Myeloperoxidase Location in Myeloid Cells
The enzyme myeloperoxidase (MPO) is present in the primary granules (azurophilic granules) of myeloid cells. 1
Cellular Location of Myeloperoxidase
Myeloperoxidase is specifically located in:
- Primary granules (azurophilic granules) of myeloid cells 1
- These granules are a type of primary lysosome found in neutrophils and monocytes 2
- MPO is considered a granulocytic marker in the diagnosis of acute myeloid leukemia (AML) 1
Importance in Cell Identification and Diagnosis
Myeloperoxidase serves as a critical marker for:
- Identification of myeloid lineage in acute leukemias 1
- Distinguishing myeloid from lymphoid leukemias in clinical practice 3
- Defining mixed phenotype acute leukemia (MPAL) when present in blast cells 1
According to the WHO classification and European LeukemiaNet recommendations, the presence of MPO in at least 3% of blasts is sufficient to indicate myeloid differentiation 1. This makes it one of the most reliable markers for establishing myeloid lineage in acute leukemias.
Expression During Myeloid Cell Development
The expression of MPO follows a specific pattern during myeloid differentiation:
- Synthesized primarily at the promyelocyte stage of differentiation 4, 5
- Initially produced as a precursor protein (pro-MPO) of approximately 75-89 kDa 2, 4
- Undergoes post-translational processing in the endoplasmic reticulum and Golgi apparatus 2
- Mature MPO is a heterodimer composed of heavy-light protomers (59-kDa and 13.5-kDa subunits) 2
- Expression decreases as cells mature to neutrophils 4, 3
Clinical Significance
The presence of MPO in primary granules has significant clinical implications:
- Used as a definitive marker for diagnosing AML and distinguishing it from acute lymphoblastic leukemia 1
- Required for optimal oxygen-dependent killing of microorganisms by neutrophils 2
- Detection methods include cytochemistry, flow cytometry, and immunohistochemistry 1
- MPO deficiency represents a common disorder in myeloid cells 2
Common Pitfalls in MPO Assessment
When evaluating MPO in clinical settings, be aware of these potential pitfalls:
- Some early myeloblasts and monoblasts may lack MPO expression despite being of myeloid lineage 1
- A small subset of lymphoid leukemias (particularly some pre-B-ALL) may show trace MPO expression 3
- The absence of MPO with positive myeloid markers defines AML with minimal differentiation 1
- MPO expression decreases during differentiation, so mature myeloid cells may show reduced MPO activity 4, 3
Therefore, while MPO is a highly specific marker for myeloid lineage, its absence does not completely exclude myeloid origin, particularly in early or poorly differentiated blast cells.