Where is the enzyme myeloperoxidase (MPO) primarily located in myeloid cells?

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Myeloperoxidase Location in Myeloid Cells

The enzyme myeloperoxidase is present in the primary granules (azurophilic granules) of myeloid cells. 1

Detailed Location and Characteristics

Myeloperoxidase (MPO) is specifically located in:

  • Primary granules (azurophilic granules) of myeloid cells 1
  • These granules function as a type of primary lysosome found in neutrophils and monocytes 1
  • MPO is NOT found in secondary granules of myeloid cells
  • MPO is exclusively found in the azurophilic granules of normal myelomonocytic cells 2

Clinical Significance of MPO

MPO serves several important clinical functions:

  • Acts as a definitive marker for myeloid lineage in acute leukemias 1
  • Used to distinguish myeloid from lymphoid leukemias in clinical practice 1
  • Presence of MPO in at least 3% of blasts indicates myeloid differentiation according to WHO classification 1
  • Considered a critical granulocytic marker in the diagnosis of acute myeloid leukemia (AML) 1

Molecular Characteristics

  • MPO is a heme-containing glycoprotein 3
  • The native molecule is a heterodimer composed of heavy-light protomers 4
  • Each protomer contains a 59-kDa and 13.5-kDa subunit 4
  • MPO undergoes significant posttranslational processing during biosynthesis 4

Important Diagnostic Considerations

  • Detection methods for MPO include cytochemistry, flow cytometry, and immunohistochemistry 1
  • Some early myeloblasts and monoblasts may lack MPO expression despite being of myeloid lineage 1
  • The absence of MPO with positive myeloid markers defines AML with minimal differentiation 1

Common Pitfalls in MPO Interpretation

  • MPO cannot be used as an absolutely lineage-specific marker for leukemic cells 2
  • Some pre-B-ALL cell lines can occasionally show trace MPO expression 2
  • MPO expression decreases during myeloid cell differentiation 5, 6
  • MPO synthesis is highest in myeloblasts and decreases significantly in differentiated granulocytes and macrophages 5

Therefore, based on the most recent and highest quality evidence, the answer is A: primary granules; myeloid.

References

1
Guideline

Myeloperoxidase in Myeloid Cells

Praxis Medical Insights: Practical Summaries of Clinical Guidelines, 2025

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Professional Medical Disclaimer

This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.

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