Comparison of the catalytic activity of plasma factor IXa and factor IXaR94S in the tenase enzyme complex. Plasma factor IXa (0.17 nmol/L, ○, ^___) or factor IXaR94S (1.5 nmol/L, •, —-) was incubated with factor X (0 to 100 nmol/L) in the presence of phospholipid vesicles (35 μmol/L), factor VIIIa (14 U/mL) and Ca²⁺ (5 mmol/L) in TBS containing 0.1% BSA at 37°C. At discrete time intervals, aliquots of the reactions were quenched with 50 mmol/L EDTA and factor Xa quantitated using CHG-GR-pNA. The data has been normalized with respect to enzyme concentration by expressing the ordinate axis as initial velocity, v_i, divided by the factor IXa or factor IXaR94S concentration. The lines represent the best fit of the data to the Michaelis-Menten equation using nonlinear regression analysis. The kinetic parameters are summarized in Table 1.