Mechanism of inhibition of myeloperoxidase by anti-inflammatory drugs

Combining Double-Dose and High-Dose Pulsed Dapsone Combination Therapy for Chronic Lyme Disease/Post-Treatment Lyme Disease Syndrome and Co-Infections, Including Bartonella: A Report of 3 Cases and a Literature Review

Three patients with relapsing and remitting borreliosis, babesiosis, and bartonellosis, despite extended anti-infective therapy, were prescribed double-dose dapsone combination therapy (DDDCT) for 8 weeks, followed by one or several two-week courses of pulsed high-dose dapsone combination therapy (HDDCT). We discuss these patients' cases to illustrate three important variables required for long-term remission. First, diagnosing and treating active co-infections, including Babesia and Bartonella were important. Babesia required rotations of multiple anti-malarial drug combinations and herbal therapies, and Bartonella required one or several 6-day HDDCT pulses to achieve clinical remission. Second, all prior oral, intramuscular (IM), and/or intravenous (IV) antibiotics used for chronic Lyme disease (CLD)/post-treatment Lyme disease syndrome (PTLDS), irrespective of the length of administration, were inferior in efficacy to short-term pulsed biofilm/persister drug combination therapy i.e., dapsone, rifampin, methylene blue, and pyrazinamide, which improved resistant fatigue, pain, headaches, insomnia, and neuropsychiatric symptoms. Lastly, addressing multiple factors on the 16-point multiple systemic infectious disease syndrome (MSIDS) model was important in achieving remission. In conclusion, DDDCT with one or several 6-7-day pulses of HDDCT, while addressing abnormalities on the 16-point MSIDS map, could represent a novel effective clinical and anti-infective strategy in CLD/PTLDS and associated co-infections including Bartonella.

Peroxidasin Inhibition by Phloroglucinol and Other Peroxidase Inhibitors

Human peroxidasin (PXDN) is a ubiquitous peroxidase enzyme expressed in most tissues in the body. PXDN represents an interesting therapeutic target for inhibition, as it plays a role in numerous pathologies, including cardiovascular disease, cancer and fibrosis. Like other peroxidases, PXDN generates hypohalous acids and free radical species, thereby facilitating oxidative modifications of numerous biomolecules. We have studied the inhibition of PXDN halogenation and peroxidase activity by phloroglucinol and 14 other peroxidase inhibitors. Although a number of compounds on their own potently inhibited PXDN halogenation activity, only five were effective in the presence of a peroxidase substrate with IC50 values in the low μM range. Using sequential stopped-flow spectrophotometry, we examined the mechanisms of inhibition for several compounds. Phloroglucinol was the most potent inhibitor with a nanomolar IC50 for purified PXDN and IC50 values of 0.95 μM and 1.6 μM for the inhibition of hypobromous acid (HOBr)-mediated collagen IV cross-linking in a decellularized extracellular matrix and a cell culture model. Other compounds were less effective in these models. Most interestingly, phloroglucinol was identified to irreversibly inhibit PXDN, either by mechanism-based inhibition or tight binding. Our work has highlighted phloroglucinol as a promising lead compound for the design of highly specific PXDN inhibitors and the assays used in this study provide a suitable approach for high-throughput screening of PXDN inhibitors.

Superoxide: The enigmatic chemical chameleon in neutrophil biology

The burst of superoxide produced when neutrophils phagocytose bacteria is the defining biochemical feature of these abundant immune cells. But 50 years since this discovery, the vital role superoxide plays in host defense has yet to be defined. Superoxide is neither bactericidal nor is it just a source of hydrogen peroxide. This simple free radical does, however, have remarkable chemical dexterity. Depending on its environment and reaction partners, superoxide can act as an oxidant, a reductant, a nucleophile, or an enzyme substrate. We outline the evidence that inside phagosomes where neutrophils trap, kill, and digest bacteria, superoxide will react preferentially with the enzyme myeloperoxidase, not the bacterium. By acting as a cofactor, superoxide will sustain hypochlorous acid production by myeloperoxidase. As a substrate, superoxide may give rise to other forms of reactive oxygen. We contend that these interactions hold the key to understanding the precise role superoxide plays in neutrophil biology. State-of-the-art techniques in mass spectrometry, oxidant-specific fluorescent probes, and microscopy focused on individual phagosomes are needed to identify bactericidal mechanisms driven by superoxide. This work will undoubtably lead to fascinating discoveries in host defense and give a richer understanding of superoxide's varied biology.

Shared and Related Molecular Targets and Actions of Salicylic Acid in Plants and Humans

Salicylic acid (SA) is a phenolic compound produced by all plants that has an important role in diverse processes of plant growth and stress responses. SA is also the principal metabolite of aspirin and is responsible for many of the anti-inflammatory, cardioprotective and antitumor activities of aspirin. As a result, the number of identified SA targets in both plants and humans is large and continues to increase. These SA targets include catalases/peroxidases, metabolic enzymes, protein kinases and phosphatases, nucleosomal and ribosomal proteins and regulatory and signaling proteins, which mediate the diverse actions of SA in plants and humans. While some of these SA targets and actions are unique to plants or humans, many others are conserved or share striking similarities in the two types of organisms, which underlie a host of common biological processes that are regulated or impacted by SA. In this review, we compare shared and related SA targets and activities to highlight the common nature of actions by SA as a hormone in plants versus a therapeutic agent in humans. The cross examination of SA targets and activities can help identify new actions of SA and better explain their underlying mechanisms in plants and humans.

A comprehensive insight into the anti-inflammatory properties of dapsone

The 4,4'-diaminodiphenyl sulfone (DDS), also known as dapsone, is traditionally used as a potent anti-bacterial agent in clinical management of leprosy. For decades, dapsone has been among the first-line medications used in multidrug treatment of leprosy recommended by the World Health Organization (WHO). Shortly after dapsone's discovery as an antibiotic in 1937, the dual function of dapsone (anti-microbial and anti-inflammatory) was elucidated. Dapsone exerts its anti-bacterial effects by inhibiting dihydrofolic acid synthesis, leading to inhibition of bacterial growth, while its anti-inflammatory properties are triggered by inhibiting reactive oxygen species (ROS) production, reducing the effect of eosinophil peroxidase on mast cells and downregulating neutrophil-mediated inflammatory responses. Among the leading mechanisms associated with its anti-microbial/anti-protozoal effects, dapsone clearly has multiple antioxidant, anti-inflammatory, and anti-apoptotic functions. In this regard, it has been described in treating a wide variety of inflammatory and infectious skin conditions. Previous reports have explored different molecular targets for dapsone and provided insight into the anti-inflammatory mechanism of dapsone. This article reviews several basic, experimental, and clinical approaches on anti-inflammatory effect of dapsone.

