Notes

The Function of MPO 222
The protein mpo 222 is the major myeloperoxidase produced by neutrophil leukocytes. It plays a critical role in the production of reactive oxygen species, which are essential for innate immune defences against pathogens. The peroxidase activity of MPO enables it to directly oxidize various organic substrates, including phenols and proteins. The ability of MPO to oxidize phenols and proteins renders them non-toxic and allows for their destruction by the bactericidal action of H2O2.

Human myeloperoxidase is a cationic 145 kDa dimer with high conformational and thermal stability, consisting of two glycosylated heavy (73 kDa) and one light polypeptide chain of 15 kDa each. The dimer is held by a single disulfide bridge between symmetry-related halves of the enzyme. Each subunit contains a derivative of protoporphyrin IX in which the methyl groups on pyrrole rings A and C are linked via ester bonds to the carboxyl groups of highly conserved aspartate and glutamate residues. This distorted structure confers unique spectral and redox properties to MPO [20].

In the cytoplasm, MPO is inactive due to the fact that it is bound to a complex of heme and iron (compound I). When released into the extracellular space or into phagosomes, MPO becomes active by a one-electron oxidation of substrates, converting them to a free radical metabolite (compound II) and dioxygen. The oxidation of substrates in this way renders them non-toxic and bactericidal, thus allowing MPO to destroy bacteria and fungi.

Hydrogen peroxide treatment of MPO rapidly inactivates the enzyme as evidenced by a sharp decrease in the Soret absorbance at 430 nm and the loss of 10% of activity (insets to Fig. 3A). Treatment with 250 mM hydrogen peroxide results in the consumption of the entire excess of H2O2 within 2 min and loss of the absorbance at 430 nm by 6.2 +- 2.8%. The formation of Compound III is also observed. The recombination of dioxygen and H2O2 to form superoxide is slow, but eventually occurs, producing the burst phase kinetics observed.

MPO is found in several tissues, but is best known for its release from astrocytes and microglia in the central nervous system where it is involved in neuroinflammation. The redox activity of MPO in the brain is implicated in neuroinflammation and neurodegeneration associated with Alzheimer's disease, multiple sclerosis, stroke or cerebral ischemia, Parkinson's disease and depression or bipolar disorder, as well as in bacterial meningitis. In addition, MPO has been shown to be a key molecule in the activation of macrophages. Moreover, it is believed to play a critical role in nitration reactions and chlorination reactions, which result in Cl-Tyr and Nitro-Tyr respectively. These redox reactions are important for the activation of macrophages and other cell types and impact signaling pathways in a variety of cell types. mpo 222 The inhibition of MPO function by pharmacological agents can be used in the treatment of diseases associated with these redox processes. MPO is also a target for the development of anticancer therapies as it has been shown that inactivation of this protein significantly inhibits tumor growth in mouse models.
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