Heme-linked ionization and chloride binding in intestinal peroxidase and lactoperoxidase

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.

Binding of iodide to Arthromyces ramosus peroxidase investigated with X-ray crystallographic analysis, 1H and 127I NMR spectroscopy, and steady-state kinetics

The site and characteristics of iodide binding to Arthromyces ramosus peroxidase were examined by x-ray crystallographic analysis, 1H and 127I NMR, and kinetic studies. X-ray analysis of an A. ramosus peroxidase crystal soaked in a KI solution at pH 5.5 showed that a single iodide ion is located at the entrance of the access channel to the distal side of the heme and lies between the two peptide segments, Phe90-Pro91-Ala92 and Ser151-Leu152-Ile153, 12.8 A from the heme iron. The distances between the iodide ion and heme peripheral methyl groups were all more than 10 A. The findings agree with the results obtained with 1H NMR in which the chemical shift and intensity of the methyl groups in the hyperfine shift region of A. ramosus peroxidase were hardly affected by the addition of iodide, unlike the case of horseradish peroxidase. Moreover, 127I NMR and steady-state kinetics showed that the binding of iodide depends on protonation of an amino acid residue with a pKa of about 5.3, which presumably is the distal histidine (His56), 7.8 A away from the iodide ion. The mechanism of electron transfer from the iodide ion to the heme iron is discussed on the basis of these findings.

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.

Spectroscopic investigations on the highly purified lactoperoxidase Fe(III)-heme catalytic site

Purification of the lactoperoxidase (LPO) major cationic isoenzyme was significantly improved by the use of preparative chromatographic and electrophoretic methods combined with analytical electrophoretic techniques and image processing. A detailed report is given of the experimental procedure. Furthermore, electron paramagnetic resonance has played a fundamental role in evaluating the enzyme purity against lactoferrin and minor LPO isoenzyme components in setting the final steps of the purification. With the aim to completely clarify the Fe(III)-heme high-spin nature of the native LPO, two samples of lactoperoxidase, LPO1 and LPO2 (RZ = 0.95) from farm and commercial milk, respectively, were purified and characterized in particular by electron paramagnetic resonance (EPR) spectroscopy, in comparison with a commercial preparation (LPOs). The LPO1 EPR spectrum, at physiological pH, is clearly indictive of the presence of an iron(III)-heme high-spin catalytic site in the native enzyme. On the contrary, in the LPO2 spectrum a thermal equilibrium between high- and low-spin iron(III)-heme species is present. The low-spin component of the spectrum has been assigned to an LPO-NO2- adduct due to the presence of some nitrite impurities originating either from commercial unpasteurized milk or from external sources. The LPOs EPR spectrum shwos the presence of some spurious lines in the g approximately equal to 6 and 4 regions due to the minor LPO isoenzyme components and to lactoferrin, respectively. The LPO EPR spectra previously reported in the literature contain a variable number of spurious lines in the g approximately equal to 4 and 2 regions as a consequence of lactoferrin impurity and LPO low-spin adducts with endogenous or exogenous anions. Furthermore, the interaction of LPO with its native substrate (the thiocyanate anion), which previously was shown by NMR and EPR (at high substrate concentration) spectroscopies, has been confirmed by EPR at low temperature and low substrate concentration and by optical spectroscopy at room temperature and high substrate concentration as a function of pH. The LPO activity at optimum pH (approximately equal to 4-5) has been measured in phosphate and acetate buffer using as an oxidizable substrate the system dimethylamino benzoic acid 3-methyl-2-benzothiazolinone hydrazone hydrochloride monohydrate (DMAB-MBTH), which was considered a good chromogen for other peroxidases such as HRP and zucchini peroxidases. The LPO vs SCN- activity at optimum pH (approximately equal to 5.5) has been measured in phosphate and acetate buffer.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of intestinal peroxidase activity by nonsteroidal antiinflammatory drugs

