for nitric oxide (NO), but is also activated by carbon monoxide in the presence of an allosteric modulator. Resonance Raman studies on the structure-function relations of sGC are reviewed with a focus on the CO-adduct in the presence and absence of allosteric modulator, YC-1, and substrate analogues. It is demonstrated that the sGC isolated from bovine lung contains one species with a five-coordinate (5c) ferrous high-spin heme with the Fe-His stretching mode at 204 cm(-1), but its CO adduct yields two species with different conformations about the heme pocket with the Fe-CO stretching (nuFe-CO) mode at 473 and 489 cm(-1), both of which are His- and CO-coordinated 6c ferrous adducts. Addition of YC-1 to it changes their population and further addition of GTP yields one kind of 6c (nuFe-CO=489 cm(-1)) in addition to 5c CO-adduct (nuFe-CO=521 cm(-1)). Under this condition the enzymatic activity becomes nearly the same level as that of NO adduct. Addition of gamma-S-GTP yields the same effect as GTP does but cGMP and GDP gives much less effects. Unexpectedly, ATP cancels the effects of GTP. The structural meaning of these spectroscopic observations is discussed in detail.

NO Reduction by Nitric-oxide Reductase from Denitrifying Bacterium Pseudomonas aeruginosa

Nitric-oxide reductase (NOR) of a denitrifying bacterium catalyzes NO reduction to N(2)O at the binuclear catalytic center consisting of high spin heme b(3) and non-heme Fe(B). The structures of the reaction intermediates in the single turnover of the NO reduction by NOR from Pseudomonas aeruginosa were investigated using optical absorption and EPR spectroscopies combined with an originally designed freeze-quench device. In the EPR spectrum of the sample, in which the fully reduced NOR was mixed with an NO solution and quenched at 0.5 ms after the mixing, two characteristic signals for the ferrous Fe(B)-NO and the penta-coordinated ferrous heme b(3)-NO species were observed. The CO inhibition of its formation indicated that two NO molecules were simultaneously distributed into the two irons of the same binuclear center of the enzyme in this state. The time- and temperature-dependent EPR spectral changes indicated that the species that appeared at 0.5 ms is a transient reaction intermediate prior to the N(2)O formation, in good agreement with the so-called "trans" mechanism. It was also found that the final state of the enzyme in the single turnover cycle is the fully oxidized state, in which the mu-oxo-bridged ligand is absent between the two irons of its binuclear center, unlike the resting form of NOR as isolated. On the basis of these present findings, we propose a newly developed mechanism for the NO reduction reaction conducted by NOR.

Resonance Raman study on synergistic activation of soluble guanylate cyclase by imidazole, YC-1 and GTP

Soluble guanylate cyclase (sGC), a physiological nitric oxide (NO) receptor, is a heme-containing protein and catalyzes the conversion of GTP to cyclic GMP. We found that 200 mM imidazole moderately activated sGC in the coexistence with 3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole (YC-1), although imidazole or YC-1 alone had little effect for activation. GTP facilitated this process. Resonance Raman spectra of imidazole complex of native sGC and CO-bound sGC (CO-sGC) have demonstrated that a simple heme adduct with imidazole at the sixth coordination position is not present for both sGC and CO-sGC below 200 mM of the imidazole concentration and that the Fe-CO stretching band (nuFe-CO)) appears at 492 cm(-1) in the presence of imidazole compared with 473 cm(-1) in its absence. Both frequencies fall on the line of His-coordinated heme proteins in the nuFe-CO vs nuC-O plot. However, it is stressed that the CO-heme of sGC becomes apparently photo-inert in a spinning cell in the presence of imidazole, suggesting the formation of five-coordinate CO-heme or of six-coordinate heme with a very weak trans ligand. These observations suggest that imidazole alters not only the polarity of heme pocket but also the coordination structure at the fifth coordination side presumably by perturbing the heme-protein interactions at propionic side chains. Despite the fact that the isolated sGC stays in the reduced state and is not oxidized by O(2), sGC under the high concentration of imidazole (1.2 M) yielded nu4 at 1373 cm(-1) even after its removal by gel-filtration, but addition of dithionite gave the strong nu4 band at 1360 cm(-1). This indicated that imidazole caused autoxidation of sGC.

The source of heme for vascular heme oxygenase II: de novo heme biosynthesis in rat aorta

Heme is an essential prosthetic group or substrate for many proteins, including hemoglobin, and hemo enzymes such as nitric oxide synthase, soluble guanylyl cyclase, and heme oxygenase (HO). HO is responsible for the breakdown of heme into equimolar amounts of biliverdin, iron, and carbon monoxide, the latter of which is thought to play a role in the regulation of vascular tone. It is not clear whether the source of heme for cardiovascular functions is derived from uptake from the extracellular milieu or synthesis. In this study, we tested the hypothesis that blood vessels obtain their supply of heme for HO through de novo synthesis. Adult male Sprague–Dawley rat aorta was incubated at 37 °C in Krebs' solution with 1 µM [14C]δ-aminolevulinic acid (ALA). [14C]ALA uptake was linear for about 30 min and reached a plateau at approximately 100 min. The radioactivity was incorporated into porphyrins and heme as determined by esterification of14C-labelled metabolites and thin-layer chromatography. The first and rate-limiting step of heme biosynthesis is catalyzed by ALA synthase (ALA-S), the activity of which was determined in rat aorta using a radiometric assay, ~250 nmol·(g wet mass)–1·h–1. Inducing HO-1 in rat aorta with S-nitroso-N-acetyl penicil la mine (500 µM) did not increase ALA-S activity as compared with basal activity levels of the enzyme. It appears that there is a sufficient amount of heme available under basal ALA-S activity conditions to meet the increased demand for heme resulting from HO-1 induction. These observations indicate that the complete enzymatic pathway for de novo heme biosynthesis resides in rat aorta and furthermore indicate that de novo heme synthesis is capable of supplying a substantial portion of the heme substrate for HO in the aorta.Key words: heme biosynthesis, vasculature, carbon monoxide, heme oxygenase, δ-aminolevulinic acid synthase.

