Mode of binding of the tuberculosis prodrug isoniazid to heme peroxidases: binding studies and crystal structure of bovine lactoperoxidase with isoniazid at 2.7 A resolution

Structural evidence of the conversion of nitric oxide (NO) to nitrite ion (NO2-) by lactoperoxidase (LPO): Structure of the complex of LPO with NO2- at 1.89 Å resolution

Lactoperoxidase (LPO) is a heme containing mammalian enzyme which uses hydrogen peroxide (H2O2) to catalyze the conversion of substrates into oxidized products. LPO is found in body fluids and tissues such as milk, saliva, tears, mucosa and other body secretions. The previous structural studies have shown that LPO converts substrates, thiocyanate (SCN-) and iodide (I-) ions into oxidized products, hypothiocyanite (OSCN-) and hypoiodite (IO-) ions respectively. We report here a new structure of the complex of LPO with an oxidized product, nitrite (NO2-). This product was generated from NO using the two step reaction of LPO by adding hydrogen peroxide (H2O2) in the solution of LPO in 0.1 M phosphate buffer at pH 6.8 as the first step. In the second step, NO gas was added to the above mixture. This was crystallized using 20% (w/v) PEG-3350 and 0.2 M ammonium iodide at pH 6.8. The structure determination showed the presence of NO2- ion in the distal heme cavity of the substrate binding site of LPO. The structure also showed that the propionate group which is linked to pyrrole ring D of the heme moiety was disordered. Similarly, the side chain of Asp108, which is covalently linked to heme moiety, was also split into two components. As a result of these changes, the conformation of the side chain of Arg255 was altered allowing it to form new interactions with the disordered carboxylic group of propionate moiety. These structural changes are indicative of an intermediate state in the catalytic reaction pathway of LPO.

One-Pot Synthesis and in Silico Molecular Docking Studies of Arylselanyl Hydrazides as Potential Antituberculosis Agents

The present study reports a simple two-step method for the synthesis of arylselanyl hydrazide derivatives using hypophosphorous acid and polyethylene glycol (H₃ PO₂ /PEG-400) as an alternative reducing system and hydrazine hydrate (NH₂ NH₂ ⋅xH₂ O/50-60 %). This single-vessel procedure was employed with methyl acrylate 2a and methyl bromoacetate 2b using diaryl diselenides to generate the nucleophile species to produce, respectively, 3-(arylselanyl)propane-hydrazides 4a-e and 2-(arylselanyl)acetohydrazides 5a-e with good yields by accelerating the reduction of -Se-Se- bond, when compared to available methods. The synthesized molecules are structurally similar to the isoniazid (INH). Therefore, we perform in silico molecular docking studies, using the lactoperoxidase enzyme, in order to verify whether the INH Se derivatives could interact in a similar way to INH at the active site of the mammalian enzyme. The in silico results indicated a similar type of interaction of the arylselanyl hydrazide derivatives with that of INH. In view of the similar in silico interaction of the selenium derivatives of INH, the arylselanyl hydrazide derivatives reported here should be tested against Mycobacterium tuberculosis in vitro.

Structural evidence of the oxidation of iodide ion into hyper-reactive hypoiodite ion by mammalian heme lactoperoxidase

Lactoperoxidase (1.11.1.7, LPO) is a mammalian heme peroxidase found in the extracellular fluids of mammals including plasma, saliva, airway epithelial lining fluids, nasal lining fluid, milk, tears, gastric juices, and intestinal mucosa. To perform its innate immune action against invading microbes, LPO utilizes hydrogen peroxide (H₂ O₂ ) to convert thiocyanate (SCN^- ) and iodide (I^- ) ions into the oxidizing compounds hypothiocyanite (OSCN^- ) and hypoiodite (IO^- ). Previously determined structures of the complexes of LPO with SCN^- , OSCN^- , and I^- show that SCN^- and I^- occupy appropriate positions in the distal heme cavity as substrates while OSCN^- binds in the distal heme cavity as a product inhibitor. We report here the structure of the complex of LPO with IO^- as the first structural evidence of the conversion of iodide into hypoiodite by LPO. To obtain this complex, a solution of LPO was first incubated with H₂ O₂ , then mixed with ammonium iodide solution and the complex crystallized by the addition of PEG-3350, 20% (wt/vol). These crystals were used for X-ray intensity data collection and structure analysis. The structure determination revealed the presence of four hypoiodite ions in the substrate binding channel of LPO. In addition to these, six other hypoiodite ions were observed at different exterior sites. We surmise that the presence of hypoiodite ions in the distal heme cavity blocks the substrate binding site and inhibits catalysis. This was confirmed by activity experiments with the colorimetric substrate, ABTS (2,2'-azino-bis(3-ethylbenzthiazoline-sulfonic acid)), in the presence of hypoiodite and iodide ions.