Neutrophil Myeloperoxidase-Mediated N-Demethylation of Quetiapine Leads to N-Desalkylquetiapine, a Pharmacologically Active Cytochrome P450 Metabolite

The atypical antipsychotic drugs, quetiapine and clozapine, are associated with idiosyncratic drug reactions (such as agranulocytosis or neutropenia) that are thought to involve reactive metabolites. Neutrophil myeloperoxidase (MPO) metabolism of quetiapine is not well-studied, but is metabolized by cytochrome P450. Based on structural similarity to clozapine, we hypothesized that quetiapine can be metabolized by MPO and that there is overlap between cytochrome P450 and MPO metabolism of quetiapine. The interaction of quetiapine and clozapine with MPO and MPO chlorination activity was studied using UV-vis spectrophotometry. The metabolites were characterized using liquid chromatography-mass spectrometry (LC-MS), and electron paramagnetic resonance (EPR) spectroscopy was used for detecting drug-catalyzed glutathione oxidation. In the presence of quetiapine, MPO compound II accumulated for about 7.5 min, whereas in the presence of clozapine, MPO compound II was not observed as it was rapidly reduced back to the resting state. Increasing quetiapine concentrations resulted in a decrease in MPO chlorination activity, while the opposite result was found in the case of clozapine. UV-vis spectral studies showed no change when quetiapine was oxidized in the absence and presence of chloride anion (Cl^-, to catalyze chlorination reactions). Significant changes, however, were observed in the same assay with clozapine, where Cl^- appeared to hinder the rate of clozapine metabolism. The MPO-catalyzed hydroxylated and dealkylated metabolites of quetiapine and hydroxylated metabolites of clozapine were observed from the LC-MS analyses, particularly when Cl^- was included in the reaction. In addition, hydroxylated, dealkylated, and a proposed sulfoxide metabolite of quetiapine were also observed in the reaction catalyzed by human microsomes/NADPH. Lastly, compared to quetiapine, clozapine metabolism by MPO/H₂O₂ and glutathione produced more glutathionyl radicals using EPR spin trapping. In conclusion, MPO/H₂O₂/Cl^- was shown to metabolize quetiapine to S-oxidation and P450-like dealkylation products, and quetiapine metabolites were generally less reactive than clozapine.

Automatic Identification of Myeloperoxidase Natural Inhibitors in Plant Extracts

The aim of this study is the development of an automated method for myeloperoxidase activity evaluation and its application in testing the inhibitory action of different plant extracts on the activity of the enzyme. This enzyme has its concentration increased in inflammatory and infectious processes, so it is a possible target to limit these processes. Therefore, an automatic sequential in-jection analysis (SIA) system was optimized and demonstrated that it is possible to obtain results with satisfactory accuracy and precision. With the developed method, plant extracts were studied, as promising candidates for MPO inhibition. In the group of selected plant extracts, IC50 values from 0.029 ± 0.002 mg/mL to 35.4 ± 3.5 mg/mL were obtained. Arbutus unedo L. proved to be the most inhibitory extract for MPO based on its phenolic compound content. The coupling of an automatic SIA method to MPO inhibition assays is a good alternative to other conventional methods, due to its simplicity and speed. This work also supports the pharmacological use of these species that inhibit MPO, and exhibit activity that may be related to the treatment of infection and inflammation.

Propofol inhibits the myeloperoxidase activity by acting as substrate through a redox process

BACKGROUND

Propofol (2,6-diisopropylphenol) is frequently used as intravenous anesthetic agent, especially in its injectable form (Diprivan), to initiate and maintain sedative state during surgery or in intensive care units. Numerous studies have reported the antioxidant and anti-inflammatory effect of propofol. The oxidant enzyme myeloperoxidase (MPO), released from activated neutrophils, plays a key role in host defense. An increase of the circulating MPO concentration has been observed in patients admitted in intensive care unit and presenting a systemic inflammatory response related to septic shock or trauma.

METHODS

This study investigates the immunomodulatory action of propofol and Diprivan as inhibitor of the oxidant activity of MPO. The understanding of the redox action mechanism of propofol and Diprivan on the myeloperoxidase chlorination and peroxidase activities has been refined using the combination of fluorescence and absorption spectroscopies with docking and cyclic voltammetry.

RESULTS

Propofol acts as a reversible MPO inhibitor. The molecule interacts as a reducing substrate in the peroxidase cycle and promotes the accumulation of compound II. At acidic pH (5.5), propofol and Diprivan do not inhibit the chlorination activity, but their action increases at physiological pH (7.4). The main inhibitory action of Diprivan could be attributed to its HOCl scavenging property.

GENERAL SIGNIFICANCE

Propofol can act as a reversible MPO inhibitor at clinical concentrations. This property could, in addition to other previously proven anti-inflammatory actions, induce an immunomodulatory action, beneficial during clinical use, particularly in the treatment of systemic inflammation response syndrome.

Limitations of drug concentrations used in cell culture studies for understanding clinical responses of NSAIDs

In this review, the in vitro cellular effects of six nonsteroidal anti-inflammatory drugs (NSAIDs), salicylate, ibuprofen, naproxen, indomethacin, celecoxib and diclofenac, are examined. Inhibition of prostanoid synthesis in vitro generally occurs within the therapeutic range of plasma concentrations that are observed in vivo, consistent with the major action of NSAIDs being inhibition of prostanoid production. An additional probable cellular action of NSAIDs has been discovered recently, viz. decreased oxidation of the endocannabinoids, 2-arachidonoyl glycerol and arachidonyl ethanolamide. Many effects of NSAIDs, other than decreased oxidation of arachidonic acid and endocannabinoids, have been put forward but almost all of these additional processes are observed at supratherapeutic concentrations when the concentration of albumin, the major protein that binds NSAIDs, is taken into account. However, one exception is salicylate, a very potent inhibitor of the neutrophilic enzyme, myeloperoxidase, the inhibition of which leads to reduced production of the inflammatory mediator, hypochlorous acid, and inhibition of the inflammation associated with rheumatoid arthritis.