Activation of heme-regulated eukaryotic initiation factor 2alpha kinase by nitric oxide is induced by the formation of a five-coordinate NO-heme complex: optical absorption, electron spin resonance, and resonance raman spectral studies

Heme-regulated eukaryotic initiation factor 2alpha kinase (HRI) regulates the synthesis of hemoglobin in reticulocytes in response to heme availability. HRI contains a tightly bound heme at the N-terminal domain. Earlier reports show that nitric oxide (NO) regulates HRI catalysis. However, the mechanism of this process remains unclear. In the present study, we utilize in vitro kinase assays, optical absorption, electron spin resonance (ESR), and resonance Raman spectra of purified full-length HRI for the first time to elucidate the regulation mechanism of NO. HRI was activated via heme upon NO binding, and the Fe(II)-HRI(NO) complex displayed 5-fold greater eukaryotic initiation factor 2alpha kinase activity than the Fe(III)-HRI complex. The Fe(III)-HRI complex exhibited a Soret peak at 418 nm and a rhombic ESR signal with g values of 2.49, 2.28, and 1.87, suggesting coordination with Cys as an axial ligand. Interestingly, optical absorption, ESR, and resonance Raman spectra of the Fe(II)-NO complex were characteristic of five-coordinate NO-heme. Spectral findings on the coordination structure of full-length HRI were distinct from those obtained for the isolated N-terminal heme-binding domain. Specifically, six-coordinate NO-Fe(II)-His was observed but not Cys-Fe(III) coordination. It is suggested that significant conformational change(s) in the protein induced by NO binding to the heme lead to HRI activation. We discuss the role of NO and heme in catalysis by HRI, focusing on heme-based sensor proteins.

Synthesis, characterization and molecular structures of six-coordinate manganese nitrosyl porphyrins

Manganese(II) porphyrins are isoelectronic with iron(III) porphyrins, and previously reported work suggests that manganese nitrosyl porphyrins are good structural models for their kinetically unstable and biologically relevant ferric-NO analogues. We have prepared a new set of six-coordinate manganese nitrosyl porphyrins of the general form (por)Mn(NO)(L)(por = TTP, T(p-OCH3)PP; L = piperidine, methanol, 1-methylimidazole) in moderate to high yields. The (por)Mn(NO)(pip) complexes were prepared from the reductive nitrosylation of the (por)MnCl compounds with NO in the presence of piperidine. The IR spectra of the (por)Mn(NO)(pip) compounds as KBr pellets show new strong bands at 1746 cm(-1)(for TTP) and 1748 cm(-1)(for (T(p-OCH3)PP) due to the NO ligands. Attempted crystallization of one of these compounds (por = TTP) from dichloromethane-methanol resulted in the generation of the methanol complex (TTP)Mn(NO)(CH3OH). Reaction of the (por)Mn(NO)(pip) compounds with excess 1-methylimidazole gave the (por)Mn(NO)(1-MeIm) derivatives in good yields. The IR spectra of these compounds show nu(NO) bands that are approximately 12 cm(-1) lower than those of the (por)Mn(NO)(pip) precursors, indicative of greater Mn-->NO pi-backdonation in the 1-MeIm derivatives. X-Ray crystal structures of three of these compounds, namely (TTP)Mn(NO)(CH3OH), (TTP)Mn(NO)(1-MeIm) and (T(p-OCH3)PP)Mn(NO)(1-MeIm) were obtained, and reveal that the NO ligands in these complexes are linear.

Identification of crucial histidines for heme binding in the N-terminal domain of the heme-regulated eIF2alpha kinase

The heme-regulated eukaryotic initiation factor-2alpha (eIF2alpha) kinase (HRI) regulates the initiation of protein synthesis in reticulocytes. The binding of NO to the N-terminal heme-binding domain (NTD) of HRI positively modulates its kinase activity. By utilizing UV-visible absorption, resonance Raman, EPR and CD spectroscopies, two histidine residues have been identified that are crucial for the binding of heme to the NTD. The UV-visible absorption and resonance Raman spectra of all the histidine to alanine mutants constructed were similar to those of the unmutated NTD. However, the change in the CD spectra of the NTD construct containing mutation of His78 to Ala (H78A) indicated loss of the specific binding of heme. The EPR spectrum for the ferric H78A mutant was also substantially perturbed. Thus, His78 is one of the axial ligands for the NTD of HRI. Significant changes in the EPR spectrum of the H123A mutant were also observed, and heme readily dissociated from both the H123A and the H78A NTD mutants, suggesting that His123 was also an axial heme ligand. However, the CD spectrum for the Soret region of the H123A mutant indicated that this mutant still bound heme specifically. Thus, while both His78 and His123 are crucial for stable heme binding, the effects of their mutations on the structure of the NTD differed. His78 appears to play the primary role in the specific binding of heme to the NTD, acting analogously to the "proximal histidine" ligand of globins, while His123 appears to act as the "distal" heme ligand.