Structure of dye-decolorizing peroxidase from Bacillus subtilis in complex with veratryl alcohol

Dye-decolorizing peroxidases (DyPs) are heme-containing peroxidases, which have promising application in biodegradation of phenolic lignin compounds and in detoxification of dyes. In this study, the crystal structure of BsDyP- veratryl alcohol (VA) complex delves deep into the binding of small substrate molecules within the DyP heme cavity. The biochemical analysis shows that BsDyP oxidizes the VA with a turnover number of 0.065 s^-1, followed by the oxidation of 2,6-dimethoxyphenol (DMP) and guaiacol with a comparable turnover number (k_cat) of 0.07 s^-1 and 0.07 s^-1, respectively. Moreover, biophysical and computational studies reveal the comparable binding affinity of substrates to BsDyP and produce lower-energy stable BsDyP-ligand(s) complexes. All together with our previous findings, we are providing a complete structural description of substrate-binding sites in DyP. The structural insight of BsDyP helps to modulate its engineering to enhance the activity towards the oxidation of a wide range of substrates.

Structure of a ternary complex of lactoperoxidase with iodide and hydrogen peroxide at 1.77 Å resolution

Lactoperoxidase (LPO) is a mammalian heme peroxidase which catalyzes the conversion of thiocyanate (SCN¯) and iodide (I^-) by hydrogen peroxide (H₂O₂) into antimicrobial hypothiocyanite (OSCN¯) and hypoiodite (IO^-). The prosthetic heme group is covalently attached to LPO through two ester linkages involving conserved glutamate and aspartate residues. On the proximal side, His351 is coordinated to heme iron while His 109 is located in the substrate binding site on the distal heme side. We report here the first structure of the ternary complex of LPO with iodide (I^-) and H₂O₂ at 1.77 Å resolution. LPO was crystallized with ammonium iodide and the crystals were soaked in the reservoir solution containing H₂O_2. Structure determination showed the presence of an iodide ion and a H₂O₂ molecule in the substrate binding site. The iodide ion occupied the position which is stabilized by the interactions with heme moiety, His109, Arg255 and Glu258 while H₂O₂ was held between the heme iron and His109. The presence of I^- in the distal heme cavity seems to screen the positive charge of Arg255 thus suppressing the proton transfer from H₂O₂ to His109. This prevents compound I formation and allows trapping of a stable enzyme-substrate (LPO-I^--H₂O₂₎ ternary complex. This stable geometrical arrangement of H₂O₂ in the distal heme cavity of LPO is similar to that of H₂O₂ in the structure of the transient intermediate of the palm tree heme peroxidase. The biochemical studies showed that the catalytic activity of LPO decreased when the samples of LPO were preincubated with ammonium iodide.

