Exploitation of the unusual thermodynamic properties of human myeloperoxidase in inhibitor design

Myeloperoxidase as a therapeutic target for oxidative damage in Alzheimer's disease

Alzheimer's disease (AD) is a major neurodegenerative disorder more common in older adults. One of the leading AD hypotheses involves the amyloid beta (A) production, it is associated to oxidative stress, neuroinflammation, and neurovascular damage. The interaction of A with the blood vessel wall contributes to the disruption of the blood-brain barrier (BBB), allowing neutrophil infiltration containing the myeloperoxidase enzyme (MPO), which produces hypochlorous acid (HOCl) a potent oxidant. Also, MPO could be released from the microglia cells and interact with the amyloid beta plaques. This review aims to study the role of MPO in the progression of AD, in particular its contribution to oxidative stress and neuroinflammation. Furthermore, to explore the MPO-potential as AD-biomarker to evaluate the therapeutic potential of its inhibitors to mitigate the neurotoxicity. Finally, revise MPO inhibitors that could act as dual inhibitors acting on MPO and acetylcholinesterase and or another target involved in AD.

In Situ Imaging of Cellular Inflammatory Response to Antibiotic Exposure with a DNAzyme Nanorobot

Antibiotic-induced inflammation involves the release of myeloperoxidase (MPO), an enzyme whose expression in tissues is associated with the inflammatory pathway. However, existing methods for detecting MPO in cells are limited. In this study, a DNAzyme nanorobot was developed using a scaffold of gold nanoparticles (AuNPs) decorated with functional DNAzyme strands and their fluorophore-labeled substrate strands. The DNAzyme remains inactive due to a self-assembled hairpin structure, with a phosphorothioate (PT) modification inserted into the stem domain. When MPO is present, it triggers a halogenation process that generates hypochlorous acid (HClO). HClO specifically catalyzes the cleavage of the PT-site, releasing free DNAzyme strands to cleave their substrates and generating an increasing fluorescent signal. The detection limit for MPO and its primary product, HClO, were determined to be 0.038 μg/mL and 0.013 μM, respectively. The DNAzyme nanorobot can be readily introduced into cells and function autonomously to differentiate increased MPO/HClO levels caused by antibiotics. This approach was applied to image RAW264.7 cells exposed to four prevalent antibiotics found in the environment (phorbol 12-myristate 13-acetate, erythromycin, penicillin, and tetracycline) as well as antibiotic production wastewater. This nanorobot offers novel strategies for monitoring inflammation to evaluate the health impacts of antibiotic exposure.

The Roles of Neutrophil-Derived Myeloperoxidase (MPO) in Diseases: The New Progress

Myeloperoxidase (MPO) is a heme-containing peroxidase, mainly expressed in neutrophils and, to a lesser extent, in monocytes. MPO is known to have a broad bactericidal ability via catalyzing the reaction of Cl- with H2O2 to produce a strong oxidant, hypochlorous acid (HOCl). However, the overproduction of MPO-derived oxidants has drawn attention to its detrimental role, especially in diseases characterized by acute or chronic inflammation. Broadly speaking, MPO and its derived oxidants are involved in the pathological processes of diseases mainly through the oxidation of biomolecules, which promotes inflammation and oxidative stress. Meanwhile, some researchers found that MPO deficiency or using MPO inhibitors could attenuate inflammation and tissue injuries. Taken together, MPO might be a promising target for both prognostic and therapeutic interventions. Therefore, understanding the role of MPO in the progress of various diseases is of great value. This review provides a comprehensive analysis of the diverse roles of MPO in the progression of several diseases, including cardiovascular diseases (CVDs), neurodegenerative diseases, cancers, renal diseases, and lung diseases (including COVID-19). This information serves as a valuable reference for subsequent mechanistic research and drug development.

Microbiota-derived tryptophan metabolism: Impacts on health, aging, and disease

The intricate interplay between gut microbiota and the host is pivotal in maintaining homeostasis and health. Dietary tryptophan (TRP) metabolism initiates a cascade of essential endogenous metabolites, including kynurenine, kynurenic acid, serotonin, and melatonin, as well as microbiota-derived Trp metabolites like tryptamine, indole propionic acid (IPA), and other indole derivatives. Notably, tryptamine and IPA, among the indole metabolites, exert crucial roles in modulating immune, metabolic, and neuronal responses at both local and distant sites. Additionally, these metabolites demonstrate potent antioxidant and anti-inflammatory activities. The levels of microbiota-derived TRP metabolites are intricately linked to the gut microbiota's health, which, in turn, can be influenced by age-related changes. This review aims to comprehensively summarize the cellular and molecular impacts of tryptamine and IPA on health and aging-related complications. Furthermore, we explore the levels of tryptamine and IPA and their corresponding bacteria in select diseased conditions, shedding light on their potential significance as biomarkers and therapeutic targets.