Dapsone Ameliorates Colitis through TLR4/NF-kB Pathway in TNBS Induced Colitis Model in Rat

BACKGROUND

Crohn's disease (CD), a type of inflammatory bowel disease (IBD), emerges with severe gastrointestinal (GI) tract inflammation, sometimes known as hostile abdomen. Conventional treatment of CD has several limitations such as insufficient response to treatment, and intolerable side effects of drugs. In addition, the high cost of biologic drugs prevents patients from continuing their treatment. Dapsone showed vigorous anti-inflammatory effects on the skin diseases, lung diseases and inflammatory diseases of the nervous system. Hence, we decided to investigate the effect of dapsone on animal model of CD.

METHODS

In this study, colitis was induced by instillation of 2,4,6-trinitrobenzenesulfonic acid (TNBS) 100 mg/kg. Rats were treated with daily gavage of dapsone (10, 12.5 and 20 mg/kg). Seven days after induction of colitis, specimens were collected for pathological and molecular assessments.

RESULTS

Dapsone (12.5 and 20 mg/kg) preserved the histologic architecture of the colon and prevented crypts irregularity. Additionally, it decreased tissue edema and hindered inflammatory cells infiltration. Besides, all doses of dapsone decreased tissue concentration of tumor necrosis factor α (TNF-α) and interferon γ (INFγ). Western blot revealed that dapsone could attenuate inflammation via downregulation of toll-like receptor 4 (TLR4) and dephosphorylation of nuclear factor kB (NF-kB).

CONCLUSION

Based on these findings, dapsone attenuates inflammation and decreases TNF-α and INF-γ in animal model of CD. It acts through TLR4/NF-kB pathway to exert these effects.

Chalcones and their B-aryl analogues as myeloperoxidase inhibitors: In silico, in vitro and ex vivo investigations

In the present study, a series of chalcones and their B-aryl analogues were prepared and evaluate as inhibitors of myeloperoxidase (MPO) chlorinating activity, using in vitro and ex vivo assays. Among these, B-thiophenyl chalcone (analogue 9) demonstrated inhibition of in vitro and ex vivo MPO chlorinating activity, exhibiting IC₅₀ value of 0.53 and 19.2 µM, respectively. Potent ex vivo MPO inhibitors 5, 8 and 9 were not toxic to human neutrophils at 50 µM, as well as displayed weak 2,2-diphenyl-1-pycrylhydrazyl radical (DPPH•) and hypochlorous acid (HOCl) scavenger abilities. Docking simulations indicated binding mode of MPO inhibitors, evidencing hydrogen bonds between the amino group at 4'position (ring A) of chalcones with Gln91, Asp94, and Hys95 MPO residues. In this regard, the efficacy and low toxicity promoted aminochalcones and arylic analogues to the rank of hit compounds in the search for new non-steroidal anti-inflammatory compounds.

Integration of an Inhibitor-like Rule and Structure-based Virtual Screening for the Discovery of Novel Myeloperoxidase Inhibitors

Myeloperoxidase (MPO) is an attractive therapeutic target against inflammation. Herein, we developed an inhibitor-like rule, based on known MPO inhibitors, and generated a target database containing 6546 molecules with privileged inhibitory properties. Using a structure-based approach validated by decoys, robust statistical metrics, redocking, and cross-docking, we selected 10 putative MPO inhibitors with high chemical diversity. At 20 μM, six of these 10 compounds (i.e., 60% success rate) inhibited more than 20% of the chlorinating activity of the enzyme. Additionally, we found that compound ZINC9089086 forms hydrogen bonds with Arg233 and with the hemic carboxylate. It makes a π-stacking interaction with the heme group and displays a high affinity for the enzyme active site. When incubated with purified MPO, ZINC9089086 inhibited the chlorinating activity of the enzyme with an IC₅₀ of 2.2 ± 0.1 μM in a reversible manner. Subsequent experiments revealed that ZINC9089086 inhibited hypochlorous acid production in dHL-60 cells and human neutrophils. Furthermore, the theoretical ADME/Tox profile indicated that this compound exhibits low toxicity risks and adequate pharmacokinetic parameters, thus making ZINC9089086 a very promising candidate for preclinical anti-inflammatory studies. Overall, our study shows that integrating an inhibitor-like rule with a validated structure-based methodology is an excellent approach for improving the success rate and molecular diversity of novel MPO inhibitors with good pharmacokinetics and toxicological profiles. By combining these tools, it was possible to increase the assurance rate, which ultimately diminishes the costs and time needed for the acquisition, synthesis, and evaluation of new compounds.

The Dual Role of Myeloperoxidase in Immune Response

The heme protein myeloperoxidase (MPO) is a major constituent of neutrophils. As a key mediator of the innate immune system, neutrophils are rapidly recruited to inflammatory sites, where they recognize, phagocytose, and inactivate foreign microorganisms. In the newly formed phagosomes, MPO is involved in the creation and maintenance of an alkaline milieu, which is optimal in combatting microbes. Myeloperoxidase is also a key component in neutrophil extracellular traps. These helpful properties are contrasted by the release of MPO and other neutrophil constituents from necrotic cells or as a result of frustrated phagocytosis. Although MPO is inactivated by the plasma protein ceruloplasmin, it can interact with negatively charged components of serum and the extracellular matrix. In cardiovascular diseases and many other disease scenarios, active MPO and MPO-modified targets are present in atherosclerotic lesions and other disease-specific locations. This implies an involvement of neutrophils, MPO, and other neutrophil products in pathogenesis mechanisms. This review critically reflects on the beneficial and harmful functions of MPO against the background of immune response.

