The relative amidolytic activity of Hageman factor (factor XII) and its fragments. The effect of high-molecular-weight kininogen and kaolin.

The effect of HMW kininogen upon the amidolytic properties of two species of activated HF (factor XII), designated HFea and HFf, was studied by using the synthetic substrate S2238. HFea is an agent present in the fluid phase after mixture of HF and Sephadex gels to which ellagic acid has been adsorbed, whereas HFf is the carboxy-terminal fragment separated from HF by limited proteolysis. The specific amidolytic activity of HFea was significantly greater than that of HFf. HMW kininogen enhanced amidolysis by both HFea and HFf. In contrast, kaolin inhibited amidolysis by these enzymes, an effect counteracted by pretreatment of kaolin with HMW kininogen. The mechanisms responsible for these effects of HMW kininogen are uncertain. Its enhancement of amidolysis by fluid-phase HFea ad HFf may be related in part to its capacity to counteract substrate inhibition by S2238. The countervailing effect of HMW kininogen upon the inhibitory properties of kaolin may reflect its capacity to inhibit partially the adsorption of HFea or HFf to kaolin, allowing expression of the amidolytic properties of these enzymes in the fluid phase.