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.

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.

A topical formulation containing quercetin-loaded microcapsules protects against oxidative and inflammatory skin alterations triggered by UVB irradiation: enhancement of activity by microencapsulation

Ultraviolet B (UVB) irradiation causes free radical production, increase inflammation and oxidative stress, thus, supporting the use of antioxidants by topical administration as therapeutic approaches. Quercetin (QC) is a flavonoid with antioxidant activity, however, high liposolubility makes it difficult to remain in the viable skin layer. Thus, this study evaluated whether microencapsulation of QC would enhance its activity in comparison with the same dose of free QC (non-active dose) and unloaded-microcapsules added in formulation for topical administration in a mouse model of UVB irradiation targeting the skin. Topical formulation containing Quercetin-loaded microcapsules (TFcQCMC) presents physico-chemical (colour, consistence, phase separation and pH) and functional antioxidant stability at 4 °C, room temperature and 40 °C for 6 months. TFcQCMC inhibited the UVB-triggered depletion of antioxidants observed by GSH (reduced glutathione), ability to reduce iron, ability to scavenge 2,2'-azinobis radical and catalase activity. TFcQCMC also inhibited markers of oxidation (lipid hydroperoxides and superoxide anion production). Concerning inflammation, TFcQCMC reduced the production of inflammatory cytokines, matrix metalloproteinase-9 activity, skin edoema, collagen fibre damage, myeloperoxidase activity/neutrophil recruitment, mast cell and sunburn cell counts. The pharmacological activity of TFcQCMC was not shared by the same pharmaceutical form containing the same dose of free QC or unloaded control microcapsules.

Inhibition of Myeloperoxidase

Myeloperoxidase participates in innate immune defense mechanism through formation of microbicidal reactive oxidants and diffusible radical species. A unique activity is its ability to use chloride as a cosubstrate with hydrogen peroxide to generate chlorinating oxidants such as hypochlorous acid, a potent antimicrobial agent. However, chronic MPO activation can lead to indiscriminate protein modification causing tissue damage, and has been associated with chronic inflammatory diseases, atherosclerosis, and acute cardiovascular events. This has attracted considerable interest in the development of therapeutically useful MPO inhibitors. Today, based on the profound knowledge of structure and function of MPO and its biochemical and biophysical differences with the other homologous human peroxidases, various rational and high-throughput screening attempts were performed in developing specific irreversible and reversible inhibitors. The most prominent candidates as well as MPO inhibitors already studied in clinical trials are introduced and discussed.

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.

Targeting neutrophil extracellular traps in severe acute pancreatitis treatment

Severe acute pancreatitis (SAP) is a critical abdominal disease associated with high death rates. A systemic inflammatory response promotes disease progression, resulting in multiple organ dysfunction. The functions of neutrophils in the pathology of SAP have been presumed traditionally to be activation of chemokine and cytokine cascades accompanying the inflammatory process. Recently, since their discovery, a new type of antimicrobial mechanism, neutrophil extracellular traps (NETs), and their role in SAP, has attracted widespread attention from the scientific community. Significantly different from phagocytosis and degranulation, NETs kill extracellular microorganisms by releasing DNA fibers decorated with granular proteins. In addition to their strong antimicrobial functions, NETs participate in the pathophysiological process of many noninfectious diseases. In SAP, NETs injure normal tissues under inflammatory stress, which is associated with the activation of inflammatory cells, to cause an inflammatory cascade, and SAP products also trigger NET formation. Thus, due to the interaction between NET generation and SAP, a treatment targeting NETs might become a key point in SAP therapy. In this review, we summarize the mechanism of NETs in protecting the host from pathogen invasion, the stimulus that triggers NET formation, organ injury associated with SAP involving NETs, methods to interrupt the harmful effects of NETs, and different therapeutic strategies to preserve the organ function of patients with SAP by targeting NETs.