The peroxidase activity of the mitochondrial fraction of rat intestine is inhibited in vitro by non-steroidal antiinflammatory drugs (NSAIDs), such as indomethacin (IMN) and acetylsalicylic acid (ASA), the former being more potent than the latter. The peroxidase was solubilised by cetab-NH4Cl extraction and purified to apparent homogeneity by Sephadex G-150 gel filtration and affinity chromatography on Con-A Sepharose. The purified enzyme activity was 80% inhibited by 150 microM IMN and 50% by 2.67 mM ASA. IMN could also inhibit lactoperoxidase activity to the same extent but not the horseradish peroxidase activity. The inhibition of peroxidase-catalysed iodide oxidation by IMN and ASA was optimal at pH 5.5 and 4.5, respectively. Kinetic studies revealed that the inhibition by IMN was competitive with respect to iodide or guaiacol, while the inhibition by ASA was noncompetitive and reversible in nature. Studies of some structural analogues showed that indole-3-acetic acid was as effective as IMN, while salicylic acid was more potent than ASA. Spectral studies showed a small bathochromic shift of the Soret band of the enzyme by IMN, suggesting its possible interaction at or near the heme moiety. The competitive nature of IMN may be explained as due to its oxidation by the peroxidase to a product absorbing at 412 nm, the formation of which is inhibited by iodide. We suggest that IMN inhibits intestinal peroxidase activity by acting as a competitive substrate for the enzyme. As intestinal peroxidase is mainly contributed by the invading eosinophils, NSAIDs may affect the host defence mechanism by inhibiting the activity of the enzyme.

Haloperoxidase activity of Phanerochaete chrysosporium lignin peroxidases H2 and H8

Monochlorodimedone (MCD), commonly used as a halogen acceptor for haloperoxidase assays, was oxidized by hydrogen peroxide in the presence of lignin peroxidase isoenzymes H2 and H8. When oxidized, it produced a weak absorption band with an intensity that varied with pH. This absorbance was used as a simple method for the product analysis because it disappeared when MCD was brominated or chlorinated. We assessed the activity of the lignin peroxidases for oxidation of bromide by measuring the bromination of MCD, the formation of tribromide, the bromide-mediated oxidation of glutathione, and the bromide-mediated catalase-like activity. We analyzed the reaction products of MCD and the halide-mediated oxidation of glutathione when bromide was replaced by chloride. These enzymes demonstrated no significant activity for oxidation of chloride. Unlike other peroxidases, the lignin peroxidases exhibited similar pH-activity curves for the iodide and bromide oxidations. The optimum pH for activity was about 2.5. Surprisingly, this pH dependence of lignin peroxidase activity for the halides was nearly the same in the reactions with hydrogen donors, such as hydroquinone and guaiacol. The results suggested that protonation of the enzymes with pKa approximately 3.2 is necessary for the catalytic function of lignin peroxidases, irrespective of whether the substrates are electron or hydrogen donors. These unique reaction profiles of lignin peroxidases are compared to those of other peroxidases, such as lactoperoxidase, bromoperoxidase, chloroperoxidase, and horseradish peroxidase. Isozyme H2 was more active than isozyme H8, but isozyme H8 was more stable at very acidic pH.

Lactoperoxidase-catalyzed oxidation of thiocyanate ion: a carbon-13 nuclear magnetic resonance study of the oxidation products

Products formed from the lactoperoxidase (LPO) catalyzed oxidation of thiocyanate ion (SCN-) with hydrogen peroxide (H2O2) have been studied by 13C-NMR at pH 6 and pH 7. Ultimate formation of hypothiocyanite ion (OSCN-) as the major product correlates well with the known optical studies. The oxidation rate of SCN- appears to be greater at pH < or = 6.0. At [H2O2]/[SCN-] ratios of < or = 0.5, OSCN- is not formed immediately, but an unidentified intermediate is produced. At [H2O2]/[SCN-] > 0.5, SCN- appears to be directly oxidized to OSCN-. Once formed, OSCN- slowly degrades over a period of days to carbon dioxide (CO2), bicarbonate ion (HCO3-), and hydrogen cyanide (HCN). An additional, previously unrecognized product also appears after formation of OSCN-. On the basis of carbon-13 chemical shift information this new species is suggested to result from rearrangement of OSCN- to yield the thiooxime isomer, SCNO- or SCNOH.

