Modeling the Reactions of Superoxide and Myeloperoxidase in the Neutrophil Phagosome

Abstract

Neutrophils kill bacteria by ingesting them into phagosomes where superoxide and cytoplasmic granule constituents, including myeloperoxidase, are released. Myeloperoxidase converts chloride and hydrogen peroxide to hypochlorous acid (HOCl), which is strongly microbicidal. However, the role of oxidants in killing and the species responsible are poorly understood and the subject of current debate. To assess what oxidative mechanisms are likely to operate in the narrow confines of the phagosome, we have used a kinetic model to examine the fate of superoxide and its interactions with myeloperoxidase. Known rate constants for reactions of myeloperoxidase have been used and substrate concentrations estimated from neutrophil morphology. In the model, superoxide is generated at several mM/s. Most react with myeloperoxidase, which is present at millimolar concentrations, and rapidly convert the enzyme to compound III. Compound III turnover by superoxide is essential to maintain enzyme activity. Superoxide stabilizes at ϳ25 M and hydrogen peroxide in the low micromolar range. HOCl production is efficient if there is adequate chloride supply, but further knowledge on chloride concentrations and transport mechanisms is needed to assess whether this is the case. Low myeloperoxidase concentrations also limit HOCl production by allowing more hydrogen peroxide to escape from the phagosome. In the absence of myeloperoxidase, superoxide increases to >100 M but hydrogen peroxide to only ϳ30 M. Most of the HOCl reacts with released granule proteins before reaching the bacterium, and chloramine products may be effectors of its antimicrobial activity. Hydroxyl radicals should form only after all susceptible protein targets are consumed.

Direct measurement of oxidative and nitrosative stress dynamics in Salmonella inside macrophages

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NADPH oxidases in lung biology and pathology: Host defense enzymes, and more

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Intrinsic mutagenic properties of 5-chlorocytosine: A mechanistic connection between chronic inflammation and cancer

“…This type of damage is the result of the infiltration and activation of neutrophil granulocytes (the most abundant leukocyte), which produce myeloperoxidase, an enzyme capable of synthesizing HOCl from hydrogen peroxide and chloride anions ( Fig. 1) (11)(12)(13)(14). The main biomarkers found in cells exposed to HOCl are chlorinated tyrosines (in proteins) and chlorinated nucleobases (in nucleic acids and nucleotide pools) (15,16).…”