On the Clinical Pharmacology of Reactive Oxygen Species

Reactive oxygen species (ROS) have been correlated with almost every human disease. Yet clinical exploitation of these hypotheses by pharmacological modulation of ROS has been scarce to nonexistent. Are ROS, thus, irrelevant for disease? No. One key misconception in the ROS field has been its consideration as a rather detrimental metabolic by-product of cell metabolism, and thus, any approach eliminating ROS to a certain tolerable level would be beneficial. We now know, instead, that ROS at every concentration, low or high, can serve many essential signaling and metabolic functions. This likely explains why systemic, nonspecific antioxidants have failed in the clinic, often with neutral and sometimes even detrimental outcomes. Recently, drug development has focused, instead, on identifying and selectively modulating ROS enzymatic sources that in a given constellation cause disease while leaving ROS physiologic signaling and metabolic functions intact. As sources, the family of NADPH oxidases stands out as the only enzyme family solely dedicated to ROS formation. Selectively targeting disease-relevant ROS-related proteins is already quite advanced, as evidenced by several phase II/III clinical trials and the first drugs having passed registration. The ROS field is expanding by including target enzymes and maturing to resemble more and more modern, big data-enhanced drug discovery and development, including network pharmacology. By defining a disease based on a distinct mechanism, in this case ROS dysregulation, and not by a symptom or phenotype anymore, ROS pharmacology is leaping forward from a clinical underperformer to a proof of concept within the new era of mechanism-based precision medicine. SIGNIFICANCE STATEMENT: Despite being correlated to almost every human disease, nearly no ROS modulator has been translated to the clinics yet. Here, we move far beyond the old-fashioned misconception of ROS as detrimental metabolic by-products and suggest 1) novel pharmacological targeting focused on selective modulation of ROS enzymatic sources, 2) mechanism-based redefinition of diseases, and 3) network pharmacology within the ROS field, altogether toward the new era of ROS pharmacology in precision medicine.

The role of neutrophils in innate immunity-driven nonalcoholic steatohepatitis: lessons learned and future promise

The enrichment of innate immune cells and the enhanced inflammation represent the hallmark of non-alcoholic steatohepatitis (NASH), the advanced subtype with a significantly increased risk of progression to end-stage liver diseases within the spectrum of non-alcoholic fatty liver disease. Neutrophils are traditionally recognized as key components in the innate immune system to defend against pathogens. Recently, a growing body of evidence supports neutrophils as emerging key player in mediating the transition from steatosis to NASH, which is largely inspired by the histological findings in human liver biopsy indicating the enhanced infiltration of neutrophils as one of the key histological features of NASH. In this review, we discuss data regarding histological perspectives of hepatic infiltration of neutrophils in NASH. We also highlight the pathophysiological role of neutrophils in promoting metabolic inflammation in the liver through the release of a vast array of granule proteins, the interaction with other pro-inflammatory immune cells, and the formation of neutrophil extracellular traps. Neutrophil granule proteins possess pleiotropic effects on regulating neutrophil biology and functions. A variety of granule proteins (including lipocalin-2, myeloperoxidase, proteinase 3, neutrophil elastase, etc.) produced by neutrophils enhance liver metabolic inflammation, thereby promoting NASH progression by mediating neutrophil-macrophage interaction. Therapeutically, pharmacological inhibitors targeting neutrophil granule proteins hold promise to combat NASH. In addition, this article also summarizes potentials of neutrophils and its derived various granule proteins for the accurate, even non-invasive diagnosis of NASH.

Myeloperoxidase - A bridge linking inflammation and oxidative stress with cardiovascular disease

Myeloperoxidase (MPO) is a member of the superfamily of heme peroxidases that is mainly expressed in neutrophils and monocytes. MPO-derived reactive species play a key role in neutrophil antimicrobial activity and human defense against various pathogens primarily by participating in phagocytosis. Elevated MPO levels in circulation are associated with inflammation and increased oxidative stress. Multiple lines of evidence suggest an association between MPO and cardiovascular disease (CVD) including coronary artery disease, congestive heart failure, arterial hypertension, pulmonary arterial hypertension, peripheral arterial disease, myocardial ischemia/reperfusion-related injury, stroke, cardiac arrhythmia and venous thrombosis. Elevated MPO levels are associated with a poor prognosis including increased risk for overall and CVD-related mortality. Elevated MPO may signify an increased risk for CVD for at least 2 reasons. First, low-grade inflammation and increased oxidative stress coexist with many metabolic abnormalities and comorbidities and consequently an elevated MPO level may represent an increased cardiometabolic risk in general. Second, MPO produces a large number of highly reactive species which can attack, destroy or modify the function of every known cellular component. The most common MPO actions relevant to CVD are generation of dysfunctional lipoproteins with an increased atherogenicity potential, reduced NO availability, endothelial dysfunction, impaired vasoreactivity and atherosclerotic plaque instability. These actions strongly suggest that MPO is directly involved in the pathophysiology of CVD. In this regard MPO may be seen as a mediator or an instrument through which inflammation promotes CVD at molecular and cellular level. Clinical value of MPO therapeutic inhibition remains to be tested.

The development of myeloperoxidase inhibitors

Myeloperoxidase (MPO), an abundant hemoprotein present in neutrophils and monocytes, plays a significant role in immune surveillance and host defense mechanisms. However, increased MPO activity has been linked to a number of pathologies with compelling evidence in initiation and progression of inflammatory events. As a result, search for active compounds that can efficiently inhibit MPO activity and subsequently decrease inflammatory events has been focus of the current research. This perspective provides an overview of the development of MPO inhibitors, their mechanism of action and the review of molecules that were in clinical trials as promising MPO inhibitors.