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.

Axial ligand coordination in intestinal peroxidase

EPR spectra of intestinal peroxidase are reported for the first time. The resting state of intestinal peroxidase exhibits only a high spin EPR spectrum with pH-dependent rhombicity. Addition of chloride shifts the equilibrium between an acidic and a neutral form of the enzyme. In contrast, resting lactoperoxidase shows EPR spectra of both low spin and high spin species, indicating a different heme environment between these two peroxidases. The high spin signal of lactoperoxidase consists of multiple components; the major component exhibits pH-dependent rhombicity similar to intestinal peroxidase and the equilibrium between the acidic and the neutral forms is also shifted by chloride ion. EPR features of the low spin cyanide complex of intestinal peroxidase and lactoperoxidase are compared with those of other hemeproteins, whose proximal axial ligands are known to be histidine residues. The g-values of the cyanide adducts of the mammalian peroxidases are similar. The relationship between the g-value anisotropy and imidazolate character of the proximal histidine is discussed.

Membrane peroxidases

It is well known that the partial reduction of oxygen can result in the formation of highly reactive oxygen products. Hydrogen peroxide is one of these metabolites of oxygen. Peroxidases utilize this metabolite for a variety of functions. It is the purpose of this treatise to review the nature and function of various membrane peroxidases in the body.

Purification and characterization of rat intestinal peroxidase. Its activity towards 2-t-butyl-4-methoxyphenol (BHA)

Impure preparations of rat intestinal peroxidase were shown to aggregate at low ionic strengths and to disaggregate at higher values. This aggregation was accompanied by a decrease in specific activity, which could lead to hysteretic behaviour of reaction progress curves. Advantage was taken of this reversible aggregation to obtain a relatively pure extract, which was subsequently purified to apparent homogeneity by affinity chromatography on concanavalin A-Sepharose followed by hydrophobic chromatography. The purified enzyme did not show the ionic-strength-dependent aggregation behaviour, behaving as a monomer of Mr 50,000. The purified enzyme was shown to catalyse the peroxidatic conversion of the commonly used antioxidant 2-t-butyl-4-methoxyphenol (butylated hydroxyanisole, BHA) to form 3,3'-di-t-butyl-2,2'-dihydroxy-5,5'-dimethoxybiphenyl, with a Km value of 176 microM and a maximum velocity of 8 mumol/min per mg. The specificity constant, kcat./Km, for this substrate was similar to that shown towards the substrate guaiacol.

Purification, characterization and origin of rat gastric peroxidase

A membrane-bound peroxidase (EC 1.11.1.7) from rat stomach has been solubilized by 0.2% cetyltrimethylammonium bromide in the presence of 1.2 M NH4Cl. The enzyme was purified 3355-fold to apparent homogeneity as judged by acid polyacrylamide gel electrophoresis and appears to be a cationic protein. In sodium dodecyl sulfate gel electrophoresis, the enzyme shows single polypeptide band of Mr 45,000. In gel permeation, the Mr has been estimated as 47,000. Spectral properties indicate the presence of Soret band at 412 nm which shifts to 425 nm on complexation with CN- and to 430 nm on reduction with dithionite. The velocity constant, k1 for the reaction of the peroxidase with H2O2 is 1.38 X 10(7) M-1 s-1 and Km for H2O2 is 0.1 mM. The enzyme contains active sulphydryl groups and is inhibited by sulphydryl reagents of which p-hydroxymercuribenzoate is more reactive than mersalyl or N-ethylmaleimide. The enzyme is very resistant to thermal denaturation up to 65 degrees C and also to chaotropic reagents at least up to 2 M above which it is inactivated. The enzyme shows similarity with the intestinal eosinophil peroxidase as regards the molecular mass, spectral, kinetic and some of the catalytic properties. However, they differ significantly in terms of their interaction with fluoride ion, sulphydryl reagents, chaotropic reagent and also with the antiserum against the gastric peroxidase. Histochemically, the gastric peroxidase is shown to be localised in the gastric gland proper of the fundic stomach, rich in parietal and chief cells.

