Myeloperoxidase Location in Myeloid Cells
The enzyme myeloperoxidase is present in the primary granules (azurophilic granules) of myeloid cells, making option A the correct answer. 1
Detailed Explanation
Myeloperoxidase (MPO) is a critical enzyme found specifically in the primary granules, also known as azurophilic granules, of myeloid cells. These primary granules are a type of primary lysosome found in neutrophils and monocytes 1. The localization of MPO to these primary granules is well-established in clinical guidelines and research.
Evidence Supporting Primary Granule Localization
Multiple studies confirm that MPO is found exclusively in the azurophilic (primary) granules:
- MPO is a lysosomal enzyme present specifically in the azurophilic granules of human neutrophils and monocytes 2
- MPO is found exclusively in the azurophilic granules (primary lysosomes) of normal myelomonocytic cells 3
- MPO is a heme-containing glycoprotein found in the primary granules of human polymorphonuclear leukocytes 4
Clinical Significance of MPO
The presence of MPO in primary granules has important diagnostic implications:
- MPO serves as a granulocytic marker in the diagnosis of acute myeloid leukemia (AML) 1
- The presence of MPO in at least 3% of blasts is sufficient to indicate myeloid differentiation according to WHO classification 1
- MPO is used as a definitive marker for distinguishing AML from acute lymphoblastic leukemia 1
- Detection methods for MPO include cytochemistry, flow cytometry, and immunohistochemistry 1
MPO Biosynthesis and Processing
The biosynthesis of MPO involves:
- Initial synthesis as a larger precursor (pro-MPO) of approximately 75-77 kDa 5
- Processing through the endoplasmic reticulum and Golgi apparatus 2
- Final maturation into a heterodimer composed of heavy and light subunits (59 kDa and 13.5 kDa) 2
Important Caveats
- 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
- While MPO is primarily associated with myeloid cells, rare cases of MPO expression have been reported in some pre-B-ALL cell lines, though at much lower levels than in myeloid cells 3
In summary, myeloperoxidase is definitively located in the primary (azurophilic) granules of myeloid cells, not in secondary granules or lymphoid cell nucleoli.