Kinetic and thermodynamic analysis of leech-derived tryptase inhibitor interaction with bovine tryptase and bovine trypsin

The interaction of leech-derived tryptase inhibitor (LDTI) with bovine liver capsule tryptase (BLCT) and bovine trypsin has been studied using both thermodynamic and kinetic approaches. Several differences were detected: (i) the equilibrium affinity of LDTI for BLCT (K-a = 8.9 x 10(5) M-1) is about 600-fold lower than that for bovine trypsin (K-a = 5.1 x 10(8) M-1); (ii) LDTI behaves as a purely non-competitive inhibitor of BLCT, while it is a purely competitive inhibitor of bovine trypsin. These functional data are compared with those previously reported for the LDTI binding to human tryptase, where tight inhibition occurs at two of the four active sites of the tetramer (K-a = 7.1 x 10(8) M-1). Amino acid sequence alignment of BLCT, human beta II-tryptase and bovine trypsin allows us to infer some possible structural basis for the observed functional differences.