Reduction of lactoperoxidase by the dithionite anion monomer

The reduction of lactoperoxidase with sodium dithionite has been studied by means of stopped-flow spectrophotometry in an anaerobic system. Under pseudo-first-order conditions the rate constant was found to be linearly dependent on the square root of the dithionite concentration, which confirms the monomeric radical, SO2- as the reducing species. The second-order rate constant is moderately influenced by increased ionic strength but drastically increased at lower pH. The pH dependence supports the previously suggested existence of a carboxyl group, essential to the different enzymatic functions of lactoperoxidase. The second-order rate constant for the reduction of lactoperoxidase at pH 7.0 (kappa 1 = 1.3 X 10(5) M-1 s-1) was about three times higher than the rate constant for the reduction of cyanide-bound lactoperoxidase and two times the rate constant for the reduction of the fluoride-lactoperoxidase complex.

A kinetic study of the reaction between human myeloperoxidase, hydroperoxides and cyanide. Inhibition by chloride and thiocyanate

The reaction between myeloperoxidase (donor:hydrogen-peroxide oxidoreductase, EC 1.11.1.7), hydrogen peroxide and ethyl hydroperoxide was investigated using the stopped-flow technique. Like other peroxidases, myeloperoxidase forms two sequential peroxide compounds. The pH-dependence of the apparent second-order rate constant of compound I formation shows that there is an acid/base group on the enzyme with a pKa of 4.30 +/- 0.15, which - when protonated - prevents the reaction of the enzyme with peroxides. The rate constants for the formation of compound I by hydrogen peroxide and ethyl hydroperoxide are (2.3 +/- 0.1) X 10(7) M-1 X s-1 and (2.8 +/- 0.3) X 10(5) M-1 X s-1, respectively. The binding of cyanide to myeloperoxidase (k1 = (1.30 +/- 0.05) X 10(6) M-1 X s-1) is also regulated by an acid/base group with a pKa of 4.00 +/- 0.05 as is the case with hydrogen peroxide; also, only the protonated uncharged form of cyanide reacts with the enzyme. From their effects on the binding of cyanide to the enzyme it is concluded that chloride and thiocyanate bind to myeloperoxidase only when the acid/base group is protonated. The pH-dependence of the dissociation constant of the myeloperoxidase-chloride complex obtained from the spectral changes induced by chloride is the same as observed in the inhibition by chloride of the binding of cyanide. It is concluded that hydrogen peroxide, cyanide, chloride and thiocyanate bind at the same site on the enzyme.

Lactoperoxidase, a dithionite ion dismutase

The dithionite ion is catalytically disproportionated by lactoperoxidase with Km = 0.36 mM in 100 mM glycine HCl pH 3.0. The products formed are thiosulfate and hydrogensulfite ions. The rate of reaction is considerably increased at low pH with a pKa at 3-3.5 possibly indicating the involvement of a carboxyl group. The reaction is competitively inhibited by hydrogensulfite, Ki = 5.5 mM in 100 mM glycine HCl pH 3.50. Four different spectral forms of reduced lactoperoxidase appear during the reaction. The first two forms are found during the lag phase of the reaction. The third form, which is interpreted as a ternary complex, exists under the dismutation phase. After exhaustion of the substrate a visible spectrum similar to that of lactoperoxidase H2O2 compound III appears. A mechanistic model for the lactoperoxidase dismutation of the dithionite ion is proposed and discussed.

