Regulation of Human Mast Cell β-Tryptase: Conversion of Inactive Monomer to Active Tetramer at Acid pH

Abstract

At neutral pH, human mast cell β-tryptase is stabilized in its enzymatically active, tetrameric form by heparin, and resists inhibition by biologic protease inhibitors. After dissociation of β-tryptase from heparin, active tetramers rapidly convert to inactive monomers in an isotonic, neutral pH environment. Although reversible transition states probably exist during this conversion, once inactive monomers form, addition of heparin fails to reconstitute active tetramer at neutral pH. The current study shows that complete reactivation of inactive monomers can occur at acidic pH in a heparin-independent manner. The respective rate-determining steps for formation of tetramer and active enzyme from inactive monomers exhibit second and first order kinetics based on an analysis of initial reaction rates. The optimal pH for tetramer formation and reactivation is about 6, suggesting His residues play a critical role. The optimal ionic strength equivalent is 160 mM NaCl; and the optimal temperature range is 22°C to 37°C. We propose a sequential three-step reactivation process at acidic pH, dimerization of monomers (rate-determining second order step), rapid formation of inactive tetramers, and slow formation of active tetramers (overall rate-determining first order step). Whether reactivation of human β-tryptase occurs at extracellular or intracellular sites, where the pH is acidic in vivo, should be considered.