r Ixs S17 is a moderate inhibitor of trypsin, rat trypsin IV and factor Xa.

Stoichiometry inhibition (SI) analysis calculated the amount of r Ixs S17 needed to inhibit one molecule of bovine trypsin (A), rat trypsin IV (B), and factor Xa (C). Various molar ratios of r Ixs S17 to proteases (0, 2.5, 5, 10, 20, 25, 50) were incubated for 15 min at 37°C with constant concentration of bovine trypsin (1.5 nM) or rat trypsin IV (2.0 nM) or factor Xa (13.9 nM). In the presence of appropriate substrates, residual enzymatic activity was measured and plotted against r Ixs S17: protease molar ratio. The SI was determined by extrapolating to the r Ixs S17: protease ratio where protease activity is zero (Y axis = 0). The inhibition rate ( k a ) of r Ixs S17 was determined against bovine trypsin (D), rat trypsin IV (E) and factor Xa (F). Different concentrations of r Ixs S17 (50, 100, 200, 400, 600 and 1000 nM) were incubated with constant amounts of bovine trypsin (14.6 nM), trypsin IV (12.5 nM) or factor Xa (13.9 nM) for different periods of time (0, 1, 2, 4, 6, 8, 10 and 15 min) at 37°C. The residual protease activity was measured and plotted against time to determine the pseudo-first order constant, k obs . Consequently, the second-rate constant ( k a ) was determined by the best fit line slope of the k obs values that were plotted against r Ixs S17 concentration.