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The transformation of nascent phagosomes into forms capable of interacting with antimicrobial organelles of phagocytes, peroxisomes, depends on certain interactions between phagosomes and other vacuolar organelles. Phagosomes repeatedly interact with early and late endosomes through temporary contacts, which allows them to gain and lose complex sets of proteins. In addition, certain polypeptides are eliminated from phagosomes through recycling. New proteins enter phagosomes from the organelles of the biosynthetic pathway or are recruited from the cytoplasm. In addition, phagosomes receive proteins in the process of interaction with endosomes. The overall result of such transformation is acquiring new properties that make possible their interaction with peroxisomes.

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Myeloperoxidase is the main peroxisomal protein of neutrophils, monocytes, and a subpopulation of tissue macrophages; it plays the key role in protective and inflammatory responses of the organism. This role is mediated by various diffusible radicals formed during oxidative reactions catalyzed by the enzyme heme. Myeloperoxidase and nitric oxide synthase are stored in peroxisomes. Nitric oxide reacts with the heme of myeloperoxidase. Low nitric oxide concentrations increase peroxidase activity through reduction of Compound II to native myeloperoxidase. Conversely, high nitric oxide concentrations inhibit the catalytic activity of myeloperoxidase through formation of inactive nitrosyl-heme complexes. Such effect of nitric oxide on catalytic activity of myeloperoxidase has various consequences for infectious and local inflammatory processes. Another oxide of nitrogen, nitrite, is a good substrate for myeloperoxidase Compound I but slowly reacts with Compound II. Nitrogen dioxide is formed after nitrite oxidation by myeloperoxidase. Formation of nitrogen dioxide is another protective mechanism and nitration of microbial proteins by myeloperoxidase can represent an additional protective response of peroxisomes.

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The neutrophil contains numerous granules of various composition and structure. For decades, the neutrophil was believed to contain only two granule types, peroxisomes (peroxidase-positive granules) and neutralohydrolasosomes (peroxidase-negative granules). Later existence of the third type distinguished by the presence of gelatinase hydrolyzing collagen and gelatin. Gelatinase was found in the granules that are lighter as compared to neutralohydrolasosomes and represent a subpopulation of peroxidase-negative granules. In addition to gelatinase, these granules contain beta-2 microglobulin, cytochrome b558, as well as receptor and adhesion proteins. Upon stimulation by inflammatory mediators, the gelatinase granules are secreted faster than the neutralohydrolasosomes. Their exocytosis mediates delivery of new adhesion proteins to the plasma membrane, which is required for maintenance of permanent and fast cell adhesion to the endothelium. The released gelatinase allows the neutrophil to penetrate through the basement membrane of the endothelium.

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Peroxisomal myeloperoxidase plays a key role in synthesis of oxidants by neutrophilic leukocytes. This heme protein consists of two subunits connected by a disulfide bond. The enzyme uses H2O2 and Cl- for synthesis of HOCl--the major oxidant produced by neutrophils. Besides the chlorination reaction, myeloperoxidase exhibits some other properties depending on its oxidation state. The enzyme significantly affects synthesis of oxidants in the cells depending on the competing substrate concentrations and other factors. O2.- is also a physiological substrate of myeloperoxidase. Its reaction with the enzyme determines how the cells utilize O2.- for pathogen elimination. O2.- affects the chlorinating and peroxidase activities of myeloperoxidase. In addition, O2.- reacts with the enzyme yielding the catalytically active compound III that hydroxylates phenols.

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Myeloperoxidase plays the key role in antimicrobial of phagocytes. This enzyme uses hydrogen peroxide and chloride to catalyze hypochlorous acid formation. HOCl is the most probable agent in the oxygen-dependent bactericidal activity in the phagocyte phagosome. Chlorination markers indicate HOCl generation in the quantities lethal for bacteria. Enzymatic assay for myeloperoxidase indicates proceeding of other reactions involved in bactericidal activity. Superoxide integrates many activities of this kind and is important for physiological function of myeloperoxidase. Elucidation of phagosomes biochemistry can help us to understand why certain pathogens survive in such unfavorable environment.

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Myeloperoxidase plays the key role in antimicrobial oxygen-dependent activity of neutrophils. This heme-containing enzyme catalyzes HOCl formation from H2O2 and Cl-. HOCl is a strong oxidation agent produced at the significant level by neutrophils. Myeloperoxidase easily oxidizes thiocyanate to hypothiocyanate and Br- to HOBr, which are involved in protective reactions. Myeloperoxidase reacts quickly with nitric oxide and peroxynitrite in inflammation foci. All these reactions affect neutrophil-induced oxidative stress.

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Peroxisomes of neutrophils are formed in promyelocytes. In addition to myeloperoxidase constituting 35% peroxisomes, they contain nonenzymatic antimicrobial cationic peptides and polypeptides, several serine proteases, as well as some other hydrolases and additional components. Similar to serine proteases, these hydrolases can serve as natural antibiotics. Their function can complement the main oxidation function of neutrophilic myeloperoxidase in the protective response. The peroxisomes contain acid glycosaminoglycans functioning as an anionic carrier that reversibly binds cationic proteins, including hydrolases.

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Data on the existence of organelles, binding and inactivating xenobiotics on the subcellular level in animal and plant cells is presented. The morphology of these organelles both the enzymatic constituents and their expected physiological function in immunological homeostasis are discussed. The data presented suggests that the previously discussed organelles such as chloragosomes, cadmosomes, metallogranules, keratinosomes, lamellar and myeloid bodies of animal and plant cells belong to the united class of organelles--antixenosomes which segregate, bind and inactivate xenobiotics protecting both cells and the entire organism from their harmful effect.

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Functional properties of peroxidase-containing granules (peroxidasosomes) of eosinophiles are discussed. Findings are reported on the activities of peroxidase systems and of non-enzymic cationic proteins which occur in the eosinophilic peroxidasosomes in normal state and in various pathological states as well as their antimicrobial and antiparasitic activities.

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Findings suggesting a vast variety of non-enzymic cationic proteins and peptides in human and animal leucocytes are reported. These cationic proteins and peptides demonstrate their antimicrobial and cytotoxic properties due to their detergent characteristics and their capacity to change the permeability of cellular wall and membrane by electrostatic interactions. Information on their homology with some serine proteases is presented.

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