Phylogeny of the bacterial superfamily of Crp-Fnr transcription regulators: exploiting the metabolic spectrum by controlling alternative gene programs

The Crp-Fnr regulators, named after the first two identified members, are DNA-binding proteins which predominantly function as positive transcription factors, though roles of repressors are also important. Among over 1200 proteins with an N-terminally located nucleotide-binding domain similar to the cyclic adenosine monophosphate (cAMP) receptor protein, the distinctive additional trait of the Crp-Fnr superfamily is a C-terminally located helix-turn-helix motif for DNA binding. From a curated database of 369 family members exhibiting both features, we provide a protein tree of Crp-Fnr proteins according to their phylogenetic relationships. This results in the assembly of the regulators ArcR, CooA, CprK, Crp, Dnr, FixK, Flp, Fnr, FnrN, MalR, NnrR, NtcA, PrfA, and YeiL and their homologs in distinct clusters. Lead members and representatives of these groups are described, placing emphasis on the less well-known regulators and target processes. Several more groups consist of sequence-derived proteins of unknown physiological roles; some of them are tight clusters of highly similar members. The Crp-Fnr regulators stand out in responding to a broad spectrum of intracellular and exogenous signals such as cAMP, anoxia, the redox state, oxidative and nitrosative stress, nitric oxide, carbon monoxide, 2-oxoglutarate, or temperature. To accomplish their roles, Crp-Fnr members have intrinsic sensory modules allowing the binding of allosteric effector molecules, or have prosthetic groups for the interaction with the signal. The regulatory adaptability and structural flexibility represented in the Crp-Fnr scaffold has led to the evolution of an important group of physiologically versatile transcription factors.

Histidine 386 and its role in cyclooxygenase and peroxidase catalysis by prostaglandin-endoperoxide H synthases

Prostaglandin-endoperoxide H synthases (PGHSs) have a cyclooxygenase that forms prostaglandin (PG) G2 from arachidonic acid (AA) plus oxygen and a peroxidase that reduces the PGG2 to PGH2. The peroxidase activates the cyclooxygenase. This involves an initial oxidation of the peroxidase heme group by hydroperoxide, followed by oxidation of Tyr385 to a tyrosyl radical within the cyclooxygenase site. His386 of PGHS-1 is not formally part of either active site, but lies in an extended helix between Tyr385, which protrudes into the cyclooxygenase site, and His388, the proximal ligand of the peroxidase heme. When His386 was substituted with alanine in PGHS-1, the mutant retained <2.5% of the native peroxidase activity, but >20% of the native cyclooxygenase activity. However, peroxidase activity could be restored (10-30%) by treating H386A PGHS-1 with cyclooxygenase inhibitors or AA, but not with linoleic acid; in contrast, mere occupancy of the cyclooxygenase site of native PGHS-1 had no effect on peroxidase activity. Heme titrations indicated that H386A PGHS-1 binds heme less tightly than does native PGHS-1. The low peroxidase activity and decreased affinity for heme of H386A PGHS-1 imply that His386 helps optimize heme binding. Molecular dynamic simulations suggest that this is accomplished in part by a hydrogen bond between the heme D-ring propionate and the N-delta of Asn382 of the extended helix. The structure of the extended helix is, in turn, strongly supported by stable hydrogen bonding between the N-delta of His386 and the backbone carbonyl oxygens of Asn382 and Gln383. We speculate that the binding of cyclooxygenase inhibitors or AA to the cyclooxygenase site of ovine H386A PGHS-1 reopens the constriction in the cyclooxygenase site between the extended helix and a helix containing Gly526 and Ser530 and restores native-like structure to the extended helix. Being less bulky than AA, linoleic acid is apparently unable to reopen this constriction.

CO binding study of mouse heme-regulated eIF-2alpha kinase: kinetics and resonance Raman spectra

Heme-regulated eukaryotic initiation factor (eIF)-2alpha kinase (HRI) regulates the synthesis of globin chains in reticulocytes with heme availability. In the present study, CO binding kinetics to the 6-coordinated Fe(II) heme of the amino-terminal domain of mouse HRI and resonance Raman spectra of the Fe(II)-CO complex are examined to probe the character of the heme environment. The CO association rate constant, k(on)', and CO dissociation rate constant, k(off), were 0.0029 microM(-1)s(-1) and 0.003 s(-1), respectively. These values are very slow compared with those of mouse neuroglobin and sperm whale myoglobin, while the k(off) value of HRI was close to those of the 6-coordinated hemoglobins from Chlamydomonas and barley (0.0022 and 0.0011 s(-1)). The dissociation rate constant of an endogenous ligand, which occurs prior to CO association, was 18.3 s(-1), which was lower than those (197 and 47 s(-1)) of the same 6-coordinated hemoglobins. Resonance Raman spectra suggest that the Fe-C-O adopts an almost linear and upright structure and that the bound CO interacts only weakly with nearby amino acid residues.