Peroxidase Activity of Human Hemoproteins: Keeping the Fire under Control

The heme in the active center of peroxidases reacts with hydrogen peroxide to form highly reactive intermediates, which then oxidize simple substances called peroxidase substrates. Human peroxidases can be divided into two groups: (1) True peroxidases are enzymes whose main function is to generate free radicals in the peroxidase cycle and (pseudo)hypohalous acids in the halogenation cycle. The major true peroxidases are myeloperoxidase, eosinophil peroxidase and lactoperoxidase. (2) Pseudo-peroxidases perform various important functions in the body, but under the influence of external conditions they can display peroxidase-like activity. As oxidative intermediates, these peroxidases produce not only active heme compounds, but also protein-based tyrosyl radicals. Hemoglobin, myoglobin, cytochrome c/cardiolipin complexes and cytoglobin are considered as pseudo-peroxidases. Рeroxidases play an important role in innate immunity and in a number of physiologically important processes like apoptosis and cell signaling. Unfavorable excessive peroxidase activity is implicated in oxidative damage of cells and tissues, thereby initiating the variety of human diseases. Hence, regulation of peroxidase activity is of considerable importance. Since peroxidases differ in structure, properties and location, the mechanisms controlling peroxidase activity and the biological effects of peroxidase products are specific for each hemoprotein. This review summarizes the knowledge about the properties, activities, regulations and biological effects of true and pseudo-peroxidases in order to better understand the mechanisms underlying beneficial and adverse effects of this class of enzymes.

Topical formulation containing Ilex Paraguariensis extract increases metalloproteinases and myeloperoxidase activities in mice exposed to UVB radiation

Ultraviolet B radiation represents 10% of the total UV radiation that reaches the Earth's surface, being the primary responsible for the biological effects related to skin cancer and photoaging. Ilex Paraguariensis A. St. Hil., known as Yerba mate (YM), is a native tree of South America whose polyphenols in its leaves are described to exhibit photochemoprotective effect and are employed in the treatment of cancer. Additionally, the polyphenols are used to prevent lipid peroxidation and reduce the UV-induced damage, which ultimately decreases the oxidative stress. Thus, the present study aimed to characterize a new YM extract, evaluate the extract cytotoxicity and develop a formulation containing YM extract to prevent UVB-induced damage in mice skin. The YM extract showed high levels of polyphenols, flavonoids, and tannins and exhibited excellent antioxidant activity. Its main components were suggested as chlorogenic acid (1.92%) and caffeic acid (0.41%). Besides, YM extract did not exhibit cytotoxicity in fibroblasts and decreased the activity of myeloperoxidase and metalloproteinase-2 after acute UVB exposure. As a result, the formulation containing the YM extract showed a potential photochemoprotective.

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.

Thioxo-dihydroquinazolin-one Compounds as Novel Inhibitors of Myeloperoxidase

Myeloperoxidase (MPO) is a key antimicrobial enzyme, playing a normal role in host defense, but also contributing to inflammatory conditions including neuroinflammatory diseases such as Parkinson's and Alzheimer's. We synthesized and characterized more than 50 quinazolin-4(1H)-one derivatives and showed that this class of compounds inhibits MPO with IC50 values as low as 100 nM. Representative compounds showed partially reversible inhibition that was competitive with respect to Amplex Red substrate and did not result in the accumulation of MPO Compound II. Members of this group show promise for therapeutic development for the treatment of diseases in which inflammation plays a pathogenic role.

Hydrophobicity and antioxidant activity acting together for the beneficial health properties of nordihydroguaiaretic acid

Nordihydroguaiaretic acid (NDGA) and rosmarinic acid (RA), phenolic compounds found in various plants and functional foods, have known antioxidant and anti-inflammatory properties. In the present study, we comparatively investigated the importance of hydrophobicity and oxidisability of NDGA and RA, regarding their antioxidant and pharmacological activities. Using a panel of cell-free antioxidant protocols, including electrochemical measurements, we demonstrated that the anti-radical capacities of RA and NDGA were similar. However, the relative capacity of NDGA as an inhibitor of NADPH oxidase (ex vivo assays) was significantly higher compared to RA. The inhibitory effect on NADPH oxidase was not related to simple scavengers of superoxide anions, as confirmed by oxygen consumption by the activated neutrophils. The higher hydrophobicity of NDGA was also a determinant for the higher efficacy of NDGA regarding the inhibition of the release of hypochlorous acid by PMA-activated neutrophil and cytokine (TNF-α and IL-10) production by Staphylococcus aureus-stimulated peripheral blood mononuclear cells. In conclusion, although there have been extensive studies about the pharmacological properties of NDGA, our study showed, for the first time, the importance not only of its antioxidant activity, but also its hydrophobicity as a crucial factor for pharmacological action.

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.

