A nuclear Overhauser effect study of the active site of myeloperoxidase. Structural similarity of the prosthetic group to that on lactoperoxidase

Disruption of heme-peptide covalent cross-linking in mammalian peroxidases by hypochlorous acid

Myeloperoxidase (MPO), lactoperoxidase (LPO) and eosinophil peroxidase (EPO) play a central role in oxidative damage in inflammatory disorders by utilizing hydrogen peroxide and halides/pseudo halides to generate the corresponding hypohalous acid. The catalytic sites of these enzymes contain a covalently modified heme group, which is tethered to the polypeptide chain at two ester linkages via the methyl group (MPO, EPO and LPO) and one sulfonium bond via the vinyl group (MPO only). Covalent cross-linking of the catalytic site heme to the polypeptide chain in peroxidases is thought to play a protective role, since it renders the heme moiety less susceptible to the oxidants generated by these enzymes. Mass-spectrometric analysis revealed the following possible pathways by which hypochlorous acid (HOCl) disrupts the heme-protein cross-linking: (1) the methyl-ester bond is cleaved to form an alcohol; (2) the alcohol group undergoes an oxygen elimination reaction via the formation of an aldehyde intermediate or undergoes a demethylation reaction to lose the terminal CH2 group; and (3) the oxidative cleavage of the vinyl-sulfonium linkage. Once the heme moiety is released it undergoes cleavage at the carbon-methyne bridge either along the δ-β or a α-γ axis to form different pyrrole derivatives. These results indicate that covalent cross-linking is not enough to protect the enzymes from HOCl mediated heme destruction and free iron release. Thus, the interactions of mammalian peroxidases with HOCl modulates their activity and sets a stage for initiation of the Fenton reaction, further perpetuating oxidative damage at sites of inflammation.

Competitive cobalt for zinc substitution in mammalian methionine sulfoxide reductase B1 overexpressed in E. coli: structural and functional insight

Expression of the mammalian enzyme methionine sulfoxide reductase B1 (MsrB1) in Escherichia coli growing in cobalt-containing media resulted in the reproducible appearance of the stable cobalt-containing protein MsrB1-Co. NMR studies and biocomputing using the programs AnisoFit and Amber allowed us to generate a structure of MsrB1-Co sharing the overall fold with the native zinc-containing protein MsrB1-Zn. Our data suggest that the N-terminus containing resolving cysteine tends to be closer to the protein’s catalytic center than was previously reported. It is argued that this proximity supports the proposed catalytic mechanism and ensures high catalytic efficiency of MsrB1. Functional studies showed that both MsrB1-Zn and MsrB1-Co exhibit similar levels of activity, in agreement with the structural studies performed. The proposed metal ion substitution approach may have a methodological significance in determining whether methionine sulfoxide reductase B proteins contain a metal ion.

Kinetic evidence supports the existence of two halide binding sites that have a distinct impact on the heme iron microenvironment in myeloperoxidase

Myeloperoxidase (MPO) structural analysis has suggested that halides and pseudohalides bind to the distal binding site and serve as substrates or inhibitors, while others have concluded that there are two separate sites. Here, evidence for two distinct binding sites for halides comes from the bell-shaped effects observed when the second-order rate constant of nitric oxide (NO) binding to MPO was plotted versus Cl- concentration. The chloride level used in the X-ray structure that produced Cl- binding to the amino terminus of the helix halide binding site was insufficient to populate either of the two sites that appear to be responsible for the two phases. Biphasic effects were also observed when the I-, Br-, and SCN- concentrations were plotted against the NO combination rate constants. Interestingly, the trough concentrations obtained from the bell-shaped curves are comparable to normal plasma levels of halides and pseudohalides, suggesting the potential relevance of these molecules in modulating MPO function. The second-order rate constant of NO binding in the presence of plasma levels of I-, Br-, and SCN- is 1-2-fold lower compared to that obtained in the absence of these molecules and remains unaltered through the Cl- plasma level. When Cl- exceeded the plasma level, the NO combination rate becomes indistinguishable from the second phase of the bell-shaped curve that was obtained in the absence of halides. Our results are consistent with two halide binding sites that could be populated by two halides in which both display distinct effects on the MPO heme iron microenvironment.