A constitutively activated mutant of human soluble guanylyl cyclase (sGC): implication for the mechanism of sGC activation

Heterodimeric alphabeta soluble guanylyl cyclase (sGC) is a recognized receptor for nitric oxide (NO) and mediates many of its physiological functions. Although it has been clear that the heme moiety coordinated by His-105 of the beta subunit is crucial for mediating the activation of the enzyme by NO, it is not understood whether the heme moiety plays any role in the function of the enzyme in the absence of NO. Here we analyze the effects of biochemical and genetic removal of heme and its reconstitution on the activity of the enzyme. Detergent-induced loss of heme from the wild-type alphabeta enzyme resulted in several-fold activation of the enzyme. This activation was inhibited after hemin reconstitution. A heme-deficient mutant alphabetaCys-105 with Cys substituted for His-105 was constitutively active with specific activity approaching the activity of the wild-type enzyme activated by NO. However, reconstitution of mutant enzyme with heme and/or DTT treatment significantly inhibited the enzyme. Mutant enzyme reconstituted with ferrous heme was activated by NO and CO alone and showed additive effects between gaseous effectors and the allosteric activator 5-cyclopropyl-2-[1-(2-fluoro-benzyl)-1H-pyrazolo[3,4-b]pyridin-3-yl]-pyrimidin-4-ylamine. We propose that the heme moiety through its coordination with His-105 of the beta subunit acts as an endogenous inhibitor of sGC. Disruption of the heme-coordinating bond induced by binding of NO releases the restrictions imposed by this bond and allows the formation of an optimally organized catalytic center in the heterodimer.

Heme proteins and nitric oxide (NO): the neglected, eloquent chemistry in NO redox signaling and regulation

The role of nitric oxide (NO) in cellular physiology and signaling has been an important aspect in biomedical science over the last decade. As NO is a small uncharged radical, the chemistry of NO within the redox environment of the cell dictates the majority of its biological effects. The mechanisms that have received the most attention from a biological perspective involve reactions with oxygen and superoxide, despite the rich literature of metal-NO chemistry. However, NO and its related species participate in important chemistry with metalloproteins. In addition to the well known direct interactions of NO with heme proteins such as soluble guanylate cyclase and oxyhemoglobin, there is much important, but often underappreciated, chemistry between other nitrogen oxides and heme/metal proteins. Here the basic chemistry of nitrosylation and the interactions of NO and other nitrogen oxides with metal-oxo species such as found in peroxidases and monoxygenases are discussed.

Trace elements and nitric oxide function

Nitric oxide (NO) has emerged over the last 15 y as a mammalian metabolic intermediate that is involved in the regulation of critical physiological functions such as blood vessel homeostasis, neuronal transmission and host response to infection. NO is synthesized by the enzyme nitric oxide synthase, which converts the amino acid L-arginine to citrulline and NO. NO functions in biological systems in two very important ways. First, it has been found to be a messenger by which cells communicate with one another (signal transduction), and second, it plays a critical role in the host response to infection. In this second function, it appears that the toxic properties of NO have been harnessed by the immune system to kill or at least slow the growth of invading organisms. The nonspecific chemical reactivity with key cellular targets is responsible for this action. In signaling, NO directly activates the enzyme soluble guanylate cyclase (sGC). Once activated, sGC converts GTP to cGMP and pyrophosphate. The cGMP formed is responsible for the well-documented actions of NO such as blood vessel dilation. With the initial discovery of NO signaling, several important questions emerged that centered largely on the issue of how a signaling system functions when the signaling agent is chemically reactive (short lived), highly diffusible and toxic. Critical, especially in signaling, are the control of NO biosynthesis and interaction with the biological receptors at a concentration that will not harm the host. Why did Nature choose NO for the roles it has? That question engenders only speculation. How does NO work (i.e., what does NO do, and how does it do it without harm yet with specificity)? Answers to these questions can now be offered as the molecular level details emerge to form an interesting picture.

YC-1 facilitates release of the proximal His residue in the NO and CO complexes of soluble guanylate cyclase

The benzylindazole compound YC-1 has been shown to activate soluble guanylate cyclase by increasing the sensitivity toward NO and CO. Here we report the action of YC-1 on the coordination of CO- and NO-hemes in the enzyme and correlate the events with the activation of enzyme catalysis. A single YC-1-binding site on the heterodimeric enzyme was identified by equilibrium dialysis. To explore the affect of YC-1 on the NO-heme coordination, the six-coordinate NO complex of the enzyme was stabilized by dibromodeuteroheme substitution. Using the dibromodeuteroheme enzyme, YC-1 converted the six-coordinate NO-heme to a five-coordinate NO-heme with a characteristic EPR signal that differed from that in the absence of YC-1. These results revealed that YC-1 facilitated cleavage of the proximal His-iron bond and caused geometrical distortion of the five-coordinate NO-heme. Resonance Raman studies demonstrated the presence of two iron-CO stretch modes at 488 and 521 cm(-1) specific to the YC-1-bound CO complex of the native enzyme. Together with the infrared C-O stretching measurements, we assigned the 488-cm(-1) band to the iron-CO stretch of a six-coordinate CO-heme and the 521-cm(-1) band to the iron-CO stretch of a five-coordinate CO-heme. These results indicate that YC-1 stimulates enzyme activity by weakening or cleaving the proximal His-iron bond in the CO complex as well as the NO complex.