Discovery of a novel indole pharmacophore for the irreversible inhibition of myeloperoxidase (MPO)

Myeloperoxidase (MPO) activity and subsequent generation of hypochlorous acid has been associated with the killing of host-invading microorganisms (e.g. bacteria, viruses, and fungi). However, during oxidative stress, high MPO activity can damage host tissue and is linked to several chronic inflammatory conditions. Herein, we describe the development of a novel biaryl, indole-pyrazole series of irreversible mechanism-based inhibitors of MPO. Derived from an indole-containing high-throughput screen hit, optimization efforts resulted in potent and selective 6-substituted indoles with good oral bioavailability and in vivo activity.

Treatment of granuloma annulare and related granulomatous diseases with sulphasalazine: a series of 16 cases

BACKGROUND

Granuloma annulare (GA) and the related annular elastolytic giant cell granuloma (AEGCG) and interstitial granulomatous dermatitis (IGD) are idiopathic histiocytic inflammatory disorders, which are frequently recalcitrant to treatment.

OBJECTIVES

Evaluate the efficacy of sulphasalazine in treating GA, AEGCG and IGD.

METHODS

Sixteen patients were identified with granulomatous disease who were treated with sulphasalazine between September 2015 and September 2019. Outcomes were based on patients' and providers' subjective evaluations.

RESULTS

Sixteen patients were included in the study (ages 56-89, four male and twelve female). Previous treatments were attempted in fifteen patients. Clinical improvement was seen in fourteen patients (87.5%). Initial improvement was noted within a mean (SD) of 66.4 (35.1) days after starting therapy, with increasing benefits over time. Ten patients (62.5%) reported complete or near-complete clearance, three patients (18.8%) reported significant improvement, and one (6.3%) reported partial improvement. Twelve patients elected to stop or reduce therapy, resulting in relapse or worsening in five patients.

CONCLUSIONS

Sulphasalazine may be considered as treatment for GA and GA-related conditions.

Methoxyphenol derivatives as reversible inhibitors of myeloperoxidase as potential antiatherosclerotic agents

Aim: To evaluate new chemical entities, based on ferulic acid scaffolds, as reversible myeloperoxidase inhibitors (MPOI). Methodology & results:In silico docking studies are performed with MPO protein as a target for several ferulic acid analogs followed by multiple in vitro assays to validate this approach. Two lead compounds 2a and 3 are identified with optimum docking and IC50 values: -7.95 kcal/mol, 0.9 μM and -8.35 kcal/mol, 8.5 μM, respectively. These MPOIs are able to inhibit oxidation of high-density lipoprotein and further promoted functionality of high-density lipoprotein. Conclusion: Lead analogs are potent MPOIs that exert specific effects on MPO-mediated oxidation as well as inflammatory pathways. It also acts as promoters of cholesterol efflux that sheds light on pharmacological approach in atherosclerosis treatment.

Dapsone modulates lipopolysaccharide-activated bone marrow cells by inducing cell death and down-regulating tumor necrosis factor-α production

Dapsone, an antibiotic, has been used to cure leprosy. It has been reported that dapsone has anti-inflammatory activity in hosts; however, the anti-inflammatory mechanism of dapsone has not been fully elucidated. The present study investigated the anti-inflammatory effects of dapsone on bone marrow cells (BMs), especially upon exposure to lipopolysaccharide (LPS). We treated BMs with LPS and dapsone, and the treated cells underwent cellular activity assay, flow cytometry analysis, cytokine production assessment, and reactive oxygen species assay. LPS distinctly activated BMs with several characteristics including high cellular activity, granulocyte changes, and tumor necrosis factor alpha (TNF-α) production increases. Interestingly, dapsone modulated the inflammatory cells, including granulocytes in LPS-treated BMs, by inducing cell death. While the percentage of Gr-1 positive cells was 57% in control cells, LPS increased that to 75%, and LPS plus dapsone decreased it to 64%. Furthermore, dapsone decreased the mitochondrial membrane potential of LPS-treated BMs. At a low concentration (25 µg/mL), dapsone significantly decreased the production of TNF-α in LPS-treated BMs by 54%. This study confirmed that dapsone has anti-inflammatory effects on LPS-mediated inflammation via modulation of the number and function of inflammatory cells, providing new and useful information for clinicians and researchers.

Interactions of staphyloxanthin and enterobactin with myeloperoxidase and reactive chlorine species

When neutrophils engulf bacteria, myeloperoxidase converts hydrogen peroxide to hypochlorous acid, which is toxic to all micro-organisms. It has been suggested that some pathogens have virulence factors that target myeloperoxidase to dampen the oxidative reactions of neutrophils. These virulence factors include staphyloxanthin, the golden pigment of Staphylococcus aureus, and enterobactin - a siderophore released by gram-negative bacteria. We investigated the potential of staphyloxanthin and enterobactin to shield bacteria from hypochlorous acid and related chloramines. Clinical strains of S. aureus with high levels of staphyloxanthin and related carotenoids were in general more resistant to low doses of hypochlorous acid than non-pigmented bacteria. But some non-pigmented strains were also resistant to the oxidant. Doses of reactive chlorine species that killed bacteria also bleached their carotenoids. Hypochlorous acid, NH₂Cl, and NHCl₂ bleached purified staphyloxanthin. When S. aureus were phagocytosed by neutrophils there was no discernible loss of staphyloxanthin. These data suggest that staphyloxanthin is capable of protecting bacteria from low doses of reactive chlorine species formed inside phagosomes. Enterobactin was not an inhibitor of myeloperoxidase. We conclude that staphyloxanthin may protect some bacterial strains against oxidative killing by neutrophils, but enterobactin will not inhibit the production of hypochlorous acid.

