Structural and Functional Characteristics of Proteolytically Modified Bovine Factor X and Xa (Des Light Chain Residues 1-41)

Incubation of bovine Factor X with α-chymotrypsin produces a peptide, residues 1-41 of the light chain and a modified Factor X designated “headless Factor X”. Clotting activity of “headless Factor X” is virtually zero by one stage assay. Composition, chromatographic elution characteristics and molecular weight estimates by SDS gel electrophoresis form the bases for the aforedesignated structure of “headless Factor X”. Activation of “headless Factor X” by the Russell’s viper venom activator requires Ca²⁺ as does normal Factor X, however, the rate is much slower. After full activation the specific activity of “headless Factor Xa” and normal Factor Xa differed by less than 20% using BOC-L-Val-L-Leu-Gly-L-Arg-pNA as substrate. However, clotting specific activity is less than 0.002% of normal Factor Xa in the one stage clotting assay. The activation peptides released from “headless Factor X” (residues 1-51 and 291-307) of the heavy chain were identical to those released from normal Factor X. Similar rapid, highly selective chymotryptic cleavage of Prothrombin Fragment 1, and the similarity in the amino acid sequences of the light chain of Factor X and Fragment 1 suggests that the region around the susceptible peptide bond must lie on the surface of both these molecules and perhaps exist in a “hinge” region connecting the Gla containing domain and the reamining structural domain of these portions of prothrombin and Factor X. (Supported by HL12820).