Myeloperoxidase (MPO) is a key enzyme of the microbicidal activity of neutrophils. It produces a variety of antimicrobial toxins, including oxidants and hydrogen peroxide.

MPO has been shown to be involved in the development of many inflammatory diseases. Its inhibition limits DC activation, Ag uptake/processing and migration to LNs. This inhibits the generation of adaptive immunity.

Biological Activity

Myeloperoxidase (MPO) is a glycoprotein produced by neutrophils and plays an important role in the innate immune system. It is a potent and bactericidal oxidant and is released in response to bacterial invasions by phagolysosomes. During this process, MPO-produced H2O2 interacts with Cl- to form hypochlorous acid (HOCl). This oxidant is a very efficient bactericide and has been shown to be an important marker for inflammatory diseases such as rheumatoid arthritis [8,29].

In addition to its antibacterial activity, MPO has been shown to mediate the production of a wide range of reactive oxygen and nitrogen species. These ROS and RNS can also induce lipid peroxidation, protein nitration, and thiol-mediated crosslinking. These effects can be mediated by MPO in various cell types including macrophages and granulocytes.

The oxidant and nitrosyl radical production of MPO is regulated by its substrate specificity, which depends on the availability of H2O2 in the cellular environment. Supplementation of the enzyme with alternative substrates, such as NO2- or SCN-, can limit the extent to which it reacts with HOCl. Inhibition of NADPH oxidase complexes that generate O2-* is also a potential strategy for limiting MPO-mediated oxidative damage.

MPO is an adduct of an iron-semicarbazide molecule and can undergo one-electron reduction to give a ferric (Fe2+) state. MPO FA This ferric state can be reduced to a soluble superoxide radical anion (O2*-) by further reduction or reaction with an O2 addition. This is referred to as the peroxidase cycle.

A large number of naturally occurring antioxidant and antihistaminic compounds exhibit inhibitory activities against MPO, which can be attributed to their ability to interfere with the peroxidase cycle and thereby reduce MPO catalytic activity. Examples of such natural compounds include ferulic acid, caffeic acid, resveratrol, indomethacin, and flufenamic acid.

Inhibition of MPO is an effective therapeutic approach in a variety of human disease states including cardiovascular disease, neurodegenerative diseases, and pulmonary conditions. Some of the known MPO inhibitors are resveratrol, indomethacin, caffeic acid, diclofenac, and resveratrol-derived compounds, such as KYC. These compounds have been shown to improve blood pressure in sickle cell disease mice and to inhibit tumor formation in a murine lung cancer model.

Biological Significance

MPO FA is a critical component of human innate immunity, playing an important role in frontline defense against microbial infection. In the presence of MPO, neutrophils generate oxidants that kill and degrade ingested microbes.

The oxidants are produced by NADPH oxidase and delivered to the phagosome by the fusion of MPO-containing granules with nascent phagosomes [9, 10]. This oxidative system is vital for neutrophil antimicrobial activity because it produces de novo toxins that are able to penetrate cell membranes and deliver these to phagocyte lipid membranes where they are used for rapid, efficient and effective bactericidal killing.

In addition to their oxidative capacity, neutrophils have a number of antimicrobial properties that include the expression of various enzymes and proteins involved in cellular defense against pathogens. These antimicrobial agents include a variety of defensins, serine proteases, alkaline phosphatases, NADPH oxidase, cytochrome C, and granular proteins such as MPO.

Because of their antimicrobial properties, neutrophils are the first line of cellular defense in many diseases, such as sepsis, cystic fibrosis, and autoimmune disorders. Although MPO is a key component of the oxidant system that supports this function, it has not been fully understood in detail.

However, it is believed that MPO contributes to the development of inflammatory pathologies by producing HOCl and other reactive species. The oxidative activity of MPO can lead to the production of procarcinogens and other reactive species that are capable of transforming tissue into a tumor microenvironment through multiple pathways (e.g., formation of genotoxic intermediates or acrolein-protein adducts).

