Abstract
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.
Highlights
Reactive oxygen species (ROS) are involved in many physiological processes, including signal transduction, cell proliferation, gene expression, angiogenesis, and aging
The major sources of O2 – in plasma are NADPH oxidases on the surface of phagocytes and endothelial cells, xanthine oxidase bound to endothelial cells, and leakage from the mitochondrial respiratory chain [4,5,6]
The standard reduction potential of the redox couple Compound I/ferric enzyme of human true peroxidases falls in the order of MPO (1.16 V) > eosinophyl peroxidase (EPO) (1.10 V) > LPO (1.09 V), which is proportional to the reduction potential of oxidants the peroxidases can produce in the halogenation cycle (Table 1) [22,96,130]
Summary
Reactive oxygen species (ROS) are involved in many physiological processes, including signal transduction, cell proliferation, gene expression, angiogenesis, and aging. Hydrogen peroxide is one of the major in vivo oxidants. It is a two-electron acceptor with reduction potential Eo (H2 O2 /H2 O) of 1.32 V. Various forms of iron react with hydrogen peroxide to form ROS. The rate constants of the reaction of H2 O2 with free iron or free heme are low. A system of ceruloplasmin-transferrin is responsible for the transport of free iron ions in plasma; intracellular iron redox activity is arrested by ferritin [10].
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