Kinetic analysis of plasminogen activation by purified plasma kallikrein

Kinetics of plasminogen activation by purified activated plasma kallikrein have been studied in a purified system using Glu-plasminogen as a substrate. A synthetic paranitroanilide substrate was used for quantification of the formed plasmin. In that system kallikrein cleaved plasminogen with a Km value of 0.56 microM, a kcat of 1.6 X 10(-4) s-1 and a catalytic efficiency kcat/Km of 2.7 X 10(-4) s-1 microM-1. Addition of CNBr fibrinogen fragments resulted in an increase of Km to 1.18 microM, an increase of kcat to 5.1 X 10(-4) s-1 and an increase in the catalytic rate constant kcat/Km to 4.3 X 10(-4) s-1 microM-1. Addition of purified high molecular weight kininogen had no effect on the kinetics of plasminogen activation whether or not stimulating fibrinogen fragments were present. A stimulating effect of fibrinogen fragments could also be shown for the cleavage of the low molecular weight paranitroanilide substrate H-D-Pro-Phe-Arg-pNA by kallikrein; in that system the kcat for substrate cleavage by kallikrein increased from 200 s-1 to 280 s-1, while the Km value remained unchanged. From these data it can be concluded that based on enzyme kinetic studies plasminogen activator activity of purified plasma kallikrein is about 1/1000 of that of high molecular weight urokinase and is only slightly influenced by addition of stimulating fibrinogen fragments. Addition of high molecular weight kininogen does not affect plasminogen activator activity of purified plasma kallikrein.