Thiocyanate modulates the catalytic activity of mammalian peroxidases

We investigated the potential role of the co-substrate, thiocyanate (SCN-), in modulating the catalytic activity of myeloperoxidase (MPO) and other members of the mammalian peroxidase superfamily (lactoperoxidase (LPO) and eosinophil peroxidase (EPO)). Pre-incubation of SCN- with MPO generates a more complex biological setting, because SCN- serves as either a substrate or inhibitor, causing diverse impacts on the MPO heme iron microenvironment. Consistent with this hypothesis, the relationship between the association rate constant of nitric oxide binding to MPO-Fe(III) as a function of SCN- concentration is bell-shaped, with a trough comparable with normal SCN- plasma levels. Rapid kinetic measurements indicate that MPO, EPO, and LPO Compound I formation occur at rates slower than complex decay, and its formation serves to simultaneously catalyze SCN- via 1e- and 2e- oxidation pathways. For the three enzymes, Compound II formation is a fundamental feature of catalysis and allows the enzymes to operate at a fraction of their possible maximum activities. MPO and EPO Compound II is relatively stable and decays gradually within minutes to ground state upon H2O2 exhaustion. In contrast, LPO Compound II is unstable and decays within seconds to ground state, suggesting that SCN- may serve as a substrate for Compound II. Compound II formation can be partially or completely prevented by increasing SCN- concentration, depending on the experimental conditions. Collectively, these results illustrate for the first time the potential mechanistic differences of these three enzymes. A modified kinetic model, which incorporates our current findings with the mammalian peroxidases classic cycle, is presented.

Interrogation of heme pocket environment of mammalian peroxidases with diatomic ligands

Recent studies demonstrate that myeloperoxidase (MPO), eosinophil peroxidase (EPO), and lactoperoxidase (LPO), homologous members of the mammalian peroxidase superfamily, can all serve as catalysts for generating nitric oxide- (nitrogen monoxide, NO) derived oxidants. These enzymes contain heme prosthetic groups that are ligated through a histidine nitrogen and use H(2)O(2) as the electron acceptor in the catalysis of oxidative reactions. Here we show that heme reduction of these peroxidases results in distinct electronic and/or conformational changes in their heme pockets using a combination of rapid kinetics measurements, optical absorbance, and diatomic ligand binding studies. Addition of reducing agent to each peroxidase at ground state [Fe(III) state] causes immediate buildup of the corresponding Fe(II) complexes. Spectral changes indicate that two LPO-Fe(II) species are present in solution at equilibrium. Analyses of stopped-flow traces collected when EPO, MPO, or LPO solutions rapidly mixed with NO were accurately fit by single-exponential functions. Plots of the apparent rate constants as a function of NO concentration for all Fe(III) and Fe(II) forms were linear with positive intercepts, consistent with NO binding to each form in a simple reversible one-step mechanism. Fe(II) forms of MPO and LPO, but not EPO, displayed significantly lower affinity toward NO compared to Fe(III) forms, suggesting that heme reduction causes a dramatic change in the heme pocket electronic environment that alters the affinity and/or accessibility of heme iron toward NO. Optical absorbance spectra indicate that CO binds to the Fe(II) forms of both LPO and EPO, but not with MPO, and generates their respective low-spin six-coordinate complexes. Kinetic analyses indicate that the binding of CO to EPO is monophasic while CO binding to LPO is biphasic. Collectively, these results illustrate for the first time functional differences in the heme pocket environments of Fe(II) forms of EPO, LPO, and MPO toward binding of diatomic ligands. Our results suggest that, upon reduction, the heme pocket of MPO collapses, LPO adopts two spectroscopically and kinetically distinguishable forms (one partially open and the other relatively closed), and EPO remains open.