Studies on mammalian intestinal peroxidase

A peroxidase, purified from rat small intestine to apparent homogeneity as judged by polyacrylamide-gel electrophoresis, exhibited an absorbance ratio (A412/A280) of 0.783. Its Mr (44000 +/- 1000) and spectral properties were similar to those of the pig intestinal enzyme. The velocity constant for the reaction between rat intestinal peroxidase and hydrogen peroxide was found to be 1.8 x 10(7) M-1 . s-1. Benzhydroxamic acid inhibited the peroxidative oxidation of guaiacol by intestinal peroxidase from both species but the concentration required to cause half-inhibition of the enzyme from the rat was higher by one order of magnitude than for the pig enzyme. The amino acid composition of highly-purified pig intestinal peroxidase showed a relative abundance of basic amino acids (lysine and arginine) and was similar to that of lactoperoxidase, but not that of myeloperoxidase. The initial ten amino acid residues of this enzyme (the first reported partial sequence for a mammalian peroxidase) were also determined.

The halide complexes of myeloperoxidase and the mechanism of the halogenation reactions

The spectral changes caused by the addition of halides to myeloperoxidase (donor:hydrogen-peroxide oxidoreductase, EC 1.11.1.7) have been investigated and the dissociation constants of the enzyme-halide complexes have been determined. The pH dependence of the dissociation constants suggests that halide binding is associated with a protonation step in myeloperoxidase. Myeloperoxidase catalyzes the peroxidative chlorination and bromination of monochlorodimedone. It is shown that at low pH, chloride acts as a competitive inhibitor with respect to H2O2, whereas at higher pH, H2O2 inhibits the chlorination reaction. The dissociation constant (Kd) of the spectroscopically detectable complex and the Km for chloride are considerably smaller than the inhibition constant (Ki) for chloride. These halogenation reactions are strongly pH dependent, the logarithm of the Km for chloride varies linearly with pH. The position of the pH optimum of the chlorination and bromination reaction is a linear function of the logarithm of the [halide]/[H2O2] ratio. A mechanism of the chlorination and bromination reaction is suggested with substrate inhibition for both hydrogen peroxide and the halide.

Bromoperoxidases from Penicillus capitatus, Penicillus lamourouxii and Rhipocephalus phoenix

The isolation and purification of bromoperoxidases from three marine subtropical green algae is described. In the presence of KBr and H2O2, each halide-specific enzyme catalyses the bromination of monochlorodimedone (2-chloro-5,5-dimethylcyclohexane-1,3-dione) to bromochlorodimedone (2-bromo-2-chloro-5,5-dimethylcyclohexane-1,3-dione). The enzymes also catalyse the oxidation of pyrogallol, o-phenylenediamine and I- to I3-. Preliminary characterization of these enzymes reveals acidic pH optima, high thermal stability, sensitivity to higher H2O2 concentrations, and apparent molecular weights ranging from 48000 to 60000.

The interaction of myeloperoxidase with ligands as studied by EPR

1. The reaction of myeloperoxidase with fluoride, chloride and azide has been studied by EPR. 2. Fluoride decreases the rhombicity of the high-spin heme signal of myeloperoxidase and the nuclear spin of the fluoride atom induces a splitting in g parallel of 35 G. This observation demonstrates that fluoride binds as an axial ligand to the heme iron of the enzyme. 3. Addition of chloride to the fluoride-treated enzyme increases the rhombicity of the high-spin heme signal and brings about a disappearance of the splitting at g parallel. The addition of azide to the fluoride-treated enzyme changes the spin state of the heme iron from a high-to a low-spin state (gx = 2.68, gy = 2.22 and gz = 1.80). 4. Upon addition of chloride or fluoride to low-spin azido-myeloperoxidase this compound is converted into the high-spin chlorido- or fluorido-myeloperoxidase. These observations demonstrate that these ligands compete for a binding site at or close to the heme iron of myeloperoxidase.