Cyanidin-3-glucoside inhibits UVB-induced oxidative damage and inflammation by regulating MAP kinase and NF-κB signaling pathways in SKH-1 hairless mice skin

Skin cancer is one of the most commonly diagnosed cancers in the United States. Exposure to ultraviolet-B (UVB) radiation induces inflammation and photocarcinogenesis in mammalian skin. Cyanidin-3-glucoside (C3G), a member of the anthocyanin family, is present in various vegetables and fruits especially in edible berries, and displays potent antioxidant and anticarcinogenic properties. In this study, we have assessed the in vivo effects of C3G on UVB irradiation induced chronic inflammatory responses in SKH-1 hairless mice, a well-established model for UVB-induced skin carcinogenesis. Here, we show that C3G inhibited UVB-induced skin damage and inflammation in SKH-1 hairless mice. Our results indicate that C3G inhibited glutathione depletion, lipid peroxidation and myeloperoxidation in mouse skin by chronic UVB exposure. C3G significantly decreased the production of UVB-induced pro-inflammatory cytokines, such as IL-6 and TNF-α, associated with cutaneous inflammation. Likewise, UVB-induced inflammatory responses were diminished by C3G as observed by a remarkable reduction in the levels of phosphorylated MAP kinases, Erk1/2, p38, JNK1/2 and MKK4. Furthermore, C3G also decreased UVB-induced cyclooxygenase-2 (COX-2), PGE2 and iNOS levels, which are well-known key mediators of inflammation and cancer. Treatment with C3G inhibited UVB-induced nuclear translocation of NF-κB and degradation of IκBα in mice skin. Immunofluorescence assay revealed that topical application of C3G inhibited the expression of 8-hydroxy-2'-deoxyguanosine, proliferating cell nuclear antigen, and cyclin D1 in chronic UVB exposed mouse skin. Collectively, these data indicates that C3G can provide substantial protection against the adverse effects of UVB radiation by modulating UVB-induced MAP kinase and NF-κB signaling pathways.

Inhibition of UVB-induced skin damage by exopolymers from Aureobasidium pullulans SM-2001 in hairless mice

Because antioxidants from natural sources may be an effective approach to the treatment and prevention of UV radiation-induced skin damage, the effects of purified exopolymers from Aureobasidium pullulans SM-2001 ('E-AP-SM2001') were evaluated in UVB-induced hairless mice. E-AP-SM2001 consists of 1.7% β-1,3/1,6-glucan, fibrous polysaccharides and other organic materials, such as amino acids, and mono- and di-unsaturated fatty acids (linoleic and linolenic acids) and shows anti-osteoporotic and immunomodulatory effects, through antioxidant and anti-inflammatory mechanisms. Hairless mice were treated topically with vehicle, E-AP-SM2001 stock and two and four times diluted solutions once per day for 15 weeks against UVB irradiation (three times per week at 0.18 J/cm(2) ). The following parameters were evaluated in skin samples: myeloperoxidase (MPO) activity, cytokine levels [interleukin (IL)-1β and IL-10], endogenous antioxidant content (glutathione, GSH), malondialdehyde (MDA) levels, superoxide anion production; matrix metalloproteases (MMP-1, -9 and -13), GSH reductase and Nox2 (gp91phox) mRNA levels, and immunoreactivity for nitrotyrosine (NT), 4-hydroxynonenal (HNE), caspase-3, and cleaved poly(ADP-ribose) polymerase (PARP). Photoageing was induced by UVB irradiation through ROS-mediated inflammation, which was related to the depletion of endogenous antioxidants, activation of MMPs and keratinocyte apoptosis. Topical treatment with all three doses of E-AP-SM2001 and 5 nm myricetin attenuated the UV-induced depletion of GSH, activation of MMPs, production of IL-1β, the decrease in IL-10 and keratinocyte apoptosis. In this study, E-AP-SM2001 showed potent inhibitory effects against UVB-induced skin photoageing. Thus, E-AP-SM2001 may be useful as a functional ingredient in cosmetics, especially as a protective agent against UVB-induced skin photoageing.

Naturally-occurring tetrahydro-β-carboline alkaloids derived from tryptophan are oxidized to bioactive β-carboline alkaloids by heme peroxidases

β-Carbolines are indole alkaloids that occur in plants, foods, and endogenously in mammals and humans, and which exhibit potent biological, psychopharmacological and toxicological activities. They form from naturally-occurring tetrahydro-β-carboline alkaloids arising from tryptophan by still unknown way and mechanism. Results in this research show that heme peroxidases catalyzed the oxidation of tetrahydro-β-carbolines (i.e. 1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid and 1-methyl-1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid) into aromatic β-carbolines (i.e. norharman and harman, respectively). This oxidation followed a typical catalytic cycle of peroxidases through redox intermediates I, II, and ferric enzyme. Both, plant peroxidases (horseradish peroxidase, HRP) and mammalian peroxidases (myeloperoxidase, MPO and lactoperoxidase, LPO) catalyzed the oxidation in an efficient manner as determined by kinetic parameters (VMAX and KM). Oxidation of tetrahydro-β-carbolines was inhibited by peroxidase inhibitors such as sodium azide, ascorbic acid, hydroxylamine and excess of H2O2. The formation of aromatic β-carbolines by heme peroxidases can help to explain the presence and activity of these compounds in biological systems.