Potassium-induced partial inhibition of lactoperoxidase: structure of the complex of lactoperoxidase with potassium ion at 2.20 Å resolution

Lactoperoxidase, a heme-containing glycoprotein, catalyzes the oxidation of thiocyanate by hydrogen peroxide into hypothiocyanite which acts as an antibacterial agent. The prosthetic heme moiety is attached to the protein through two ester linkages via Glu258 and Asp108. In lactoperoxidase, the substrate-binding site is formed on the distal heme side. To study the effect of physiologically important potassium ion on the structure and function of lactoperoxidase, the fresh protein samples were isolated from yak (Bos grunniens) colostrum and purified to homogeneity. The biochemical studies with potassium fluoride showed a significant reduction in the catalytic activity. Lactoperoxidase was crystallized using 200 mM ammonium nitrate and 20% PEG-3350 at pH 6.0. The crystals of LPO were soaked in the solution of potassium fluoride and used for the X-ray intensity data collection. Structure determination at 2.20 Å resolution revealed the presence of a potassium ion in the distal heme cavity. Structure determination further revealed that the propionic chain attached to pyrrole ring C of the heme moiety, was disordered into two components each having an occupancy of 0.5. One component occupied a position similar to the normally observed position of propionic chain while the second component was found in the distal heme cavity. The potassium ion in the distal heme cavity formed five coordinate bonds with two oxygen atoms of propionic moiety, N^ε2 atom of His109 and two oxygen atoms of water molecules. The presence of potassium ion in the distal heme cavity hampered the catalytic activity of lactoperoxidase.

Pharmacokinetics of isoniazid: The good, the bad, and the alternatives

Although isoniazid (INH) has been successful in treating Tuberculosis (TB) since its introduction in 1952, there has been continual reports of drug-associated hepatotoxicity in TB patients. These toxic side effects may reveal more about the recipient of the drug, than the drug itself. A combination of pharmacogenetic and pharmacokinetic studies have identified polymorphisms within enzymes involved in INH metabolism and detoxification. These essential metabolic enzymes include N-acetyltransferase 2, Cytochrome P450 2E1, and glutathione S transferases. Different phenotypes of these enzymes can affect the rate of INH metabolism, resulting in production of hepatotoxic metabolites. This review is intended to elucidate the pharmacokinetics of INH by examining its Administration, Distribution, Metabolism, and Elimination, while suggesting potential alternatives within INH personalized treatment to help reduce hepatotoxicity.

Enhanced Antibacterial Activity of Lactoperoxidase-Thiocyanate-Hydrogen Peroxide System in Reduced-Lactose Milk Whey

The product of the lactoperoxidase system (LPOS) has been developed as a preservative agent to inhibit foodborne bacteria, but its action was, heretofore, limited to several original compounds in milk. This research was conducted to analyze the application of the lactoperoxidase system against Escherichia coli in fresh bovine milk and its derivative products to determine the strength of antibacterial activity. Lactoperoxidase was purified from bovine whey using the SP Sepharose Big Beads Column. The enzymatic reaction involving lactoperoxidase, thiocyanate, and hydrogen peroxide was used to generate the antibacterial agent from LPOS. This solution was then added to milk, skimmed milk, untreated whey, reduced-LPO whey, reduced-lactose whey, and high-lactose solution containing E. coli at an initial count of 6.0 log CFU/mL. LPOS showed the greatest reduction of bacteria (1.68 ± 0.1 log CFU/mL) in the reduced-lactose whey among the products tested. This result may lead to a method for enhancement of the antimicrobial activity of LPOS in milk and derived products.

Eosinophil peroxidase oxidizes isoniazid to form the active metabolite against M. tuberculosis, isoniazid-NAD. Chem Biol Interact 2019;305:48-53. [PMID: 30922765 DOI: 10.1016/j.cbi.2019.03.019]