NO-induced activation mechanism of the heme-regulated eIF2alpha kinase

The heme-regulated eukaryotic initiation factor 2alpha (eIF2alpha) kinase (HRI), which is found primarily in reticulocytes, contains an N-terminal heme-binding domain (NT-HBD). Binding of NO to the heme iron of the NT-HBD of HRI activates its eIF2alpha kinase activity, thus inhibiting the initiation of translation in reticulocyte lysate. The EPR spectrum of the NO-bound NT-HBD showed several derivative-shaped lines around g = 2.00, which is one of the well-documented signature patterns of a six-coordinate NO complex with histidine as the axial ligand. This is in sharp contrast to that of another prototypical NO-sensor protein, soluble guanylate cyclase (sGC), in which the NO binding to the heme iron disrupts the iron-histidyl bond forming a five-coordinate NO. The NO-mediated activation of HRI is, therefore, not triggered by the cleavage of the iron-histidyl bond. As evidenced by the resonance Raman spectra, two inactive forms of HRI, the ferrous ligand-unbound and the CO-bound states of the NT-HBD, contain a six-coordinate complex as found for the NO complex, indicating that the replacement of the sixth ligand of the heme iron is not sufficient to trigger the activation of HRI. Because the configuration of liganded NO is different from that of liganded CO, we propose that specific interactions between liganded NO and surrounding amino acid residues, which would not be formed in the CO complex, are responsible for the NO-induced activation of HRI.

Revisiting the kinetics of nitric oxide (NO) binding to soluble guanylate cyclase: the simple NO-binding model is incorrect

Soluble guanylate cyclase (sGC) is a ferrous iron hemoprotein receptor for nitric oxide (NO). NO binding to the heme activates the enzyme 300-fold. sGC as isolated is five-coordinate, ferrous with histidine as the axial ligand. The NO-activated enzyme is a five-coordinate nitrosyl complex where the axial histidine bond is broken. Past studies using rapid-reaction kinetics demonstrated that both the formation of a six-coordinate intermediate and the conversion of the intermediate to the activated five-coordinate nitrosyl complex depended on the concentration of NO. A model invoking a second NO molecule as a catalyst for the conversion of the six-coordinate intermediate to the five-coordinate sGC-NO complex was proposed to explain the observed kinetic data. A recent study [Bellamy, T. C., Wood, J. & Garthwaite, J. (2002) Proc. Natl. Acad. Sci. USA 99, 507-510] concluded that a simple two-step binding model explains the results. Here we show through further analysis and simulations of previous data that the simple two-step binding model cannot be used to describe our results. Instead we show that a slightly more complex two-step binding model, where NO is used as a ligand in the first step and a catalyst in the second step, can describe our results quite satisfactorily. These new simulations combined with the previous activation data lead to the conclusion that the intermediate six-coordinate sGC-NO complex has substantial activity. The model derived from our simulations also can account for the slow deactivation of sGC that has been observed in vitro.

Role of conformational changes in the heme-dependent regulation of human soluble guanylate cyclase

Soluble guanylate cyclase (sGC) is a receptor for endogenous and exogenous nitric oxide (NO) and is activated many fold upon its binding, making it a core enzyme in the nitric oxide signal transduction pathway. Much effort has been made to understand the link between binding of NO at the sGC heme and activation of the cyclase activity. We report here the first direct evidence for the role of conformational changes in transmitting the signal between the heme and cyclase domains. Using both circular dichroism (CD) and fluorescence spectroscopies, we have probed the effect that the sGC activators NO and 3-(5'-hydroxymethyl-2'-furyl)-1-benzyl-indazole (YC-1) and the inhibitor 1H-[1,2,4]-oxadiazolo-[4,3-a]-quinoxalin-1-one (ODQ) have on the structure of the protein. Surprisingly, binding of either ODQ or YC-1 to NO-bound sGC cause virtually identical changes in the far-UV CD spectra of sGC, reflecting a perturbation in the secondary structure of the enzyme. This change is absent upon binding of NO, YC-1 or ODQ alone. Using this and previous data, we propose a working model for the mechanism of activation of sGC by NO and YC-1 and inhibition by ODQ.

Characterization of NO binding to human cystathionine beta-synthase: possible implications of the effects of CO and NO binding to the human enzyme

Homocysteine is a key junction metabolite that can be converted to cystathionine in a reaction catalyzed by the heme and pyridoxal phosphate-dependent cystathionine beta-synthase. The heme has unusual spectroscopic properties and the axial ligands have been assigned as histidine and cysteine, respectively. Its role in the protein is not obvious from the chemistry of the beta-replacement reaction that is catalyzed. We have characterized the binding of the gaseous signaling molecule, NO, to cystathionine beta-synthase and examined its effect on the reactions catalyzed by the truncated dimeric form of the enzyme, W409X, which is a natural variant. Binding of NO appears to result in the formation of a five-coordinate ferrous nitrosyl species in which both endogenous ligands have been lost. This is in contrast to CO binding which is reported to displace the thiolate ligand and form a six-coordinate species. NO binds to the full-length enzyme with a K(d) of 281+/-50 microM and to the truncated enzyme with a K(d) of 350+/-44 microM. Binding of NO to the full-length enzyme inhibits activity with a K(i) of 320+/-60 microM. These studies demonstrate that as with CO, perturbation of the heme environment by NO is communicated to the active site with concomitant inhibition of enzyme activity, and suggests a regulatory role for heme in cystathionine beta-synthase.

