Influence of plasma halide, pseudohalide and nitrite ions on myeloperoxidase-mediated protein and extracellular
Shuqi Xu1, Christine Y Chuang1, Ernst Malle2
1Department of Biomedical Sciences, Panum Institute, University of Copenhagen, Denmark.
Abstract:
Myeloperoxidase (MPO) mediates pathogen destruction by generating the bactericidal oxidant hypochlorous acid (HOCl). Formation of this oxidant is however associated with host tissue damage and disease. MPO also utilizes H2O2 to oxidize other substrates, and we hypothesized that mixtures of other plasma anions, including bromide (Br-), iodide (I-), thiocyanate (SCN-) and nitrite (NO2-), at normal or supplemented concentrations, might modulate MPO-mediated HOCl damage. For the (pseudo)halide anions, only SCN- significantly modulated HOCl formation (IC50 ∼33 μM), which is within the normal physiological range, as judged by damage to human plasma fibronectin or extracellular matrix preparations detected by ELISA and LC-MS. NO2- modulated HOCl-mediated damage, in a dose-dependent manner, at physiologically-attainable anion concentrations. However, this was accompanied by increased tyrosine and tryptophan nitration (detected by ELISA and LC-MS), and the overall extent of damage remained approximately constant. Increasing NO2- concentrations (0.5-20 μM) diminished HOCl-mediated modification of tyrosine and methionine, whereas tryptophan loss was enhanced. At higher NO2- concentrations, enhanced tyrosine and methionine loss was detected. These analytical data were confirmed in studies of cell adhesion and metabolic activity. Together, these data indicate that endogenous plasma levels of SCN- (but not Br- or I-) can modulate protein modification induced by MPO, including the extent of chlorination. In contrast, NO2- alters the type of modification, but does not markedly decrease its extent, with chlorination replaced by nitration. These data also indicate that MPO could be a major source of nitration in vivo, and particularly at inflammatory sites where NO2- levels are often elevated.
Insights
Myeloperoxidase (MPO) generates hypochlorous acid (HOCl) for pathogen defense. While thiocyanate (SCN-) modulates HOCl formation, nitrite (NO2-) shifts damage from chlorination to nitration, potentially contributing to in vivo nitration.
Area of Science:
- Biochemistry
- Immunology
- Oxidative Stress Research
Background:
- Myeloperoxidase (MPO) is crucial for host defense, producing hypochlorous acid (HOCl) to kill pathogens.
- HOCl generation, while protective, can also cause host tissue damage and contribute to disease.
- MPO utilizes hydrogen peroxide (H2O2) to oxidize various substrates, suggesting potential modulation by other plasma anions.
Purpose of the Study:
- To investigate how plasma anions like bromide (Br-), iodide (I-), thiocyanate (SCN-), and nitrite (NO2-) modulate MPO-mediated HOCl damage.
- To determine the physiological relevance of these modulations based on anion concentrations and observed damage.
- To explore the potential role of MPO in vivo nitration, especially in inflammatory conditions.
Main Methods:
- Assessing MPO-mediated HOCl damage to human plasma fibronectin and extracellular matrix using ELISA and LC-MS.
- Quantifying HOCl formation and protein modifications (chlorination, nitration) in the presence of varying anion concentrations.
- Evaluating the impact of these modifications on cellular functions, including cell adhesion and metabolic activity.
Main Results:
- Thiocyanate (SCN-) significantly modulated HOCl formation at physiological concentrations (IC50 ~33 μM).
- Nitrite (NO2-) altered the type of protein modification, increasing nitration (tyrosine, tryptophan) while decreasing chlorination (tyrosine, methionine) at physiological levels.
- Cell adhesion and metabolic activity assays confirmed the functional consequences of MPO-mediated protein modifications.
Conclusions:
- Endogenous SCN- levels can modulate MPO-induced protein modification and chlorination.
- NO2- shifts MPO's activity from chlorination to nitration without significantly reducing overall damage.
- MPO may be a significant source of in vivo nitration, particularly at inflammatory sites with elevated NO2- levels.
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