The formation of isonicotinyl-nicotinamide adenine dinucleotide (INH-NAD⁺) by the mycobacterial catalase-peroxidase enzyme, KatG, was known to be the major component of the mode of action of isoniazid (INH), an anti-tuberculosis drug. However, there are other enzymes that may catalyze this reaction. We have previously reported that neutrophil myeloperoxidase (MPO) is capable of metabolizing INH through the formation of INH-NAD⁺ adduct, which could be attributed to being a possible mode of action of INH. However, eosinophilic infiltration of the lungs is more pronounced and characteristic of granulomas in Mycobacterium tuberculosis-infected patients. Thus, the aim of the present study is to investigate the role of eosinophil peroxidase (EPO), a key eosinophil enzyme, during INH metabolism and the formation of its active metabolite, INH-NAD⁺ using purified EPO and eosinophils isolated from asthmatic donors. UV-Vis spectroscopy revealed INH oxidation by EPO led to a new product (λ_max = 326 nm) in the presence of NAD⁺. This adduct was confirmed to be INH-NAD⁺ using LC-MS analysis where the intact adduct was detected (m/z = 769). Furthermore, EPO catalyzed the oxidation of INH and formed several free radical intermediates as assessed by electron paramagnetic resonance (EPR) spin-trapping; a carbon-centred radical, which is considered to be the reactive metabolite that binds with NAD⁺, was found when superoxide dismutase was included in the reaction. Our findings suggest that eosinophilic EPO may also play a role in the pharmacological activity of INH through the formation of INH-NAD⁺ adduct, and supports further evidence that human cells and enzymes are capable of producing the active metabolite involved in tuberculosis treatment.

Design of anti-thyroid drugs: Binding studies and structure determination of the complex of lactoperoxidase with 2-mercaptoimidazole at 2.30 Å resolution

Lactoperoxidase (LPO) belongs to mammalian heme peroxidase superfamily, which also includes myeloperoxidase (MPO), eosinophil peroxidase (EPO), and thyroid peroxidase (TPO). LPO catalyzes the oxidation of a number of substrates including thiocyanate while TPO catalyzes the biosynthesis of thyroid hormones. LPO is also been shown to catalyze the biosynthesis of thyroid hormones indicating similar functional and structural properties. The binding studies showed that 2-mercaptoimidazole (MZY) bound to LPO with a dissociation constant of 0.63 µM. The inhibition studies showed that the value of IC₅₀ was 17 µM. The crystal structure of the complex of LPO with MZY showed that MZY bound to LPO in the substrate-binding site on the distal heme side. MZY was oriented in the substrate-binding site in such a way that the sulfur atom is at a distance of 2.58 Å from the heme iron. Previously, a similar compound, 3-amino-1,2,4-triazole (amitrole) was also shown to bind to LPO in the substrate-binding site on the distal heme side. The amino nitrogen atom of amitrole occupied the same position as that of sulfur atom in the present structure indicating a similar mode of binding. Recently, the structure of the complex of LPO with a potent antithyroid drug, 1-methylimidazole-2-thiol (methimazole, MMZ) was also determined. It showed that MMZ bound to LPO in the substrate-binding site on the distal heme side with 2 orientations. The position of methyl group was same in the 2 orientations while the positions of sulfur atom differed indicating a higher preference for a methyl group.

Dual binding mode of antithyroid drug methimazole to mammalian heme peroxidases - structural determination of the lactoperoxidase-methimazole complex at 1.97 Å resolution

Lactoperoxidase (LPO, EC 1.11.1.7) is a member of the mammalian heme peroxidase family which also includes thyroid peroxidase (TPO). These two enzymes have a sequence homology of 76%. The structure of LPO is known but not that of TPO. In order to determine the mode of binding of antithyroid drugs to thyroid peroxidase, we have determined the crystal structure of LPO complexed with an antithyroid drug, methimazole (MMZ) at 1.97 Å resolution. LPO was isolated from caprine colostrum, purified to homogeneity and crystallized with 20% poly(ethylene glycol)‐3350. Crystals of LPO were soaked in a reservoir solution containing MMZ. The structure determination showed the presence of two crystallographically independent molecules in the asymmetric unit. Both molecules contained one molecule of MMZ, but with different orientations. MMZ was held tightly between the heme moiety on one side and the hydrophobic parts of the side chains of Arg255, Glu258, and Leu262 on the opposite side. The back of the cleft contained the side chains of Gln105 and His109 which also interacted with MMZ. In both orientations, MMZ had identical buried areas and formed a similar number of interactions. It appears that the molecules of MMZ can enter the substrate‐binding channel of LPO in two opposite orientations. But once they reach the distal heme pocket, their orientations are frozen due to equally tight packing of MMZ in both orientations. This is a novel example of an inhibitor binding to an enzyme with two orientations at the same site with nearly equal occupancies.