CooA: a heme-containing regulatory protein that serves as a specific sensor of both carbon monoxide and redox state

CooA, the heme-containing carbon monoxide (CO) sensor from the bacterium Rhodospirillum rubrum, is a transcriptional factor that activates expression of certain genes in response to CO. As with other heme proteins, CooA is unable to bind CO when the Fe heme is oxidized, consistent with the fact that some of the regulated gene products are oxygen-labile. Upon reduction, there is an unusual switch of protein ligands to the six-coordinate heme and the reduced heme is able to bind CO. CO binding stabilizes a conformation of the dimeric protein that allows sequence-specific DNA binding, and transcription is activated through contacts between CooA and RNA polymerase. CooA is therefore a novel redox sensor as well as a specific CO sensor. CooA is a homolog of catabolite responsive protein (CRP), whose transcriptionally active conformation has been known for some time. The recent solution of the crystal structure of the CO-free (transcriptionally inactive) form of CooA has allowed insights into the mechanism by which both proteins respond to their specific small-molecule effectors.

Synergism between nitric oxide and hydrogen peroxide in the inhibition of platelet function: the roles of soluble guanylyl cyclase and vasodilator-stimulated phosphoprotein

In previous studies, a strong synergism between low concentrations of hydrogen peroxide and nitric oxide in the inhibition of agonist-induced platelet aggregation has been established and may be due to enhanced formation of cyclic GMP. In this investigation, hydrogen peroxide and NO had no effect on the activity of pure soluble guanylyl cyclase or its activity in platelet lysates and cytosol. H(2)O(2) was found to increase the phosphorylation of vasodilator-stimulated phosphoprotein (VASP), increasing the amount of the 50-kDa form that results from phosphorylation at serine(157). This occurs both in the presence and in the absence of low concentrations of NO, even at submicromolar concentrations of the peroxide, which alone was not inhibitory to platelets. These actions of H(2)O(2) were inhibited to a large extent by an inhibitor of cyclic AMP-dependent protein kinase, even though H(2)O(2) did not increase cyclic AMP. This inhibitor reversed the inhibition of platelets induced by combinations of NO and H(2)O(2) at low concentrations. The results suggest that the action on VASP may be one site of action of H(2)O(2) but that this event alone does not lead to inhibition of platelets; another unspecified action of NO is required to complete the events required for inhibition.

Evaluation of islet heme oxygenase-CO and nitric oxide synthase-NO pathways during acute endotoxemia

We investigated, by a combined in vivo and in vitro approach, the temporal changes of islet nitric oxide synthase (NOS)-derived nitric oxide (NO) and heme oxygenase (HO)-derived carbon monoxide (CO) production in relation to insulin and glucagon secretion during acute endotoxemia induced by lipopolysaccharide (LPS) in mice. Basal plasma glucagon, islet cAMP and cGMP content after in vitro incubation, the insulin response to glucose in vivo and in vitro, and the insulin and glucagon responses to the adenylate cyclase activator forskolin were greatly increased after LPS. Immunoblots demonstrated expression of inducible NOS (iNOS), inducible HO (HO-1), and an increased expression of constitutive HO (HO-2) in islet tissue. Immunocytochemistry revealed a marked expression of iNOS in many beta-cells, but only in single alpha-cells after LPS. Moreover, biochemical analysis showed a time dependent and markedly increased production of NO and CO in these islets. Addition of a NOS inhibitor to such islets evoked a marked potentiation of glucose-stimulated insulin release. Finally, after incubation in vitro, a marked suppression of NO production by both exogenous CO and glucagon was observed in control islets. This effect occurred independently of a concomitant inhibition of guanylyl cyclase. We suggest that the impairing effect of increased production of islet NO on insulin secretion during acute endotoxemia is antagonized by increased activities of the islet cAMP and HO-CO systems, constituting important compensatory mechanisms against the noxious and diabetogenic actions of NO in endocrine pancreas.

Mechanism for fetal globin gene expression: role of the soluble guanylate cyclase-cGMP-dependent protein kinase pathway

Despite considerable concerns with pharmacological stimulation of fetal hemoglobin (Hb F) as a therapeutic option for the beta-globin disorders, the molecular basis of action of Hb F-inducing agents remains unclear. Here we show that an intracellular pathway including soluble guanylate cyclase (sGC) and cGMP-dependent protein kinase (PKG) plays a role in induced expression of the gamma-globin gene. sGC, an obligate heterodimer of alpha- and beta-subunits, participates in a variety of physiological processes by converting GTP to cGMP. Northern blot analyses with erythroid cell lines expressing different beta-like globin genes showed that, whereas the beta-subunit is expressed at similar levels, high-level expression of the alpha-subunit is preferentially observed in erythroid cells expressing gamma-globin but not those expressing beta-globin. Also, the levels of expression of the gamma-globin gene correlate to those of the alpha-subunit. sGC activators or cGMP analogs increased expression of the gamma-globin gene in erythroleukemic cells as well as in primary erythroblasts from normal subjects and patients with beta-thalassemia. Nuclear run-off assays showed that the sGC activator protoporphyrin IX stimulates transcription of the gamma-globin gene. Furthermore, increased expression of the gamma-globin gene by well known Hb F-inducers such as hemin and butyrate was abolished by inhibiting sGC or PKG activity. Taken together, these results strongly suggest that the sGC-PKG pathway constitutes a mechanism that regulates expression of the gamma-globin gene. Further characterization of this pathway should permit us to develop new therapeutics for the beta-globin disorders.