Nitric oxide modulates the catalytic activity of myeloperoxidase

Myeloperoxidase (MPO), an abundant protein in neutrophils, monocytes, and subpopulations of tissue macrophages, is believed to play a critical role in host defenses and inflammatory tissue injury. To perform these functions, an array of diffusible radicals and reactive oxidant species may be formed through oxidation reactions catalyzed at the heme center of the enzyme. Myeloperoxidase and inducible nitric-oxide synthase are both stored in and secreted from the primary granules of activated leukocytes, and nitric oxide (nitrogen monoxide; NO) reacts with the iron center of hemeproteins at near diffusion-controlled rates. We now demonstrate that NO modulates the catalytic activity of MPO through distinct mechanisms. NO binds to both ferric (Fe(III), the catalytically active species) and ferrous (Fe(II)) forms of MPO, generating stable low-spin six-coordinate complexes, MPO-Fe(III).NO and MPO-Fe(II).NO, respectively. These nitrosyl complexes were spectrally distinguishable by their Soret absorbance peak and visible spectra. Stopped-flow kinetic analyses indicated that NO binds reversibly to both Fe(III) and Fe(II) forms of MPO through simple one-step mechanisms. The association rate constant for NO binding to MPO-Fe(III) was comparable to that observed with other hemoproteins whose activities are thought to be modulated by NO in vivo. In stark contrast, the association rate constant for NO binding to the reduced form of MPO, MPO-Fe(II), was over an order of magnitude slower. Similarly, a 2-fold decrease was observed in the NO dissociation rate constant of the reduced versus native form of MPO. The lower NO association and dissociation rates observed suggest a remarkable conformational change that alters the affinity and accessibility of NO to the distal heme pocket of the enzyme following heme reduction. Incubation of NO with the active species of MPO (Fe(III) form) influenced peroxidase catalytic activity by dual mechanisms. Low levels of NO enhanced peroxidase activity through an effect on the rate-limiting step in catalysis, reduction of Compound II to the ground-state Fe(III) form. In contrast, higher levels of NO inhibited MPO catalysis through formation of the nitrosyl complex MPO-Fe(III)-NO. NO interaction with MPO may thus serve as a novel mechanism for modulating peroxidase catalytic activity, influencing the regulation of local inflammatory and infectious events in vivo.

The sulfonium ion linkage in myeloperoxidase. Direct spectroscopic detection by isotopic labeling and effect of mutation

The heme group of myeloperoxidase is covalently linked via two ester bonds to the protein and a unique sulfonium ion linkage involving Met(243). Mutation of Met(243) into Thr, Gln, and Val, which are the corresponding residues of eosinophil peroxidase, lactoperoxidase, and thyroid peroxidase, respectively, and into Cys was performed. The Soret band in the optical absorbance spectrum in the oxidized mutants is now found at approximately 411 nm. Both the pyridine hemochrome spectra and resonance Raman spectra of the mutants are affected by the mutation. In the Met(243) mutants the affinity for chloride has decreased 100-fold. All mutants have lost their chlorination activity, except for the M243T mutant, which still has 15% activity left. By Fourier transform infared difference spectroscopy it was possible to specifically detect the (13)CD(3)-labeled methionyl sulfonium ion linkage. We conclude that the sulfonium ion linkage serves two roles. First, it serves as an electron-withdrawing substituent via its positive charge, and, second, together with its neighboring residue Glu(242), it appears to be responsible for the lower symmetry of the heme group and distortion from the planar conformation normally seen in heme-containing proteins.

Biotransformations with peroxidases

Enzymes are chiral catalysts and are able to produce optically active molecules from prochiral or racemic substrates by catalytic asymmetric induction. One of the major challenges in organic synthesis is the development of environmentally acceptable chemical processes for the preparation of enantiomerically pure compounds, which are of increasing importance as pharmaceuticals and agrochemicals. Enzymes meet this challenge! For example, a variety of peroxidases effectively catalyze numerous selective oxidations of electron-rich substrates, which include the hydroxylation of arenes, the oxyfunctionalizations of phenols and aromatic amines, the epoxidation and halogenation of olefins, the oxygenation of heteroatoms and the enantioselective reduction of racemic hydroperoxides. In this review, we summarize the important advances achieved in the last few years on peroxidase-catalyzed transformations, with major emphasis on preparative applications.

The heme prosthetic group of lactoperoxidase. Structural characteristics of heme l and heme l-peptides