Crystal structure of halogenase PltA from the pyoluteorin biosynthetic pathway

Pyoluteorin is an antifungal agent composed of a 4,5-dichlorinated pyrrole group linked to a resorcinol moiety. The pyoluteorin biosynthetic gene cluster in Pseudomonas fluorescens Pf-5 encodes the halogenase PltA, which has been previously demonstrated to perform both chlorinations in vitro. PltA selectively accepts as a substrate a pyrrole moiety covalently tethered to a nonribosomal peptide thiolation domain PltL (pyrrolyl-S-PltL) for FAD-dependent di-chlorination, yielding 4,5-dichloropyrrolyl-S-PltL. We report a 2.75 Å-resolution crystal structure of PltA in complex with FAD and chloride. PltA is a dimeric enzyme, containing a flavin-binding fold conserved in flavin-dependent halogenases and monooxygenases, and an additional unique helical region at the C-terminus. This C-terminal region blocks a putative substrate-binding cleft, suggesting that a conformational change involving repositioning of this region is necessary to allow binding of the pyrrolyl-S-PltL substrate for its dichlorination by PltA.

Mode of binding of the antithyroid drug propylthiouracil to mammalian haem peroxidases

The mammalian haem peroxidase superfamily consists of myeloperoxidase (MPO), lactoperoxidase (LPO), eosinophil peroxidase (EPO) and thyroid peroxidase (TPO). These enzymes catalyze a number of oxidative reactions of inorganic substrates such as Cl(-), Br(-), I(-) and SCN(-) as well as of various organic aromatic compounds. To date, only structures of MPO and LPO are known. The substrate-binding sites in these enzymes are located on the distal haem side. Propylthiouracil (PTU) is a potent antithyroid drug that acts by inhibiting the function of TPO. It has also been shown to inhibit the action of LPO. However, its mode of binding to mammalian haem peroxidases is not yet known. In order to determine the mode of its binding to peroxidases, the structure of the complex of LPO with PTU has been determined. It showed that PTU binds to LPO in the substrate-binding site on the distal haem side. The IC50 values for the inhibition of LPO and TPO by PTU are 47 and 30 µM, respectively. A comparision of the residues surrounding the substrate-binding site on the distal haem side in LPO with those in TPO showed that all of the residues were identical except for Ala114 (LPO numbering scheme), which is replaced by Thr205 (TPO numbering scheme) in TPO. A threonine residue in place of alanine in the substrate-binding site may affect the affinity of PTU for peroxidases.

Rapid in vivo detection of isoniazid-sensitive Mycobacterium tuberculosis by breath test

There is urgent need for rapid, point of care diagnostic tools for tuberculosis (TB) and drug sensitivity. Current methods based on in vitro growth take weeks, while DNA amplification can neither differentiate live from dead organisms nor determine phenotypic drug resistance. Here we show the development and evaluation of a rapid breath test for isoniazid (INH)-sensitive TB based on detection of labeled N₂ gas formed specifically from labeled INH by mycobacterial KatG enzyme. In vitro data shows the assay is specific, dependent on mycobacterial abundance, and discriminates between INH-sensitive and resistant (S315T mutant KatG) TB. In vivo, the assay is rapid with maximal detection of ¹⁵N₂ in exhaled breath of infected rabbits within five to ten minutes. No increase in ¹⁵N₂ is detected in un-infected animals, and the increases in ¹⁵N₂ are dependent on infection dose. This test may allow rapid detection of INH-sensitive TB.

Lactoperoxidase: structural insights into the function,ligand binding and inhibition

Lactoperoxidase (LPO) is a member of a large group of mammalian heme peroxidases that include myeloperoxidase (MPO), eosinophil peroxidase (EPO) and thyroid peroxidase (TPO). The LPO is found in exocrine secretions including milk. It is responsible for the inactivation of a wide range of micro-organisms and hence, is an important component of defense mechanism in the body. With the help of hydrogen peroxide, it catalyzes the oxidation of halides, pseudohalides and organic aromatic molecules. Historically, LPO was isolated in 1943, nearly seventy years ago but its three-dimensional crystal structure has been elucidated only recently. This review provides various details of this protein from its discovery to understanding its structure, function and applications. In order to highlight species dependent variations in the structure and function of LPO, a detailed comparison of sequence, structure and function of LPO from various species have been made. The structural basis of ligand binding and distinctions in the modes of binding of substrates and inhibitors have been analyzed extensively.

