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Oxidative inactivation of myeloperoxidase released from human neutrophils
S W Edwards1, H L Nurcombe, C A Hart
1Department of Biochemistry, University of Liverpool, U.K.
Abstract:
Within 1 min of stimulation of human neutrophils by the chemotactic peptide (N-formyl-L-methionyl-L-leucyl-L-phenylalanine) plus cytochalasin B, myeloperoxidase (together with other granule enzymes) was secreted and detected extracellularly. In contrast with the other granule constituents assayed (vitamin B12-binding protein and beta-glucuronidase), the activity of released myeloperoxidase rapidly decreased, so that, by 10 min after stimulation, only about 5% of the total cellular activity was detected. This inactivation was shown to be dependent on oxidant generation during the respiratory burst, since inactivation was not observed (a) after stimulation of anaerobic suspensions or (b) after release from neutrophils from a patient with chronic granulomatous disease; purified myeloperoxidase was rapidly inactivated after incubation with H2O2, presumably owing to the formation of an inactive enzyme-H2O2 complex. These results show that experiments designed to assess the role of myeloperoxidase in neutrophil functions which utilize assays based on peroxidase activity will grossly underestimate this enzyme if oxidant generation during the respiratory burst has also been activated.
Insights
Human neutrophils release myeloperoxidase (MPO) upon stimulation. However, MPO activity rapidly declines due to oxidant generation during the respiratory burst, underestimating its role in neutrophil function assays.
Area of Science:
- Immunology
- Cell Biology
- Biochemistry
Background:
- Neutrophils are key immune cells involved in host defense.
- Myeloperoxidase (MPO) is a crucial enzyme released by neutrophils.
- The role of MPO in neutrophil function can be underestimated due to its inactivation.
Purpose of the Study:
- To investigate the extracellular stability and activity of myeloperoxidase (MPO) released by human neutrophils.
- To determine the factors influencing MPO inactivation following neutrophil stimulation.
- To highlight the implications of MPO inactivation for functional assays.
Main Methods:
- Stimulation of human neutrophils with N-formyl-L-methionyl-L-leucyl-L-phenylalanine and cytochalasin B.
- Assay of extracellular myeloperoxidase (MPO) activity over time.
- Comparison of MPO activity in aerobic and anaerobic conditions.
- Assessment of MPO activity in neutrophils from patients with chronic granulomatous disease.
- In vitro inactivation studies of purified MPO with hydrogen peroxide (H2O2).
Main Results:
- Extracellular myeloperoxidase (MPO) was detected within 1 minute of neutrophil stimulation.
- MPO activity rapidly decreased, with only 5% of cellular activity remaining by 10 minutes.
- MPO inactivation was dependent on oxidant generation during the respiratory burst.
- Inactivation was not observed under anaerobic conditions or in neutrophils from chronic granulomatous disease patients.
- Purified MPO was inactivated by H2O2, suggesting enzyme-H2O2 complex formation.
Conclusions:
- Neutrophil-derived myeloperoxidase (MPO) is rapidly inactivated extracellularly by oxidants produced during the respiratory burst.
- Functional studies relying on MPO activity assays may significantly underestimate the enzyme's contribution when oxidant generation is active.
- Understanding MPO inactivation is critical for accurate assessment of neutrophil function in inflammatory and infectious conditions.
