Effect Of Two Soluble "Activators" On The Activity Of Factor XII And Factor XIIa (Hageman Factor)

Abstract

Certain activators of the contact system of coagulation have been reported to induce full activity in the singlechain form (80,000 MW) of Factor XII (FXII). The effects of soluble ellagic acid (EA) and dextran sulfate (DXS) (∼500,000 MW) on purified components of the contact system were studied. Soluble EA (<40 yM) was found to exert a dose-dependent procoagulant effect on purified FXII added to FXII-deficient plasma although the maximal activity observed was much less than that elicited by kaolin in the same mixtures. 5 μM soluble EA increased the amidolytic activity of FXII against H-D-Pro-Phe-Arg-pNA from 2.5 to 5.0 mol substrate/mol enzyme/min. Purified β-FXIIa (28,000 MW) or FXII that had been preincubated with trypsin hydrolyzed 900 mol substrate/mol enzyme/min in the presence or absence of soluble EA. Thus, soluble EA induces minimal (≤0.3%) amidolytic activity in FXII. We also examined the generation of FXIa in the presence of 5 μM soluble EA, FXI, high MW kininogen, and FXII or α-FXIIa (two-chain form, 80.000 MW). The rate of cleavage of 125I-FXI in the presence of 10 μg/ml DXS, FXI, high MW kininogen, and FXII or α-FXIIa was also measured. In the presence of either DXS or soluble EA under the concentrations and conditions employed, α-FXIIa exhibited an initial rate of FXI activation that was 20 times higher than that achieved by single-chain FXII. In addition, the presence of both high MW kininogen and either EA or DXS enhanced the activity of α-FXIIa 20 fold in the activation of FXI. The results show that single chain FXII in the presence of either DXS or soluble EA expresses less than 5% of the enzymatic activities of its proteolytically derived forms whereas either “activator” greatly enhances the action of α-FXIIa on FXI in the presence of high MW kininogen.