Isoniazid inhibits the heme-based reactivity of Mycobacterium tuberculosis truncated hemoglobin N

Isoniazid represents a first-line anti-tuberculosis medication in prevention and treatment. This prodrug is activated by a mycobacterial catalase-peroxidase enzyme called KatG in Mycobacterium tuberculosis), thereby inhibiting the synthesis of mycolic acid, required for the mycobacterial cell wall. Moreover, isoniazid activation by KatG produces some radical species (e.g., nitrogen monoxide), that display anti-mycobacterial activity. Remarkably, the ability of mycobacteria to persist in vivo in the presence of reactive nitrogen and oxygen species implies the presence in these bacteria of (pseudo-)enzymatic detoxification systems, including truncated hemoglobins (trHbs). Here, we report that isoniazid binds reversibly to ferric and ferrous M. tuberculosis trHb type N (or group I; Mt-trHbN(III) and Mt-trHbN(II), respectively) with a simple bimolecular process, which perturbs the heme-based spectroscopic properties. Values of thermodynamic and kinetic parameters for isoniazid binding to Mt-trHbN(III) and Mt-trHbN(II) are K = (1.1±0.1)×10⁻⁴ M, k_on = (5.3±0.6)×10³ M⁻¹ s⁻¹ and k_off = (4.6±0.5)×10⁻¹ s⁻¹; and D = (1.2±0.2)×10⁻³ M, d_on = (1.3±0.4)×10³ M⁻¹ s⁻¹, and d_off = 1.5±0.4 s⁻¹, respectively, at pH 7.0 and 20.0°C. Accordingly, isoniazid inhibits competitively azide binding to Mt-trHbN(III) and Mt-trHbN(III)-catalyzed peroxynitrite isomerization. Moreover, isoniazid inhibits Mt-trHbN(II) oxygenation and carbonylation. Although the structure of the Mt-trHbN-isoniazid complex is not available, here we show by docking simulation that isoniazid binding to the heme-Fe atom indeed may take place. These data suggest a direct role of isoniazid to impair fundamental functions of mycobacteria, e.g. scavenging of reactive nitrogen and oxygen species, and metabolism.

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Isoniazid and thioacetazone may exhibit antitubercular activity by binding directly with the active site of mycolic acid cyclopropane synthase: Hypothesis based on computational analysis

Isoniazid and thioacetazone are the two important antitubercular drugs. In case of thioacetazone it is established that it inhibits mycolic acid cyclopropane synthase but the exact binding site accounting for such inhibition is presently unknown. In case of isoniazid its action on the said enzyme is unexplored. In this work we have analyzed the binding of isoniazid and thioacetazone with mycolic acid cyclopropane synthase (CmaA1 and CmaA2) using tools of computational biology. We have observed that thioacetazone fits well at the active site of CmaA1 and CmaA2 while isoniazid binds at the active site of CmaA1 only. We have recommended experimental validation of such results. If such results are proved to be fact it will explore the exact binding site of thioacetazone and discover a new mechanism of anti-tubercular action of isoniazid.

Crystal structure of guaiacol and phenol bound to a heme peroxidase

Guaiacol is a universal substrate for all peroxidases, and its use in a simple colorimetric assay has wide applications. However, its exact binding location has never been defined. Here we report the crystal structures of guaiacol bound to cytochrome c peroxidase (CcP). A related structure with phenol bound is also presented. The CcP-guaiacol and CcP-phenol crystal structures show that both guaiacol and phenol bind at sites distinct from the cytochrome c binding site and from the δ-heme edge, which is known to be the binding site for other substrates. Although neither guaiacol nor phenol is seen bound at the δ-heme edge in the crystal structures, inhibition data and mutagenesis strongly suggest that the catalytic binding site for aromatic compounds is the δ-heme edge in CcP. The functional implications of these observations are discussed in terms of our existing understanding of substrate binding in peroxidases [Gumiero A et al. (2010) Arch Biochem Biophys 500, 13-20].