The heme prosthetic group from the bovine milk enzyme lactoperoxidase (LPO), termed heme l, is isolated through an approach that combines proteolytic hydrolysis and reverse-phase high performance liquid chromatographic separation of the resulting digest. Application of different proteases yields either a peptide-bound heme (with trypsin and chymotrypsin) or a peptide-free heme (with proteinase K). Both heme l and heme l-peptide species were investigated by paramagnetic 1H NMR spectroscopy, electrospray mass spectrometry, and peptide sequence analysis. Paramagnetic 1H NMR experiments on the low spin bis(cyano)-Fe(III)heme l complex conclusively define the heme l structure as a 1,5-bis(hydroxymethyl) derivative of heme b. The electrospray mass spectrum of heme l confirms the two-site hydroxyl functionalization on this heme. Paramagnetic 1H NMR spectra of the high spin bis(dimethyl sulfoxide)-Fe(III) complexes of the isolated heme species provide information regarding peptide content. Sequence analyses of peptides released from two heme l-peptide species by base hydrolysis suggest that heme-protein ester linkages in lactoperoxidase occur between the two hydroxyl groups of heme l and the carboxylic side chains of glutamate 275 and aspartate 125. These results confirm the earlier reported structural proposal (Rae, T. D., and Goff, H. M. (1996) J. Am. Chem. Soc. 118, 2103-2104).

Spectroscopic and binding studies on the interaction of inorganic anions with lactoperoxidase

The interaction of several inorganic species (SCN-, I-, Br-, Cl-, F-, NO2-, N3-, CN-) with bovine lactoperoxidase was investigated through kinetic and binding studies by using UV-Vis spectroscopy. The above ligands form 1:1 complexes with the protein and can be assigned to three different groups, on the basis of the dissociation constant values (KD) of the adducts: (1) SCN-, I-, Br-, and Cl- (KD increases along the series); (2) F- (which shows a singular behavior); (3) NO2-, N3-, and CN- (that bind at the iron site). KD values for the LPO/SCN- adduct appeared to be modified in the presence of other inorganic species; a strong competition between this substrate and all other anions (with the exception of F-) was evidentiated. Binding investigations on the natural substrates SCN- and I-, at varying pH and temperature, showed that their interaction with lactoperoxidase involves the protonation of a common site in proximity of the iron (possibly distal histidine). Michaelis-Menten constants for SCN-, I-, and Br- followed roughly the same trend as KD; KM for hydrogen peroxide is strongly dependent on the cosubstrate. Computer-assisted docking simulations showed that all ligands can penetrate inside the heme pocket.

Spectral analysis of lactoperoxidase. Evidence for a common heme in mammalian peroxidases

The identity of the non-extractable heme of mammalian lactoperoxidase (LPO) has remained unsolved for over 40 years. Accepted possibilities include a constrained heme b or an 8-thiomethylene-modified heme b. Recent studies of myeloperoxidase (MPO) (Fenna, R., Zeng, J., and Davey, C. (1995) Arch. Biochem. Biophys. 316, 653-656; Taylor, K. L., Strobel, F., Yue, K. T., Ram, P., Pohl, J., Woods, A. S., and Kinkade, J. M., Jr. (1995) Arch. Biochem. Biophys. 316, 635-642) suggest possible prosthetic group similarities between MPO and LPO. To address heme identity for LPO, we used comparative magnetic circular dichroism (MCD) spectroscopy of LPO versus myoglobin (Mb), horseradish peroxidase (HRP), and MPO. MCD spectra of native Fe3+-LPO and Fe3+-CN--LPO are approximately 10 nm red shifted from analogous forms of Mb and HRP, including the formate-Mb adduct. MCD spectra of native LPO and MPO are opposite in sign, and MCD spectra of their cyanoadducts also differ. These data indicate the LPO heme is distinct from heme b of Mb and HRP as well as from "heme m" of MPO. From this work and literature analysis, we suggest that the non-extractable "heme l" of LPO has the two vinyl groups of heme b but lacks the 2-sulfonium-vinyl linkage of heme m. The observed red shifts in LPO spectra may derive from ester linkages to protein as for MPO. Strong spectral analogies between LPO and mammalian peroxidases (e.g. from saliva, eosinophils, thyroid, intestine) indicate similar prosthetic heme moieties.

Effects of mixed solvents on three elementary steps in the reactions of horseradish peroxidase and lactoperoxidase

The effects of methanol, acetone, and ethylene glycol (up to 50% v/v) on elementary steps in the reactions of horseradish peroxidase (HRP) and lactoperoxidase (LPO) were studied by means of the stopped-flow method and the difference spectrum. The rate constant (k3,app) of the oxidation reaction of p-cresol with HRP compound II was remarkably reduced in the presence of organic solvents (to 2.3%, 1.8% and 9.4% of the original value in the presence of 50% (v/v) of methanol, acetone and ethylene glycol, respectively), then to a lesser degree were decreased the rate of the oxidation reaction with LPO compound II, and the rate of the oxidation reaction with HRP compound I. These reductions in the reaction rates were not due to competitive inhibition of the solvents, but considered to be related to the degree of exposure of the electron transfer route to the medium. While the rate constant of compound I formation (k1,app) was moderately affected by organic solvents in the case of HRP, the reaction rate with LPO was scarcely affected by organic solvents, being in harmony with the compact heme crevice which probably hampers penetration of solvent molecules. The rate constant (k2,i,app) of the oxidation reaction of an iodide ion by HRP compound I was also hardly affected by the solvents. On the basis of these facts, the mechanism of electron transfer from donors to compound I and compound II is discussed.

