beta-Tryptase, a product of the TPSAB1 and TPSB2 genes, is a trypsin-like serine 
protease that is a major and selective component of the secretory granules of 
all human mast cells, accounting for as much as 25% of cell protein. Once mast 
cells are activated, beta-tryptase is released along with histamine and heparin 
proteoglycan. beta-Tryptase is a unique enzyme with a homotetrameric structure 
in which active sites face into the central cavity of the four monomers, 
stabilized by heparin-proteoglycan. This structure makes beta-tryptase resistant 
to most biological inhibitors of serine proteases. Without stabilization, at 
neutral pH beta-tryptase converts to inactive monomers. Tryptase levels are 
elevated in bronchoalveolar lavage (BAL) fluid obtained from atopic asthmatics 
and in serum during systemic anaphylactic shock. Several synthetic small 
molecular weight beta-tryptase inhibitors reduced Ag-induced airway 
hypersensitivity in animals, suggesting that beta-tryptase is involved in the 
pathogenesis of airway inflammation. Although the major biologic substrate(s) of 
beta-tryptase remain ambiguous, the protease can digest several proteins of 
potential biologic importance, including fibrinogen, fibronectin, pro-urokinase, 
pro-matrix metalloprotease-3 (proMMP-3), protease activated receptor-2 (PAR2) 
and complement component C3. Recently, monomers of beta-tryptase with enzymatic 
activity have been detected in vitro. Here we discuss how beta-tryptase monomers 
with enzymatic activity were identified as well as their potential role in vivo.