Redox Regulation of Inflammatory Processes Is Enzymatically Controlled

Redox regulation depends on the enzymatically controlled production and decay of redox active molecules. NADPH oxidases, superoxide dismutases, nitric oxide synthases, and others produce the redox active molecules superoxide, hydrogen peroxide, nitric oxide, and hydrogen sulfide. These react with target proteins inducing spatiotemporal modifications of cysteine residues within different signaling cascades. Thioredoxin family proteins are key regulators of the redox state of proteins. They regulate the formation and removal of oxidative modifications by specific thiol reduction and oxidation. All of these redox enzymes affect inflammatory processes and the innate and adaptive immune response. Interestingly, this regulation involves different mechanisms in different biological compartments and specialized cell types. The localization and activity of distinct proteins including, for instance, the transcription factor NFκB and the immune mediator HMGB1 are redox-regulated. The transmembrane protein ADAM17 releases proinflammatory mediators, such as TNFα, and is itself regulated by a thiol switch. Moreover, extracellular redox enzymes were shown to modulate the activity and migration behavior of various types of immune cells by acting as cytokines and/or chemokines. Within this review article, we will address the concept of redox signaling and the functions of both redox enzymes and redox active molecules in innate and adaptive immune responses.

Characterization of chemical features of potent myeloperoxidase inhibitors

BACKGROUND

Despite its important role in the immune system, myeloperoxidase (MPO) is implicated in a wide range of inflammatory syndromes due to its oxidative product HOCl. The oxidative damages caused by MPO make it a new target for developing promising anti-inflammatory agents. In this paper, we tried to understand the mechanism of MPO inhibition in order to facilitate the drug design, to develop more accurate virtual tests and to understand the structure-activity relationship.

RESULTS

Based on docking experiments, kinetic studies and in vitro tests, it is determined that a potent MPO inhibitor must possess an oxidizable group in addition to a high affinity with the active site. At last, a new hit was found in this work namely 4-(3-hydroxy-phenoxy)-butylamine (5) that has IC50 of 86 nM.

CONCLUSION

Hydroxy-phenoxy alkylamine derivatives were found to be promising MPO inhibitors and they may represent an important starting point in the development of more potent MPO inhibitors.

Long-Term Effects of (-)-Epigallocatechin Gallate (EGCG) on Pristane-Induced Arthritis (PIA) in Female Dark Agouti Rats

Rheumatoid arthritis (RA)--a widespread chronic inflammatory disease in industrialized countries--is characterized by a persistent and progressive joint destruction. The chronic pro-inflammatory state results from a mutual activation of the innate and the adaptive immune system, while the exact pathogenesis mechanism is still under discussion. New data suggest a role of the innate immune system and especially polymorphonuclear granulocytes (PMNs, neutrophils) not only during onset and the destructive phase of RA but also at the chronification of the disease. Thereby the enzymatic activity of myeloperoxidase (MPO), a peroxidase strongly abundant in neutrophils, may be important: While its peroxidase activity is known to contribute to cartilage destruction at later stages of RA the almost MPO-specific oxidant hypochlorous acid (HOCl) is also discussed for certain anti-inflammatory effects. In this study we used pristane-induced arthritis (PIA) in Dark Agouti rats as a model for the chronic course of RA in man. We were able to shown that a specific detection of the HOCl-producing MPO activity provides a sensitive new marker to evaluate the actual systemic inflammatory status which is only partially detectable by the evaluation of clinical symptoms (joint swelling and redness measurements). Moreover, we evaluated the long-term pharmacological effect of the well-known anti-inflammatory flavonoid epigallocatechin gallate (EGCG). Thereby only upon early and continuous oral application of this polyphenol the arthritic symptoms were considerably diminished both in the acute and in the chronic phase of the disease. The obtained results were comparable to the treatment control (application of methotrexate, MTX). As revealed by stopped-flow kinetic measurements, EGCG may regenerate the HOCl-production of MPO which is known to be impaired at chronic inflammatory diseases like RA. It can be speculated that this MPO activity-promoting effect of EGCG may contribute to the pharmacological mode of action of this polyphenol.

ESR and X-ray Structure Investigations on the Binding and Mechanism of Inhibition of the Native State of Myeloperoxidase with Low Molecular Weight Fragments

As an early visitor to the injured loci, neutrophil-derived human Myeloperoxidase (hMPO) offers an attractive protein target to modulate the inflammation of the host tissue through suitable inhibitors. We describe a novel methodology of using low temperature ESR spectroscopy (6 K) and FAST™ technology to screen a diverse series of small molecules that inhibit the peroxidase function through reversible binding to the native state of MPO. Our initial efforts to profile molecules on the inhibition of MPO-initiated nitration of the Apo-A1 peptide (AEYHAKATEHL) assay showed several potent (with sub-micro molar IC50s) but spurious inhibitors that either do not bind to the heme pocket in the enzyme or retain high (>50 %) anti oxidant potential. Such molecules when taken forward for X-ray did not yield inhibitor-bound co-crystals. We then used ESR to confirm direct binding to the native state enzyme, by measuring the binding-induced shift in the electronic parameter g to rank order the molecules. Molecules with a higher rank order-those with g-shift Rrelative ≥15-yielded well-formed protein-bound crystals (n = 33 structures). The co-crystal structure with the LSN217331 inhibitor reveals that the chlorophenyl group projects away from the heme along the edges of the Phe366 and Phe407 side chain phenyl rings thereby sterically restricting the access to the heme by the substrates like H2O2. Both ESR and antioxidant screens were used to derive the mechanism of action (reversibility, competitive substrate inhibition, and percent antioxidant potential). In conclusion, our results point to a viable path forward to target the native state of MPO to tame local inflammation.

In vitro activation of dibromoacetonitrile to cyanide by myeloperoxidase

Dibromoacetonitrile (DBAN) is a disinfection by-product classified as a potential human and animal carcinogen. This study aimed at investigating the ability of myeloperoxidase (MPO) to oxidize DBAN to cyanide (CN−) in vitro. Detection of CN− served as a marker for the possible generation of free radical intermediates implicated in DBAN-induced toxicity. Optimum conditions for the oxidation of DBAN to CN− were characterized with respect to pH, temperature, and time of incubation as well as DBAN, MPO, potassium chloride, and hydrogen peroxide (H2O2) concentrations in incubation mixtures. Maximum reaction velocity and Michaelis–Menten constant were assessed. Addition of sodium hypochlorite to the reaction mixtures significantly enhanced the rate of the reaction. Addition of the MPO inhibitors, sodium azide, 4-amino benzoic acid hydrazine, or indomethacin to the reaction mixtures significantly decreased the rate of DBAN oxidation. Inclusion of the antioxidant enzyme superoxide dismutase in the incubation mixtures significantly decreased the rate of reaction. Inclusion of the sulfhydryl compounds as reduced glutathione, N-acetylcysteine, d-penicillamine, or l-cysteine enhanced the rate of DBAN oxidation. These results demonstrate the ability of MPO/H2O2/chloride ion system to oxidize DBAN to CN− and provide insight for the elucidation of DBAN chronic toxicity.