Optical spectrum of myeloperoxidase. Origin of the red shift

The optical spectrum of reduced myeloperoxidase (EC 1.11.1.7) displays an unusual red shift of the Soret band which is at 472 nm and the alpha-band which is at 636 nm. The spectral properties of myeloperoxidase can be modified by means of acid treatment. Upon short exposure to acid (pH 1.7) the red-shifted optical absorption spectrum of the reduced enzyme (lambda max at 472 nm) was blue-shifted (lambda max at 448 nm) but the spectrum of the reduced state could be restored by increasing the pH. By contrast, the resonance Raman spectra of both the oxidized and reduced enzyme are essentially the same at both pH 1.7 and pH 7.0. This shows that the optical spectrum and the resonance Raman spectrum are not directly correlated, which we interpret to indicate that the reversible effects of lower pH primarily affect the excited-state energy levels of the macrocycle. The EPR spectrum of the oxidized enzyme showed a reversible conversion from a high-spin rhombic spectrum (gx = 6.7, gy = 5.2) at neutral pH into a more axial high-spin spectrum (gx = gy = 5.8) at low pH. Upon prolonged exposure to acid (20 min) optical absorbance spectra, EPR spectra, resonance Raman spectra and the chlorinating activity were irreversibly affected. We propose that a negatively charged protonatable residue in the proximity of a pyrrole nucleus of the haem group is present that imposes the red shift in the optical absorption spectrum. This is consistent with the available X-ray structure data.

Proton nuclear Overhauser effect study of the heme active site structure of Coprinus macrorhizus peroxidase

Proton nuclear Overhauser effect and paramagnetic relaxation measurements have been used to define more extensively the heme active site structure of Coprinus macrorhizus peroxidase, CMP (previously known as Coprinus cinereus peroxidase), as the ferric low-spin cyanide ligated complex. The results are compared with other well-characterized peroxidase enzymes. The NMR spectrum of CMPCN shows changes in the paramagnetically shifted resonances as a function of time, suggesting a significant heme disorder for CMP. The presence of proximal and distal histidine amino acid residues are common to the heme environments of both CMPCN and HRPCN. However, the upfield distal arginine signals of HRPCN are not evident in the 1H-NMR spectra of CMPCN.

X-ray crystal structure of canine myeloperoxidase at 3 A resolution

The three-dimensional structure of the enzyme myeloperoxidase has been determined by X-ray crystallography to 3 A resolution. Two heavy atom derivatives were used to phase an initial multiple isomorphous replacement map that was subsequently improved by solvent flattening and non-crystallographic symmetry averaging. Crystallographic refinement gave a final model with an R-factor of 0.257. The root-mean-square deviations from ideality for bond lengths and angles were 0.011 A and 3.8 degrees. Two, apparently identical, halves of the molecule are related by local dyad and covalently linked by a single disulfide bridge. Each half-molecule consists of two polypeptide chains of 108 and 466 amino acid residues, a heme prosthetic group, a bound calcium ion and at least three sites of asparagine-linked glycosylation. There are six additional intra-chain disulfide bonds, five in the large polypeptide and one in the small. A central core region that includes the heme binding site is composed of five alpha-helices. Regions of the larger polypeptide surrounding this core are organized into locally folded domains in which the secondary structure is predominantly alpha-helical with very little organized beta-sheet. A proximal ligand to the heme iron atom has been identified as histidine 336, which is in turn hydrogen-bonded to asparagine 421. On the distal side of the heme, histidine 95 and arginine 239 are likely to participate directly in the catalytic mechanism, in a manner analogous to the distal histidine and arginine of the non-homologous enzyme cytochrome c peroxidase. The site of the covalent linkage to the heme has been tentatively identified as glutamate 242, although the chemical nature of the link remains uncertain. The calcium binding site has been located in a loop comprising residues 168 to 174 together with aspartate 96. Myeloperoxidase is a member of a family of homologous mammalian peroxidases that includes thyroid peroxidase, eosinophil peroxidase and lactoperoxidase. The heme environment, defined by our model for myeloperoxidase, appears to be highly conserved in these four mammalian peroxidases. Furthermore, the conservation of all 12 cysteine residues involved in the six intra-chain disulfide bonds and the calcium binding loop suggests that the three-dimensional structures of members of this gene family are likely to be quite similar.