Inhibition of myeloperoxidase activity by the alkaloids of Peganum harmala L. (Zygophyllaceae)

ETHNOPHARMACOLOGICAL RELEVANCE

Seeds and aerial parts of Peganum harmala L. are widely used in Algeria as anti-inflammatory remedies. Evaluation of Peganum harmala total alkaloids extracts and pure β-carboline compounds as an anti-inflammatory treatment by the inhibition of an enzyme key of inflammatory, myeloperoxidase (MPO) and HPLC quantification of the alkaloids from the different parts of plant.

MATERIALS AND METHODS

MPO inhibition was tested using taurine chloramine test. The inhibition of LDL oxidation induced by MPO was carried out. The molecular docking analysis of Peganum harmala alkaloids on MPO was performed using the Glide XP docking protocol and scoring function and the redox potential of alkaloids was determined using an Epsilon potentiostat. The concentration of harmala alkaloids was determined using HPLC analysis.

RESULTS

The HPLC profiling of the active total alkaloids indicates that β-carboline e.g. harmine, harmaline, harmane, harmol and harmalol are major components. As β-carbolines resemble tryptamine, of which derivatives are efficient inhibitors of MPO, the harmala alkaloids were tested for their activity on this enzyme. Total alkaloids of the seeds and of the aerial parts strongly inhibited MPO at 20µg/mL (97±5% and 43±4%, respectively) whereas, at the same concentration, those of the roots showed very low inhibition (15±6%). Harmine, harmaline and harmane demonstrated a significant inhibition of MPO at IC50 of 0.26, 0.08 and 0.72µM respectively. These alkaloids exerted a similar inhibition effects on MPO-induced LDL oxidation. Molecular docking analysis of Peganum harmala alkaloids on MPO showed that all active Peganum harmala alkaloids have a high affinity on the active site of MPO (predicted free energies of binding up to -3.1kcal/mol). Measurement of redox potentials versus the normal hydrogen electrode clearly differentiated (i) the high MPO inhibitory activity of harmine, harmaline and harmane (+1014, 1014 and 1003mV, respectively); and (ii) the low activity of harmalol and harmol (+629/778 and 532/644mV, respectively). A reverse phase HPLC method has been developed to determine simultaneously five alkaloids of Peganum harmala. Seeds contained all five β-carboline derivatives with the main active alkaloids, harmaline and harmine, being up to 3.8% and 2.9%, respectively. Up to 3.2% of harmine was determined in the roots. The four β-carboline derivatives, harmine, harmaline, harmane and harmalol were identified in the aerial parts. The highest inhibitory effect observed in seeds and the moderate effect of aerial parts could be explained by their harmine and harmaline content. In contrast, the very weak inhibition of the root extract, despite the presence of harmine, may tentatively be explained by the high concentration of harmol which can reduce Compound II of MPO to the native form.

CONCLUSION

The inhibition of MPO by Peganum harmala β-carboline alkaloids, herein reported for the first time, may explain the anti-inflammatory effect traditionally attributed to its herbal medicine.

Hybrid molecules inhibiting myeloperoxidase activity and serotonin reuptake: a possible new approach of major depressive disorders with inflammatory syndrome

Objectives

Major depressive disorder (MDD) is accompanied with an imbalance in the immune system and cardiovascular impairments, such as atherosclerosis. Several mechanisms have been pointed out to underlie this rather unexpected association, and among them the activity of myeloperoxidase (MPO). The aim of our study was to find compounds that inhibit both MPO and serotonin transporter (SERT) for treating MDD associated with cardiovascular diseases.

Methods

SERT inhibition was assessed with measuring of [3H]-serotonin uptake using HEK-293 MSR cells. MPO inhibition was determined by taurine chloramine test on 3-(aminoalkyl)-5-fluoroindole derivatives and on clinically relevant antidepressants. All kinetic measurements were performed using a temperature-controlled stopped-flow apparatus (model SX-18 MV). Promising lead compounds were docked onto SERT 3D structure modelled using the LeuT structure complexed to tryptophan (PDB code 3F3A). Their toxicological profile was also assessed.

Key findings

3-(aminoalkyl)-5-fluoroindole derivative with 5 carbons on the side chain and paroxetine showed the best activity on both MPO and SERT at the nanomolar range. Paroxetine was found to be the first irreversible MPO inhibitor at nanomolar concentrations.

Conclusions

Our results put forward the first hybrid molecule (compound 25) and drug (paroxetine) that can be especially used in MDD associated with inflammatory syndrome.