Structural evidence for the order of preference of inorganic substrates in mammalian heme peroxidases: crystal structure of the complex of lactoperoxidase with four inorganic substrates, SCN, I, Br and Cl

Lactoperoxidase (LPO) is a member of the family of mammalian heme peroxidases. It catalyzes the oxidation of halides and pseudohalides in presence of hydrogen peroxide. LPO has been co-crystallized with inorganic substrates, SCN(-), I(-), Br(-) and Cl(-). The structure determination of the complex of LPO with above four substrates showed that all of them occupied distinct positions in the substrate binding site on the distal heme side. The bound substrate ions were separated from each other by one or more water molecules. The heme iron is coordinated to His-351 N(ϵ2) on the proximal side while it is coordinated to conserved water molecule W-1 on the distal heme side. W-1 is hydrogen bonded to Br(-) ion which is followed by Cl(-) ion with a hydrogen bonded water molecule W-5' between them. Next to Cl(-) ion is a hydrogen bonded water molecule W-7' which in turn is hydrogen bonded to W-8' and N atom of SCN(-). W-80 is hydrogen bonded to W-9' which is hydrogen bonded to I(-). SCN(-) ion also interacts directly with Asn-230 and through water molecules with Ser-235 and Phe-254. Therefore, according to the locations of four substrate anions, the order of preference for binding to lactoperoxidase is observed as Br(-) > Cl(-) > SCN(-) > I(-). The positions of anions are further defined in terms of subsites where Br(-) is located in subsite 1, Cl(-) in subsite 2, SCN(-) in subsite 3 and I(-) in subsite 4.

A novel metabolite of antituberculosis therapy demonstrates host activation of isoniazid and formation of the isoniazid-NAD+ adduct

One of the most effective and widely used antituberculosis (anti-TB) drugs is isoniazid (INH), a prodrug activated via oxidation that forms an adduct with NAD(+) to inhibit NADH-dependent targets of Mycobacterium tuberculosis, such as enoyl-acyl carrier protein reductase (InhA). The metabolic by-products and potentially toxic intermediates resulting from INH therapy have been identified through a large body of work. However, an INH-NAD adduct or structures related to this adduct have not been identified in specimens from human TB patients or animal models of TB. Analyses by mass spectrometry of urine collected from TB patients in a study conducted by the NIAID-funded Tuberculosis Research Unit identified 4-isonicotinoylnicotinamide (C(12)H(9)N(3)O(2)) as a novel metabolite of INH therapy. This compound was formed by M. tuberculosis strains in a KatG-dependent manner but could also be produced by mice treated with INH independent of an M. tuberculosis infection. Thus, the 4-isonicotinoylnicotinamide observed in human urine samples is likely derived from the degradation of oxidized INH-NAD adducts and provides direct evidence of host INH activation.

Effects of mono- and disaccharides on the antimicrobial activity of bovine lactoperoxidase system

The effects of mono- and disaccharides on the antimicrobial activity of the lactoperoxidase (LPO) system against Salmonella Enteritidis were investigated. The results clearly reveal that most of the sugars inhibit the antimicrobial activity of the LPO system. The inhibitory potency varies depending on the structure of sugar. L-Fructose and D-allose were strongly inhibitive to the action of the LPO system, while sucrose was the weakest inhibitor. The decreased antimicrobial activity is due to the reduction of LPO catalytic activity by sugar. An inhibitory kinetic study showed the noncompetitive inhibitor. D-Allose and L-fructose yielded strikingly low K(i) values of 0.36 and 0.42 mM, respectively, while the K(i) values of the other sugars ranged from 1.37 to 3.60 mM. Since LPO activity is inhibited by the saccharides, the sugar content in food should be considered when the LPO system is applied to the preservation of food.