Site-directed mutants of human myeloperoxidase. A topological approach to the heme-binding site

Two site-directed mutants of human promyeloperoxidase, MPO(His416----Ala) and MPO(His502----Ala), have been expressed in Chinese hamster ovary cells and purified. Overall purification yields and apparent molecular masses of the mutant proteins were similar to those of the wild-type enzyme. Both mutant species were analyzed spectroscopically to check the presence of the hemic iron in the proteins and were assayed for peroxidase activity. The data show that substitution of His502 leads to the loss, or to an inappropriate configuration, of the heme together with the loss of activity, suggesting that this residue could be the proximal His involved in the binding to the iron centers. On the other hand, substitution of His416 by alanine had no effect on either of the studied parameters.

Human hemi-myeloperoxidase. Initial chlorinating activity at neutral pH, compound II and III formation, and stability towards hypochlorous acid and high temperature

Human neutrophilic myeloperoxidase (MPO) is involved in the defence mechanism of the body against micro-organisms. The enzyme catalyses the generation of the strong oxidant hypochlorous acid (HOCl) from hydrogen peroxide and chloride ions. In normal neutrophils MPO is present in the dimeric form (140 kDa). The disulphide-linked protomers each consist of a heavy subunit and a light one. Reductive alkylation converts the dimeric enzyme into two promoters, 'hemi-myeloperoxidase'. We studied the initial activities of human dimeric MPO and hemi-MPO at the physiological pH of 7.2 and found no significant differences in chlorinating activity. These results indicate that, at least at neutral pH, the protomers of MPO function independently. The absorption spectra of MPO compounds II and III, both inactive forms concerning HOCl generation, and the rate constants of their formation were the same for dimeric MPO and hemi-MPO, but hemi-MPO required a slightly larger excess of H2O2 for complete conversion. Hemi-MPO was less stable at a high temperature (80 degrees C) as compared to the dimeric enzyme. Furthermore, the resistance of the chlorinating activity of hemi-MPO against its oxidative product hypochlorous acid was somewhat lower (IC50 = 32 microM HOCl) compared to dimeric MPO (IC50 = 50 microM HOCl). The higher stability of dimeric MPO in the presence of its oxidative product compared to that of monomeric MPO might be the reason for the occurrence of MPO as a dimer.

The active site structure of E. coli HPII catalase. Evidence favoring coordination of a tyrosinate proximal ligand to the chlorin iron

E. coli produces 2 catalases known as HPI and HPII. While the heme prosthetic group of the HPII catalase has been established to be a dihydroporphyrin or chlorin, the identity of the proximal ligand to the iron has not been addressed. The magnetic circular dichroism (MCD) spectrum of native ferric HPII catalase is very similar to those of a 5-coordinate phenolate-ligated ferric chlorin complex, a model for tyrosinate proximal ligation, as well as of chlorin-reconstituted ferric horseradish peroxidase, a model for 5-coordinate histidine ligation. However, further MCD comparisons of chlorin-reconstituted myoglobin with parallel ligand-bound adducts of the catalase clearly rule out histidine ligation in the latter, leaving tyrosinate as the best candidate for the proximal ligand.

1H-NMR characterization of cucumber peroxidases

Two peroxidase isoenzymes from Cucumber seedlings, one acidic (pI = 4) and one basic (pI = 9), were characterized by 1H-NMR spectroscopy. The NMR spectra were obtained in the native (ferric high-spin) and cyanide ligated (ferric low-spin) forms of both isoenzymes. The NMR spectral comparison of paramagnetically shifted resonances with those of the well characterized horseradish peroxidase C, HRP(C), isoenzyme indicates that both cucumber peroxidases have a protohemin IX prosthetic group with proximal histidine coordinated to the heme iron. The downfield heme 1H-NMR shift pattern is distinct for each isoenzyme, and this reflects presumably dissimilar heme active site environments. The basic isoenzyme shows less asymmetry in heme 1H-NMR signals as compared to the acidic isoenzyme or HRP(C) isoenzyme. It was also found that the acidic cucumber peroxidase exists predominantly as a monomeric species in solution with 30 kDa molecular mass as opposed to its earlier characterization as a 60 kDa dimeric protein.