Mechanism for fetal globin gene expression: Role of the soluble guanylate cyclase-cGMP-dependent protein kinase pathway

Despite considerable concerns with pharmacological stimulation of fetal hemoglobin (Hb F) as a therapeutic option for the beta-globin disorders, the molecular basis of action of Hb F-inducing agents remains unclear. Here we show that an intracellular pathway including soluble guanylate cyclase (sGC) and cGMP-dependent protein kinase (PKG) plays a role in induced expression of the gamma-globin gene. sGC, an obligate heterodimer of alpha- and beta-subunits, participates in a variety of physiological processes by converting GTP to cGMP. Northern blot analyses with erythroid cell lines expressing different beta-like globin genes showed that, whereas the beta-subunit is expressed at similar levels, high-level expression of the alpha-subunit is preferentially observed in erythroid cells expressing gamma-globin but not those expressing beta-globin. Also, the levels of expression of the gamma-globin gene correlate to those of the alpha-subunit. sGC activators or cGMP analogs increased expression of the gamma-globin gene in erythroleukemic cells as well as in primary erythroblasts from normal subjects and patients with beta-thalassemia. Nuclear run-off assays showed that the sGC activator protoporphyrin IX stimulates transcription of the gamma-globin gene. Furthermore, increased expression of the gamma-globin gene by well known Hb F-inducers such as hemin and butyrate was abolished by inhibiting sGC or PKG activity. Taken together, these results strongly suggest that the sGC-PKG pathway constitutes a mechanism that regulates expression of the gamma-globin gene. Further characterization of this pathway should permit us to develop new therapeutics for the beta-globin disorders.

Inhibition of hypoxic pulmonary vasoconstriction by carbon monoxide in dogs

OBJECTIVE

We tested the hypothesis that carbon monoxide might participate in the modulation of hypoxic pulmonary vasoconstriction (HPV) by prostacyclin (PGI2) and nitric oxide.

DESIGN

Prospective, interventional study.

SETTING

University laboratory.

SUBJECTS

Nineteen intact anesthetized mongrel dogs.

INTERVENTIONS

Right heart catheterization for the measurements of mean pulmonary artery pressure (Ppa), left atrial pressure estimated from occluded Ppa (Ppao), pulmonary capillary pressure (Pcp) calculated from the Ppa decay curve after balloon occlusion, and cardiac output (Q); inferior vena cava balloon for the control of Q by manipulation of venous return; ventilation in hyperoxia (fraction of inspired O2, 0.4) or in hypoxia (Fio2, 0.1); inhibition of cyclooxygenase by indomethacin (Indo); inhibition of nitric oxide synthase by NG-nitro-l-arginine (L-NA); inhibition of heme oxygenase by mesoporphyrin IX (SnMP); inhalation of nitric oxide (20 ppm); and inhalation of carbon monoxide (100 ppm).

MEASUREMENTS AND MAIN RESULTS

The first seven dogs were weak responders to hypoxia as assessed by a hypoxia-induced increase in the gradient between Ppa and Ppao, measured at one level of Q kept constant, by an average of only 2 mm Hg (p = NS). This HPV was markedly increased by the combined administration of Indo and L-NA. A further enhancement of HPV was observed after the addition of SnMP, leading to severe pulmonary hypertension with an average increase in Ppa to 39 mm Hg. Inhaled nitric oxide inhibited HPV only after the combined administration of Indo, L-NA, and SnMP. Inhaled carbon monoxide had no effect. The next 12 dogs were stronger responders to hypoxia, as assessed by a hypoxia-induced increase in the gradient between Ppa and Ppao, measured at several levels of Q, by an average of 3 mm Hg (p <.05). This HPV was of the same magnitude after administration of placebo (n = 6) or SnMP (n = 6). Addition of Indo enhanced HPV to the same extent in the placebo and in the SnMP groups. Addition of L-NA induced a further enhancement of HPV, which was, however, greater in the SnMP group. There was a slight increase in the capillary-venous segment relative to the arterial segment in hypoxic conditions, but the partitioning of pulmonary vascular resistance was otherwise unaffected by nitric oxide, carbon monoxide, or PGI2.

CONCLUSIONS

Endogenous carbon monoxide modulates canine HPV only in the absence of nitric oxide. The vasodilation mediated by nitric oxide, PGI2, or carbon monoxide is essentially distributed between proximal and distal sites proportionally to the degree of constriction produced during hypoxia.