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.

Mechanistic characterization of a 2-thioxanthine myeloperoxidase inhibitor and selectivity assessment utilizing click chemistry--activity-based protein profiling

Myeloperoxidase (MPO) is a heme peroxidase that catalyzes the production of hypochlorous acid. Despite a high level of interest in MPO as a therapeutic target, there have been limited reports about MPO inhibitors that are suitable for evaluating MPO in pharmacological studies. 2-Thioxanthine, 3-(2-ethoxypropyl)-2-thioxo-2,3-dihydro-1H-purin-6(9H)-one (A), has recently been reported to inhibit MPO by covalently modifying the heme prosthetic group. Here we report a detailed mechanistic characterization demonstrating that A possesses all the distinguishing features of a mechanism-based inactivator. A is a time-dependent MPO inhibitor and displays saturable inactivation kinetics consistent with a two-step mechanism of inactivation and a potency (k(inact)/K(I) ratio) of 8450 ± 780 M⁻¹ s⁻¹. MPO inactivation by A is dependent on MPO catalysis and is protected by substrate. A reduces MPO compound I to compound II with a second-order rate constant of (0.801 ± 0.056) × 10⁶ M⁻¹ s⁻¹, and its irreversible inactivation of MPO occurs prior to release of the activated inhibitory species. Despite its relatively high selectivity against a broad panel of more than 100 individual targets, including enzymes, receptors, transporters, and ion channels, we demonstrate that A labels multiple other protein targets in the presence of MPO. By synthesizing an alkyne analogue of A and utilizing click chemistry-activity-based protein profiling, we present that the MPO-activated inhibitory species can diffuse away to covalently modify other proteins, as reflected by the relatively high partition ratio of A, which we determined to be 15.6. This study highlights critical methods that can guide the discovery and development of next-generation MPO inhibitors.

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.

Efficacy of topical formulations containing Pimenta pseudocaryophyllus extract against UVB-induced oxidative stress and inflammation in hairless mice

Plants rich in antioxidant substances may be a promising strategy for preventing UV-induced oxidative and inflammatory damage of the skin. Pimenta pseudocaryophyllus is native to Brazil and presents flavonoids and other polyphenolic compounds in high concentration. Thus, the present study evaluated the possible effects of topical formulations containing P. pseudocaryophyllus ethanolic extract (PPE) at inhibiting UV-B irradiation-induced oxidative stress and inflammation. PPE was administered on the dorsal skin of hairless mice using two formulations: F1 (non-ionic emulsion with high lipid content) and F2 (anionic emulsion with low lipid content) before and after UV-B irradiation. The following parameters were evaluated in skin samples: edema, myeloperoxidase activity, cytokines levels, matrix metalloprotease-9 (MMP-9) secretion/activity, reduced glutathione (GSH), superoxide anion and lipid peroxidation levels, and mRNA expression for glutathione reductase and gp91phox. The UV-B irradiation increased all parameters, except for IL-10 levels and glutathione reductase mRNA expression, which were not altered, and GSH levels, which were reduced by exposure to UV-B light. Treatments with F1 and F2 containing PPE inhibited UV-B-induced edema formation (89% and 86%), myeloperoxidase activity (85% and 81%), IL-1β production (62% and 82%), MMP-9 activity (71% and 74%), GSH depletion (73% and 85%), superoxide anion (83% and 66%) and TBARS (100% and 100%) levels, increased glutathione reductase (2.54 and 2.55-fold) and reduced gp91phox (67% and 100%) mRNA expression, respectively. F2 containing PPE also increased IL-10 levels. Therefore, this study demonstrates the effectiveness of topical formulations containing PPE in inhibiting UV-B irradiation-induced inflammation and oxidative stress of the skin.

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.

Design, synthesis, and structure-activity relationship studies of novel 3-alkylindole derivatives as selective and highly potent myeloperoxidase inhibitors

Due to its production of potent antimicrobial oxidants including hypochlorous acid, human myeloperoxidase (MPO) plays a critical role in innate immunity and inflammatory diseases. Thus MPO is an attractive target in drug design. (Aminoalkyl)fluoroindole derivatives were detected to be very potent MPO inhibitors; however, they also promote inhibition of the serotonin reuptake transporter (SERT) at the same concentration range. Via structure-based drug design, a new series of MPO inhibitors derived from 3-alkylindole were synthesized and their effects were assessed on MPO-mediated taurine chlorination and low-density lipoprotein oxidation as well as on inhibition of SERT. The fluoroindole compound with three carbons in the side chain and one amide group exhibited a selectivity index of 35 (Ki/IC50) with high inhibition of MPO activity (IC50 = 18 nM), whereas its effect on SERT was in the micromolar range. Structure-function relationships, mechanism of action, and safety of the molecule are discussed.

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.