Posttranslational protein modifications by reactive nitrogen and chlorine species and strategies for their prevention and elimination

Proteins are subject to various posttranslational modifications, some of them being undesired from the point of view of metabolic efficiency. Prevention of such modifications is expected to provide new means of therapy of diseases and decelerate the process of aging. In this review, modifications of proteins by reactive nitrogen species and reactive halogen species, is briefly presented and means of prevention of these modifications and their sequelae are discussed, including the denitrase activity and inhibitors of myeloperoxidase.

Inhibition of myeloperoxidase- and neutrophil-mediated oxidant production by tetraethyl and tetramethyl nitroxides

The powerful oxidant HOCl (hypochlorous acid and its corresponding anion, (-)OCl) generated by the myeloperoxidase (MPO)-H2O2-Cl(-) system of activated leukocytes is strongly associated with multiple human inflammatory diseases; consequently there is considerable interest in inhibition of this enzyme. Nitroxides are established antioxidants of low toxicity that can attenuate oxidation in animal models, with this ascribed to superoxide dismutase or radical-scavenging activities. We have shown (M.D. Rees et al., Biochem. J. 421, 79-86, 2009) that nitroxides, including 4-amino-TEMPO (4-amino-2,2,6,6-tetramethylpiperidin-1-yloxyl radical), are potent inhibitors of HOCl formation by isolated MPO and activated neutrophils, with IC50 values of ~1 and ~6 µM respectively. The utility of tetramethyl-substituted nitroxides is, however, limited by their rapid reduction by biological reductants. The corresponding tetraethyl-substituted nitroxides have, however, been reported to be less susceptible to reduction. In this study we show that the tetraethyl species were reduced less rapidly than the tetramethyl species by both human plasma (89-99% decreased rate of reduction) and activated human neutrophils (62-75% decreased rate). The tetraethyl-substituted nitroxides retained their ability to inhibit HOCl production by MPO and activated neutrophils with IC50 values in the low-micromolar range; in some cases inhibition was enhanced compared to tetramethyl substitution. Nitroxides with rigid structures (fused oxaspiro rings) were, however, inactive. Overall, these data indicate that tetraethyl-substituted nitroxides are potent inhibitors of oxidant formation by MPO, with longer plasma and cellular half-lives compared to the tetramethyl species, potentially allowing lower doses to be employed.

Characterization and antioxidant properties of six Algerian propolis extracts: ethyl acetate extracts inhibit myeloperoxidase activity

Because propolis contains many types of antioxidant compounds such as polyphenols and flavonoids, it can be useful in preventing oxidative damages. Ethyl acetate extracts of propolis from several Algerian regions show high activity by scavenging free radicals, preventing lipid peroxidation and inhibiting myeloperoxidase (MPO). By fractioning and assaying ethyl acetate extracts, it was observed that both polyphenols and flavonoids contribute to these activities. A correlation was observed between the polyphenol content and the MPO inhibition. However, it seems that kaempferol, a flavonoid, contributes mainly to the MPO inhibition. This molecule is in a high amount in the ethyl acetate extract and demonstrates the best efficiency towards the enzyme with an inhibiting concentration at 50% of 4 ± 2 µM.

Flavonoids and 5-aminosalicylic acid inhibit the formation of neutrophil extracellular traps

Neutrophil extracellular traps (NETs) have been suggested to play a pathophysiological role in several autoimmune diseases. Since NET-formation in response to several biological and chemical stimuli is mostly ROS dependent, in theory any substance that inhibits or scavenges ROS could prevent ROS-dependent NET release. Therefore, in the present comprehensive study, several antioxidative substances were assessed for their capacity to inhibit NET formation of primary human neutrophils in vitro. We could show that the flavonoids (-)-epicatechin, (+)-catechin hydrate, and rutin trihydrate as well as vitamin C and the pharmacological substances N-acetyl-L-cysteine and 5-aminosalicylic acid inhibited PMA induced ROS production and NET formation. Therefore, a broad spectrum of antioxidative substances that reduce ROS production of primary human neutrophils also inhibits ROS-dependent NET formation. It is tempting to speculate that such antioxidants can have beneficial therapeutic effects in diseases associated with ROS-dependent NET formation.

Dapsone in dermatology and beyond

Dapsone (4,4′-diaminodiphenylsulfone) is an aniline derivative belonging to the group of synthetic sulfones. In 1937 against the background of sulfonamide era the microbial activity of dapsone has been discovered. Shortly thereafter, the use of dapsone to treat non-pathogen-caused diseases revealed alternate antiinflammatory mechanisms that initially were elucidated by inflammatory animal models. Thus, dapsone clearly has dual functions of both: antimicrobial/antiprotozoal effects and anti-inflammatory features similarly to non-steroidal anti-inflammatory drugs. The latter capabilities primarily were used in treating chronic inflammatory disorders. Dapsone has been investigated predominantly by in vitro methods aiming to get more insights into the effect of dapsone to inflammatory effector cells, cytokines, and/or mediators, such as cellular toxic oxygen metabolism, myoloperoxidase-/halogenid system, adhesion molecules, chemotaxis, membrane-associated phospholipids, prostaglandins, leukotrienes, interleukin-8, tumor necrosis factor α, lymphocyte functions, and tumor growth. Moreover, attention has been paid to mechanisms by which dapsone mediates effects in more complex settings like impact of lifespan, stroke, glioblastoma, or as anticonvulsive agent. Additionally, there are some dermatological investigations in human being using dapsone and its metabolites (e.g., leukotriene B₄-induced chemotaxis, ultraviolet-induced erythema). It could be established that dapsone metabolites by their own have anti-inflammatory properties. Pharmacology and mechanisms of action are determining factors for clinical use of dapsone chiefly in neutrophilic and/or eosinophilic dermatoses and in chronic disorders outside the field of dermatology. The steroid-sparing effect of dapsone is useful for numerous clinical entities. Future avenues of investigations will provide more information on this fascinating and essential agent.