Additionally, MPO can promote endothelial dysfunction and inflammation in the vasculature through various intracellular signaling cascades. For example, MPO activates Rho-kinase, which in turn promotes the induction MPO FA of calpain to impair vasorelaxation and induce pulmonary arterial hypertension. Moreover, MPO also alters the rigidity index of red blood cells by promoting alterations in plasma membrane fluidity, transmembrane potential, cell size, hemolysis sensitivity, and cellular deformability.

It has also been shown that MPO gene polymorphisms can contribute to increased risks of different forms of cancer. This is in part due to the abnormal MPO expression, which is accompanied by a higher production of ROS.

Chemical Structure

MPO FA is a key mediator of cerebral ischemia-reperfusion injury. It mediates oxidative stress and neuroinflammation, and is associated with cell death in a number of neurodegenerative diseases (146). Because of its cytotoxic activity against neurons, it plays a vital role in the development of ischemic stroke. However, despite its important role in brain health and disease, MPO remains poorly understood as a therapeutic target.

In this context, a detailed investigation of the chemical structure of MPO FA was required to better understand its biological activities and roles. The first step was to determine the crystal structure of recombinant proMPO. This X-ray structure revealed six disulfide intrachain bridges, which are found in both proMPO and mature MPO.

These bridges are essential for MPO function as they allow catalysis by halogens, including HOCl. In addition, they provide the distal Ca2+-binding site with a pentagonal bipyramidal arrangement of amino acids. The heme prosthetic group is covalently bound to the MPO active site via three amino acid linkages, and it contains the hydrophobic Arg-380 residues as well as the distal cysteine residues.

The hydrogen bonding network in the heme cavity was similar to that observed in mature MPO. This network included hydrogen bonds between the asymmetrical heme cavity and water molecules W1-W4, and to the pyrrole ring C propionate. It also contained hydrogen bonds between the distal heme cavities, which were formed by His-261 and Arg-405, as well as by Gln-257.

Furthermore, the core-protein region of proMPO exhibited an average root mean square deviation of 0.49 A over 576 superposed Ca atoms, suggesting high structural similarities with mature MPO. This comparison is further supported by the analysis of the solvent content and Matthews coefficient of one asymmetric unit per molecule, which yielded 2.28 A3/Da in proMPO and 3.53 A3/Da in mature MPO.

The X-ray structure of proMPO also revealed the presence of an oxidized side chain, which was likely a result of hydroxylation of a methyl substituent on pyrrole ring A. This is also consistent with the observed electron density at the methyl substituent on pyrrole rings A and C.

Biological Function

Myeloperoxidase (MPO) is a peroxidase enzyme that primarily produces hypochlorous acid (HOCl). It also forms hydroxyl radicals, superoxide, and other reactive species. These oxidants are produced in response to microbial or inflammatory stimuli, as well as during normal tissue injury and the pathogenesis of many chronic diseases.

In vivo, the production of MPO and HOCl by neutrophils is crucial for their antimicrobial function. It also plays a major role in the development and progression of a wide range of vascular inflammation-related conditions.

MPO oxidation of different lipoproteins is a key mechanism for this, as it alters their affinity for macrophages and the vascular wall. It also leads to nitrotyrosine formation in the endothelium and interferes with NO* bioavailability and vasodilation, which is essential for blood vessel permeability. In addition, MPO-induced endothelial NO depletion can impair calpain activation and increase vascular permeability, which increases the risk of pulmonary arterial hypertension and other cardiovascular disease.

Inhibition of MPO with cytotoxic agents and drugs that contain thiol groups can reduce the generation of oxidants by inhibiting the HOCl-catalyzed halogenation cycle. However, these agents can have side effects, such as an accelerated oxidative stress response that can increase cellular damage, thereby contributing to a variety of health problems, including heart disease and cancer. Nonetheless, a promising therapeutic strategy for MPO inhibition is currently being investigated.