Granule Enzymes and Proteoglycans

Neutral serine proteases, including tryptase and chymase, are the most abundant protein constituents of mast cell secretory granules and contribute to tissue damage in immediate hypersensitivity reactions. Tryptase is present in all human mast cells and is not known to be present in any other cell type. Consequently, the presence of tryptase in human biologic fluids is interpreted as a marker of mast cell activation. Chymase is found in some human mast cells, and its presence or absence is one criterion for characterizing human mast cell subsets, as discussed earlier. The functions of these enzymes in vivo are not known; however, several activities demonstrated in vitro suggest important biologic effects. For example, tryptase cleaves fibrinogen and activates collagenase, thereby causing tissue damage, whereas chymase can convert angiotensin I to angiotensin II, degrade epidermal basement membranes, and stimulate mucus secretion. Other enzymes found within mast cell granules include carboxypeptidase A and cathepsin G. Basophil granules also contain several enzymes, some of which are the same as those in mast cell granules, such as neutral proteases, and others are found in eosin-ophil granules, such as major basic protein and lysophospholipase.

Proteoglycans, including heparin and chondroitin sulfate, are also major constituents of both mast cell and basophil granules. These molecules are composed of a polypeptide core and multiple unbranched glycosamino-glycan side chains that impart a strong net negative charge to the molecules. Within the granules, proteogly-cans serve as storage matrices for positively charged bio-genic amines, proteases, and other mediators and prevent their accessibility to the rest of the cell. The mediators are released from the proteoglycans at different rates after granule exocytosis, biogenic amines dissociating much more rapidly than tryptase or chymase. In this way, the proteoglycans may control the kinetics of immediate hypersensitivity reactions.

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