Relaxant effects of carbon monoxide compared with nitric oxide in pulmonary and systemic vessels of newborn piglets

Nitric oxide (NO) has been implicated in a number of diverse physiologic processes, including regulation of vascular tone. Carbon monoxide (CO) is another endogenously generated diatomic gas that may play an important physiologic role in vascular smooth muscle homeostasis. The purpose of this study was to compare the responses to exogenous NO and CO in isolated vessels (pulmonary arteries, pulmonary veins, and mesenteric arteries) from 12- to 24-h-old and 2-wk-old piglets. Vessels precontracted with the thromboxane A(2) mimetic U46619 (10(-7) M) relaxed in response to CO (2 x 10(-6) to 2 x 10(-4) M) and NO (2 x 10(-9) to 2 x 10(-7) M); these effects were not affected by endothelium removal but were completely abolished by the soluble guanylate cyclase inhibitor ODQ (10(-5) M). In pulmonary arteries, the maximal relaxation to NO increased with postnatal age from 33 +/- 4% of the precontraction value to 56 +/- 5%, in 12- to 24-h-old and 2-week-old piglets, respectively (p < 0.01), but the response to CO decreased from 25 +/- 3% to 12 +/- 1%, respectively (p < 0.01). The maximal response to CO was greater in pulmonary veins than in pulmonary or mesenteric arteries for both age groups (p < 0.01). Vasorelaxation induced by endogenous NO (stimulated by acetylcholine) was also greater in pulmonary veins when compared with pulmonary arteries and increased with postnatal age in both vessels. In contrast, no age-related differences were observed in the vasorelaxation induced by the cGMP analog 8-bromo cGMP in pulmonary arteries. When the response to NO was analyzed under three different extracellular O(2) concentrations (PO(2) 4.51 +/- 0.03, 19. 32 +/- 0.17, and 86 +/- 0.62, kPa), no significant differences were found. However, in the presence of superoxide dismutase (100 U/mL). the response to CO remained unchanged, and the response to NO improved in pulmonary arteries from 2-week-old but not from newborn piglets. In conclusion, both NO and CO relaxed neonatal vessels through soluble guanylate cyclase activation. However, when compared with NO, CO exhibited a poor vasorelaxant activity. Pulmonary vasorelaxation induced by NO increased with postnatal age, whereas that induced by CO decreased. Changes in extracellular oxygen concentration did not alter the pulmonary vascular response to NO. However, the presence of superoxide dismutase improved the response to NO, indicating that oxidant activity limits the vasorelaxant response to NO but not to CO.

Redox reactions related to indoleamine 2,3-dioxygenase and tryptophan metabolism along the kynurenine pathway

The heme enzyme indoleamine 2,3-dioxygenase (IDO) oxidizes the pyrrole moiety of L-tryptophan (Trp) and other indoleamines and represents the initial and rate-limiting enzyme of the kynurenine (Kyn) pathway. IDO is a unique enzyme in that it can utilize superoxide anion radical (O2*- ) as both a substrate and a co-factor. The latter role is due to the ability of O2*- to reduce inactive ferric-IDO to the active ferrous form. Nitrogen monoxide (*NO) and H2O2 inhibit the dioxygenase and various inter-relationships between the nitric oxide synthase- and IDO-initiated amino acid degradative pathways exist. Induction of IDO and metabolism of Trp along the Kyn pathway is implicated in a variety of physiological and pathophysiological processes, including anti-microbial and anti-tumor defense, neuropathology, immunoregulation and antioxidant activity. Antioxidant activity may arise from O2*- scavenging by IDO and formation of the potent radical scavengers and Kyn pathway metabolites, 3-hydroxyanthranilic acid and 3-hydroxykynurenine. Under certain conditions, these aminophenols and other Kyn pathway metabolites may exhibit pro-oxidant activities. This article reviews findings indicating that redox reactions are involved in the regulation of IDO and Trp metabolism along the Kyn pathway and also participate in the biological activities exhibited by Kyn pathway metabolites.

Activation of soluble guanylate cyclase by carbon monoxide and nitric oxide: a mechanistic model

Soluble guanylate cyclase (GC) from bovine lung is activated 4-fold by carbon monoxide (CO) and 400-fold by nitric oxide (NO). Spectroscopic and kinetic data for ligation of CO and NO with GC are summarized and compared with similar data for myoglobin (Mb), hemoglobin (Hb), and heme model compounds. Kinetic, thermodynamic, and structural data form a basis on which to construct a model for the manner in which the two ligands affect protein structure near the heme for heme proteins in general and for GC in particular. The most significant datum is that although association rates of ligands with GC are similar to those with Mb and Hb, their dissociation rates are dramatically faster. This suggests a delicate balance between five- and six-coordinate heme iron in both NO and CO complexes. Based on these and other data, a model for GC activation is proposed: The first step is formation of a six-coordinate species concomitant with tertiary and quaternary structural changes in protein structure and about a 4-fold increase in enzyme activity. In the second step, applicable to NO, the bond from iron to the proximal histidine ruptures, leading to additional relaxation in the quaternary and tertiary structure and a further 100-fold increase in activity. This is the main event in activation, available to NO and possibly other activators or combinations of activators. It is proposed, finally, that the proximal base freed in step 2, or some other protein base suitably positioned as a result of structural changes following ligation, may provide a center for nucleophilic substitution catalyzing the reaction GTP --> cGMP. An example is provided for a similar reaction in a derivatized protoheme model compound. The reaction mechanism attempts to rationalize the relative enzymatic activities of GC, heme-deficient GC, GC-CO, and GC-NO on a common basis and makes predictions for new activators that may be discovered in the future.