Potent reversible inhibition of myeloperoxidase by aromatic hydroxamates

The neutrophil enzyme myeloperoxidase (MPO) promotes oxidative stress in numerous inflammatory pathologies by producing hypohalous acids. Its inadvertent activity is a prime target for pharmacological control. Previously, salicylhydroxamic acid was reported to be a weak reversible inhibitor of MPO. We aimed to identify related hydroxamates that are good inhibitors of the enzyme. We report on three hydroxamates as the first potent reversible inhibitors of MPO. The chlorination activity of purified MPO was inhibited by 50% by a 5 nm concentration of a trifluoromethyl-substituted aromatic hydroxamate, HX1. The hydroxamates were specific for MPO in neutrophils and more potent toward MPO compared with a broad range of redox enzymes and alternative targets. Surface plasmon resonance measurements showed that the strength of binding of hydroxamates to MPO correlated with the degree of enzyme inhibition. The crystal structure of MPO-HX1 revealed that the inhibitor was bound within the active site cavity above the heme and blocked the substrate channel. HX1 was a mixed-type inhibitor of the halogenation activity of MPO with respect to both hydrogen peroxide and halide. Spectral analyses demonstrated that hydroxamates can act variably as substrates for MPO and convert the enzyme to a nitrosyl ferrous intermediate. This property was unrelated to their ability to inhibit MPO. We propose that aromatic hydroxamates bind tightly to the active site of MPO and prevent it from producing hypohalous acids. This mode of reversible inhibition has potential for blocking the activity of MPO and limiting oxidative stress during inflammation.

N-acetyl lysyltyrosylcysteine amide inhibits myeloperoxidase, a novel tripeptide inhibitor

Myeloperoxidase (MPO) plays important roles in disease by increasing oxidative and nitrosative stress and oxidizing lipoproteins. Here we report N-acetyl lysyltyrosylcysteine amide (KYC) is an effective inhibitor of MPO activity. We show KYC inhibits MPO-mediated hypochlorous acid (HOCl) formation and nitration/oxidation of LDL. Disulfide is the major product of MPO-mediated KYC oxidation. KYC (≤4,000 μM) does not induce cytotoxicity in bovine aortic endothelial cells (BAECs). KYC inhibits HOCl generation by phorbol myristate acetate (PMA)-stimulated neutrophils and human promyelocytic leukemia (HL-60) cells but not superoxide generation by PMA-stimulated HL-60 cells. KYC inhibits MPO-mediated HOCl formation in BAEC culture and protects BAECs from MPO-induced injury. KYC inhibits MPO-mediated lipid peroxidation of LDL whereas tyrosine (Tyr) and tryptophan (Trp) enhance oxidation. KYC is unique as its isomers do not inhibit MPO activity, or are much less effective. Ultraviolet-visible spectral studies indicate KYC binds to the active site of MPO and reacts with compounds I and II. Docking studies show the Tyr of KYC rests just above the heme of MPO. Interestingly, KYC increases MPO-dependent H₂O₂ consumption. These data indicate KYC is a novel and specific inhibitor of MPO activity that is nontoxic to endothelial cell cultures. Accordingly, KYC may be useful for treating MPO-mediated vascular disease.

Measuring chlorine bleach in biology and medicine

BACKGROUND

Chlorine bleach, or hypochlorous acid, is the most reactive two-electron oxidant produced in appreciable amounts in our bodies. Neutrophils are the main source of hypochlorous acid. These champions of the innate immune system use it to fight infection but also direct it against host tissue in inflammatory diseases. Neutrophils contain a rich supply of the enzyme myeloperoxidase. It uses hydrogen peroxide to convert chloride to hypochlorous acid.

SCOPE OF REVIEW

We give a critical appraisal of the best methods to measure production of hypochlorous acid by purified peroxidases and isolated neutrophils. Robust ways of detecting it inside neutrophil phagosomes where bacteria are killed are also discussed. Special attention is focused on reaction-based fluorescent probes but their visual charm is tempered by stressing their current limitations. Finally, the strengths and weaknesses of biomarker assays that capture the footprints of chlorine in various pathologies are evaluated.

MAJOR CONCLUSIONS

Detection of hypochlorous acid by purified peroxidases and isolated neutrophils is best achieved by measuring accumulation of taurine chloramine. Formation of hypochlorous acid inside neutrophil phagosomes can be tracked using mass spectrometric analysis of 3-chlorotyrosine and methionine sulfoxide in bacterial proteins, or detection of chlorinated fluorescein on ingestible particles. Reaction-based fluorescent probes can also be used to monitor hypochlorous acid during phagocytosis. Specific biomarkers of its formation during inflammation include 3-chlorotyrosine, chlorinated products of plasmalogens, and glutathione sulfonamide.

GENERAL SIGNIFICANCE

These methods should bring new insights into how chlorine bleach is produced by peroxidases, reacts within phagosomes to kill bacteria, and contributes to inflammation. This article is part of a Special Issue entitled Current methods to study reactive oxygen species - pros and cons and biophysics of membrane proteins. Guest Editor: Christine Winterbourn.

Ceruloplasmin is an endogenous inhibitor of myeloperoxidase

Myeloperoxidase is a neutrophil enzyme that promotes oxidative stress in numerous inflammatory pathologies. It uses hydrogen peroxide to catalyze the production of strong oxidants including chlorine bleach and free radicals. A physiological defense against the inappropriate action of this enzyme has yet to be identified. We found that myeloperoxidase oxidized 75% of the ascorbate in plasma from ceruloplasmin knock-out mice, but there was no significant loss in plasma from wild type animals. When myeloperoxidase was added to human plasma it became bound to other proteins and was reversibly inhibited. Ceruloplasmin was the predominant protein associated with myeloperoxidase. When the purified proteins were mixed, they became strongly but reversibly associated. Ceruloplasmin was a potent inhibitor of purified myeloperoxidase, inhibiting production of hypochlorous acid by 50% at 25 nm. Ceruloplasmin rapidly reduced Compound I, the Fe(V) redox intermediate of myeloperoxidase, to Compound II, which has Fe(IV) in its heme prosthetic groups. It also prevented the fast reduction of Compound II by tyrosine. In the presence of chloride and hydrogen peroxide, ceruloplasmin converted myeloperoxidase to Compound II and slowed its conversion back to the ferric enzyme. Collectively, our results indicate that ceruloplasmin inhibits myeloperoxidase by reducing Compound I and then trapping the enzyme as inactive Compound II. We propose that ceruloplasmin should provide a protective shield against inadvertent oxidant production by myeloperoxidase during inflammation.

