Peroxidase-containing microgranules in human neutrophils: physical, morphological, cytochemical, and secretory...

A microgranule fraction, isolated from human neutrophils by using a novel high-resolution Percoll density gradient system contained granules with the lowest density and diameter when compared with 12 other isopycnic granule fractions. Ultrastructurally, from 34% to 50% of the microgranules showed homogeneous diaminobenzidine (DAB) staining under conditions for localizing peroxidase reactivity. The presence of myeloperoxidase (MPO) was further confirmed by biochemical and spectral analysis and immunodiffusion methods. Periodate-thiocarbohydrazide-silver proteinate (PA-TCH-SP) intensely stained vicinal glycols in the matrix of greater than 97% microgranules in contrast to the weak or absent staining seen in larger primary granules. Directly sampled segmented neutrophils contained small DAB- and PA-TCH-SP-positive granules, which often appeared in clusters. These DAB-positive microgranules selectively remained within the cells after stimulation of exocytosis with the calcium ionophore A23187. The enriched DAB-positive microgranule fraction recovered from A23187-treated cells also contained lysozyme and beta-glucuronidase but lacked vitamin B12 binding protein activity. A similar small, DAB- and PA-TCH-SP-positive granule type was also identified in normal promyelocytes and was the predominant or only granule type observed in leukemic or preleukemic myeloid cells from four patients. This study demonstrates a unique subpopulation of MPO-containing microgranules in normal and leukemic human myeloid cells that are distinguished from (other) primary granules by their extremely low density, small size, content of complex carbohydrates, and resistance to secretion.