Bovine lactoperoxidase - a versatile one- and two-electron catalyst of high structural and thermal stability

Lactoperoxidase (LPO), a member of the peroxidase-cyclooxygenase superfamily, is found in multiple human exocrine secretions and acts as a first line of defense against invading microorganisms by production of antimicrobial oxidants. Because of its ability to efficiently catalyze one- and two-electron oxidation reactions of inorganic and organic compounds, the heme peroxidase is widely used in food biotechnology, cosmetic industry, and diagnostic kits. In order to probe its structural integrity, conformational, and thermal stability, we have undertaken a comprehensive investigation by using complementary biophysical techniques including UV-Vis, circular dichroism and fluorescence spectroscopy as well as differential scanning calorimetry (DSC). The oxidoreductase exhibits a high chemical and thermal stability under oxidizing conditions but is significantly destabilized by addition of DTT. Due to its unique ester bonds between the prosthetic group and the protein as well as six intra-chain disulfides, unfolding of the central compact (-helical core occurs concomitantly with denaturation of the heme cavity. The corresponding enthalpic and entropic contributions to the free enthalpy of unfolding are presented. Together with spectroscopic data they will be discussed with respect to the known structure of bovine LPO and homologous myeloperoxidase as well as to its practical application.

Isonicotinic acid hydrazide conversion to Isonicotinyl-NAD by catalase-peroxidases

Activation of the pro-drug isoniazid (INH) as an anti-tubercular drug in Mycobacterium tuberculosis involves its conversion to isonicotinyl-NAD, a reaction that requires the catalase-peroxidase KatG. This report shows that the reaction proceeds in the absence of KatG at a slow rate in a mixture of INH, NAD(+), Mn(2+), and O(2), and that the inclusion of KatG increases the rate by >7 times. Superoxide, generated by either Mn(2+)- or KatG-catalyzed reduction of O(2), is an essential intermediate in the reaction. Elimination of the peroxidatic process by mutation slows the rate of reaction by 60% revealing that the peroxidatic process enhances, but is not essential for isonicotinyl-NAD formation. The isonicotinyl-NAD(*+) radical is identified as a reaction intermediate, and its reduction by superoxide is proposed. Binding sites for INH and its co-substrate, NAD(+), are identified for the first time in crystal complexes of Burkholderia pseudomallei catalase-peroxidase with INH and NAD(+) grown by co-crystallization. The best defined INH binding sites were identified, one in each subunit, on the opposite side of the protein from the entrance to the heme cavity in a funnel-shaped channel. The NAD(+) binding site is approximately 20 A from the entrance to the heme cavity and involves interactions primarily with the AMP portion of the molecule in agreement with the NMR saturation transfer difference results.

CO binding and ligand discrimination in human myeloperoxidase

Despite the fact that ferrous myeloperoxidase (MPO) can bind both O(2) and NO, its ability to bind CO has been questioned. UV/visible spectroscopy was used to confirm that CO induces small spectral shifts in ferrous MPO, and Fourier transform infrared difference spectroscopy showed definitively that these arose from formation of a heme ferrous-CO compound. Recombination rates after CO photolysis were monitored at 618 and 645 nm as a function of CO concentration and pH. At pH 6.3, k(on) and k(off) were 0.14 mM(-1) x s(-1) and 0.23 s(-1), respectively, yielding an unusually high K(D) of 1.6 mM. This affinity of MPO for CO is 10 times weaker than its affinity for O(2). The observed rate constant for CO binding increased with increasing pH and was governed by a single protonatable group with a pK(a) of 7.8. Fourier transform infrared spectroscopy revealed two different conformations of bound CO with frequencies at 1927 and 1942 cm(-1). Their recombination rate constants were identical, indicative of two forms of bound CO that are in rapid thermal equilibrium rather than two distinct protein populations with different binding sites. The ratio of bound states was pH-dependent (pK(a) approximately 7.4) with the 1927 cm(-1) form favored at high pH. Structural factors that account for the ligand-binding properties of MPO are identified by comparisons with published data on a range of other ligand-binding heme proteins, and support is given to the recent suggestion that the proximal His336 in MPO is in a true imidazolate state.