Spectral and enzymatic properties of human recombinant myeloperoxidase: comparison with the mature enzyme

Human recombinant myeloperoxidase (recMPO), purified from an engineered Chinese hamster ovary (CHO) cell line, has been characterized and compared to the mature enzyme isolated from polymorphonuclear leukocytes. Both molecules appear essentially similar in physicochemical enzymatic terms according to the following observations. 1. The unprocessed recombinant protein displays the characteristic light absorption spectra of ferric mature MPO and exhibits its typical spectral changes in the presence of dithionite or hydrogen peroxide. 2. The addition of 14C-labeled 5-aminolevulinic acid, a heme precursor, to the culture medium of recombinant CHO cells yields labeled recMPO, indicating the presence of a heme-like structure in the molecule. 3. Like mature MPO, recMPO has a peroxidatic activity and catalyzes the oxidation of chloride ions in the presence of hydrogen peroxide, producing hypochlorous acid as measured by the monochlorodimedon assay. For both enzymes, the chlorinating activity optimally occurs around pH 5.0 at about 100 microM of hydrogen peroxide and is strongly inhibited by methimazole. 4. Diethylpyrocarbonate significantly reduces the enzymatic activity of both molecules, suggesting that histidine residues may be of prime importance in the active site of the enzymes. 5. According to infrared spectroscopy data, both enzymes present a very similar secondary structure organization. In conclusion, the data suggest that the processing of the precursor enzyme (recMPO) into the mature form occurs without major structural and functional consequences.

Metabolism of hydroquinone by human myeloperoxidase: mechanisms of stimulation by other phenolic compounds

Hydroquinone, a metabolite of benzene, is converted by human myeloperoxidase to 1,4-benzoquinone, a highly toxic species. This conversion is stimulated by phenol, another metabolite of benzene. Here we report that peroxidase-dependent hydroquinone metabolism is also stimulated by catechol, resorcinol, o-cresol, m-cresol, p-cresol, guaiacol, histidine, and imidazole. In order to gain insights into the mechanisms of this stimulation, we have compared the kinetics of human myeloperoxidase-dependent phenol, hydroquinone, and catechol metabolism. The specificity (Vmax/Km) of hydroquinone for myeloperoxidase was found to be 5-fold greater than that of catechol and 16-fold greater than that of phenol. These specificities for myeloperoxidase-dependent metabolism inversely correlated with the respective one-electron oxidation potentials of hydroquinone, catechol, and phenol and suggested that phenol- and catechol-induced stimulation of myeloperoxidase-dependent hydroquinone metabolism cannot simply be explained by interaction of hydroquinone with stimulant-derived radicals. Phenol (100 microM), catechol (20 microM), and imidazole (50 mM) did, however, all increase the specificity (Vmax/Km) of hydroquinone for myeloperoxidase, indicating that these three compounds may be stimulating hydroquinone metabolism by a common mechanism. Interestingly, the stimulation of peroxidase-dependent hydroquinone metabolism by other phenolic compounds was pH-dependent, with the stimulating effect being higher under alkaline conditions. These results therefore suggest that the interaction of phenolic compounds, presumably by hydrogen-bonding, with the activity limiting distal amino acid residue(s) or with the ferryl oxygen of peroxidase may be an important contributing factor in the enhanced myeloperoxidase-dependent metabolism of hydroquinone in the presence of other phenolic compounds.

Interaction of halides with the cyanide complex of myeloperoxidase: a model for substrate binding to compound I

EPR spectra of the low-spin cyanide complex of myeloperoxidase have been measured in the absence and presence of halide substrates; chloride, bromide and iodide. Halide-dependent spectral changes are found at acidic pH. The electronic structure of the low-spin ferric iron in cyanide complex appears to be modulated by halide binding to a protonated amino acid in the distal heme cavity. These findings suggest halide substrates can interact with ferryl oxygen in compound I during enzyme catalysis to form hypohalous acid.