The Major Enzymes of Human Mast Cell Secretory Granules1–3

Introduction Two groups of enzymes have been clearly localized to human mast cell secretory granules, acid hydrolases and neutral proteases. Enzymes in the former group, such as P-hexosaminidase, though concentrated in mast cell granules, are ubiquitous and usually present in lysosomes; ^-hexosaminidase has been useful for quantitation of degranulation from partially purified human mast cells in vitro (1), analogous to its utilization in purified rodent mast cells (2,3). It cannot serve as a specific mast cell marker in vivo. The mast cell neutral proteases are specific markers of mast cells and mast cell subsets and represent the major protein component(s) of these cells on a weight basis. Their presence in high concentration suggests that they are crucial components of mast cell function. This review will therefore focus on the neutral proteases of human mast cells, both as specific markers for human mast cells and as important functional components of this cell type. Only 2 neutral proteases have been purified, characterized, and conclusively localized to human mast cells, tryptase and chymase. They should not be confused with either trypsin or chymotrypsin, 2 antigenically and biochemically distinct proteases. To ascertain the importance of other protease activities that have been associated with human mast cell preparations will require the purification and quantitative analysis of the responsible enzymes and determination of their cellular origin(s) and relationship(s) to tryptase and chymase.