Possible aggregatory effect on the catalytic activity of human plasma kallikrein: a cautionary note for kineticists

tively. In the presence of aprotinin, simple inhibition of the reaction did not occur, since a plot of 1 / v against \ I 1 did not give a straight line relationship, whereas a plot of l / v against 111" did, indicating that there is more than one binding site for the kininogenase (Dixon & Webb, 1964). Plots were linear when n = 2 at low substrate concentrations (S-2266, 0 . 0 2 m ~ : Bz-Arg-OEt, 4 m ~ ) and when n = 3 at higher substrate concentrations (S-2266, 0.2 mM: Bz-Arg-OEt 40 m ~ ) . It is, therefore, possible that at low substrate concentration. when only the high affinity site for substrate is occupied, there are two binding sites for inhibitor, whereas at higher substrate concentrations a conformational change takes place making a low-affinity site available for substrate and a third binding site available for the inhibitor. These results suggest that porcine pancreatic kininogenase has more than one binding site for both substrate and inhibitor.