Myeloperoxidase: a leukocyte-derived protagonist of inflammation and cardiovascular disease

SIGNIFICANCE

The heme-enzyme myeloperoxidase (MPO) is one of the major neutrophil bactericidal proteins and is stored in large amounts inside azurophilic granules of neutrophils. Upon cell activation, MPO is released and extracellular MPO has been detected in a wide range of acute and chronic inflammatory conditions. Recent ADVANCES AND CRITICAL ISSUES: Apart from its role during infection, MPO has emerged as a critical modulator of inflammation throughout the last decade and is currently discussed in the initiation and propagation of cardiovascular diseases. MPO-derived oxidants (e.g., hypochlorous acid) interfere with various cell functions and contribute to tissue injury. Recent data also suggest that MPO itself exerts proinflammatory properties independent of its catalytic activity. Despite advances in unraveling the complex action of MPO and MPO-derived oxidants, further research is warranted to determine the precise nature and biological role of MPO in inflammation.

FUTURE DIRECTIONS

The identification of MPO as a central player in inflammation renders this enzyme an attractive prognostic biomarker and a potential target for therapeutic interventions. A better understanding of the (patho-) physiology of MPO is essential for the development of successful treatment strategies in acute and chronic inflammatory diseases.

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.

Inhibition of the chlorinating activity of myeloperoxidase by tempol: revisiting the kinetics and mechanisms

The nitroxide tempol (4-hydroxy-2,2,6,6-tetramethyl piperidine-1-oxyl) reduces tissue injury in animal models of inflammation by mechanisms that are not completely understood. MPO (myeloperoxidase), which plays a fundamental role in oxidant production by neutrophils, is an important target for anti-inflammatory action. By amplifying the oxidative potential of H2O2, MPO produces hypochlorous acid and radicals through the oxidizing intermediates MPO-I [MPO-porphyrin•+-Fe(IV)=O] and MPO-II [MPO-porphyrin-Fe(IV)=O]. Previously, we reported that tempol reacts with MPO-I and MPO-II with second-order rate constants similar to those of tyrosine. However, we noticed that tempol inhibits the chlorinating activity of MPO, in contrast with tyrosine. Thus we studied the inhibition of MPO-mediated taurine chlorination by tempol at pH 7.4 and re-determined the kinetic constants of the reactions of tempol with MPO-I (k=3.5×105 M−1·s−1) and MPO-II, the kinetics of which indicated a binding interaction (K=2.0×10−5 M; k=3.6×10−2 s−1). Also, we showed that tempol reacts extremely slowly with hypochlorous acid (k=0.29 and 0.054 M−1·s−1 at pH 5.4 and 7.4 respectively). The results demonstrated that tempol acts mostly as a reversible inhibitor of MPO by trapping it as MPO-II and the MPO-II–tempol complex, which are not within the chlorinating cycle. After turnover, a minor fraction of MPO is irreversibly inactivated, probably due to its reaction with the oxammonium cation resulting from tempol oxidation. Kinetic modelling indicated that taurine reacts with enzyme-bound hypochlorous acid. Our investigation complements a comprehensive study reported while the present study was underway [Rees, Bottle, Fairfull-Smith, Malle, Whitelock and Davies (2009) Biochem. J. 421, 79–86].

Myeloperoxidase-dependent oxidation of etoposide in human myeloid progenitor CD34+ cells

Etoposide is a widely used anticancer drug successfully used for the treatment of many types of cancer in children and adults. Its use, however, is associated with an increased risk of development of secondary acute myelogenous leukemia involving the mixed-lineage leukemia (MLL) gene (11q23) translocations. Previous studies demonstrated that the phenoxyl radical of etoposide can be produced by action of myeloperoxidase (MPO), an enzyme found in developing myeloid progenitor cells, the likely origin for myeloid leukemias. We hypothesized, therefore, that one-electron oxidation of etoposide by MPO to its phenoxyl radical is important for converting this anticancer drug to genotoxic and carcinogenic species in human CD34(+) myeloid progenitor cells. In the present study, using electron paramagnetic resonance spectroscopy, we provide conclusive evidence for MPO-dependent formation of etoposide phenoxyl radicals in growth factor-mobilized CD34(+) cells isolated from human umbilical cord blood and demonstrate that MPO-induced oxidation of etoposide is amplified in the presence of phenol. Formation of etoposide radicals resulted in the oxidation of endogenous thiols, thus providing evidence for etoposide-mediated MPO-catalyzed redox cycling that may play a role in enhanced etoposide genotoxicity. In separate studies, etoposide-induced DNA damage and MLL gene rearrangements were demonstrated to be dependent in part on MPO activity in CD34(+) cells. Together, our results are consistent with the idea that MPO-dependent oxidation of etoposide in human hematopoietic CD34(+) cells makes these cells especially prone to the induction of etoposide-related acute myeloid leukemia.