Formation of reactive sulfite-derived free radicals by the activation of human neutrophils: an ESR study

The objective of this study was to determine the effect of (bi)sulfite (hydrated sulfur dioxide) on human neutrophils and the ability of these immune cells to produce reactive free radicals due to (bi)sulfite oxidation. Myeloperoxidase (MPO) is an abundant heme protein in neutrophils that catalyzes the formation of cytotoxic oxidants implicated in asthma and inflammatory disorders. In this study sulfite ((•)SO(3)(-)) and sulfate (SO(4)(•-)) anion radicals are characterized with the ESR spin-trapping technique using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) in the reaction of (bi)sulfite oxidation by human MPO and human neutrophils via sulfite radical chain reaction chemistry. After treatment with (bi)sulfite, phorbol 12-myristate 13-acetate-stimulated neutrophils produced DMPO-sulfite anion radical, -superoxide, and -hydroxyl radical adducts. The last adduct probably resulted, in part, from the conversion of DMPO-sulfate to DMPO-hydroxyl radical adduct via a nucleophilic substitution reaction of the radical adduct. This anion radical (SO(4)(•-)) is highly reactive and, presumably, can oxidize target proteins to protein radicals, thereby initiating protein oxidation. Therefore, we propose that the potential toxicity of (bi)sulfite during pulmonary inflammation or lung-associated diseases such as asthma may be related to free radical formation.

The impact of various reactive oxygen species on the formation of neutrophil extracellular traps

The formation of neutrophil extracellular traps (NETs) depends on the generation of reactive oxygen species (ROS). Previous studies revealed that both NADPH oxidase and myeloperoxidase (MPO) are required for NET release. However, the contribution of various ROS as well as the role of mitochondria-derived ROS has not been addressed so far. In the present study we aimed to investigate in a systematic and comprehensive manner the contribution of various ROS and ROS-generating pathways to the PMA-induced NET release. By using specific inhibitors, the role of both NADPH oxidase- and mitochondria-derived ROS as well as the contribution of superoxide dismutase (SOD) and MPO on the NET release was assessed. We could demonstrate that NADPH oxidase function is crucial for the formation of NETs. In addition, we could clearly show the involvement of MPO-derived ROS in NET release. Our results, however, did not provide evidence for the role of SOD- or mitochondria-derived ROS in NET formation.

2-thioxanthines are mechanism-based inactivators of myeloperoxidase that block oxidative stress during inflammation

Myeloperoxidase (MPO) is a prime candidate for promoting oxidative stress during inflammation. This abundant enzyme of neutrophils uses hydrogen peroxide to oxidize chloride to highly reactive and toxic chlorine bleach. We have identified 2-thioxanthines as potent mechanism-based inactivators of MPO. Mass spectrometry and x-ray crystal structures revealed that these inhibitors become covalently attached to the heme prosthetic groups of the enzyme. We propose a mechanism whereby 2-thioxanthines are oxidized, and their incipient free radicals react with the heme groups of the enzyme before they can exit the active site. 2-Thioxanthines inhibited MPO in plasma and decreased protein chlorination in a mouse model of peritonitis. They slowed but did not prevent neutrophils from killing bacteria and were poor inhibitors of thyroid peroxidase. Our study shows that MPO is susceptible to the free radicals it generates, and this Achilles' heel of the enzyme can be exploited to block oxidative stress during inflammation.

Scavenging of reactive oxygen and nitrogen species by the prodrug sulfasalazine and its metabolites 5-aminosalicylic acid and sulfapyridine

Sulfasalazine is a prodrug composed by a molecule of 5-aminosalicylic acid (5-ASA) and sulfapyridine (SP), linked by an azo bond, which has been shown to be effective in the therapy of inflammatory bowel diseases (IBD) such as ulcerative colitis and Crohn's disease, as well as of rheumatic diseases, such as rheumatoid arthritis and ankylosing spondylitis. The precise mechanism of action of sulfasalazine and/or its metabolites has not been completely elucidated, though its antioxidant effects are well established and are probably due to its scavenging effects against reactive oxygen and nitrogen species (ROS and RNS), as well as metal chelating properties, in association to its inhibitory effects over neutrophil oxidative burst. The present work was focused on screening and comparing the potential scavenging activity for an array of ROS (O(2)(•-), H(2)O(2), (1)O(2), ROO(•) and HOCl) and RNS ((•)NO and ONOO(-)), mediated by sulfasalazine and its metabolites 5-ASA and SP, using validated in vitro screening systems. The results showed that both 5-ASA and sulfasalazine were able to scavenge all the tested ROS while SP was practically ineffective in all the assays. For HOCl, (1)O(2), and ROO(•), 5-ASA showed the best scavenging effects. A new and important finding of the present study was the strong scavenging effect of 5-ASA against (1)O(2). 5-ASA was shown to be a strong scavenger of (•)NO and ONOO(-). Sulfasalazine was also able to scavenge these RNS, although with a much lower potency than 5-ASA. SP was unable to scavenge (•)NO in the tested concentrations but was shown to scavenge ONOO(-), with a higher strength when the assay was performed in the presence of 25 mM bicarbonate, suggesting further scavenging of oxidizing carbonate radical. In conclusion, the ROS- and RNS-scavenging effects of sulfasalazine and its metabolites shown in this study may contribute to the anti-inflammatory effects mediated by sulfasalazine through the prevention of the oxidative/nitrative/nitrosative damages caused by these species.