The mechanism of activation of bovine prothrombin by an activator isolated from Echis carinatus venon and characterization of the new active intermediates.

Bovine prothrombin was activated, in both the absence and presence of dissopropyphosphofluoridate (DEP) and benzamidine, by an activator which was highly purified from the venom of Echis carinatus (saw-scaled viper, ECV). The process of activation was monitored by sodium dodecysulfate (SDS)-polyacrylamide gel electrophoresis, and the reaction products were isolated and chemically characterized. In the absence of the inhibitors, prothrombin yielded two fragments with molecular weights of 28,000 and 57,000, of which the former was the N-terminal fragment of the zymogen and the latter was intermediate 1, consisting of a single polypeptide chain. Intermediate 1 was subsequently converted to an active intermediate, named intermediate ECV, without decrease of molecular weight. This new intermediate ECV, which showed little clotting activity but a strong alpha-N-tosyl-L-arginine methyl ester (TAME)-esterolytic activity and which bound with hirudin or antithrombin III, consisted of two polypeptide chains with molecular weights of 35,000 of 27,000 daltons. The former was indentified as the thrombin B chain with the N-terminal sequence Ile-Val-Glu-Gly and C-terminal serine, and the latter was a fragment with N-terminal Ser-Gly-Gly, linked to the thrombin A chain. On prolonged incubation, intermediate ECV autocaralytically yielded a fragment (inner fragment) of 14,000 daltons with N-terminal serine and the clotting enzyme alpha-thrombin [EC 3.4.21.5], which consists of A and B chains. In the presence of the inhibitors, intermediate ECV and the N-terminal fragment were accumulated in the activation mixture. On the other hand, when prothrombin was activated by the venom activator in the presence of hirudin, antithrombin III, or p-nitrophenyl p'-guanidinobenzoate, it did not yield any fragments but was converted to a derivative with two polypeptide chains having molecular weights of 51,000 and 34,000 daltons, of which the former consisted of N-terminal fragment, the inner fragment, and thrombin A chain, and the latter was thrombin B chain. This new prothrombin derivative, named prothrombin ECV, formed a high-molecular-weight complex, associating with antithrombin III. The complex was not dissociable even in the presence of SDS. Moreover, prothrombin ECV reacted with p-nitrophenyl p'-guanidinobenzoate. On the basis of the results described above, the mechanism of activaton of prothrombin by Echis carinatus venom activator can be summarized as follows: The venom activator first cleaves an Arg-Ile bond liniking thrombin A and B chains in the zymogen molecule, forming an active derivative, prothrombin ECV. This active derivative converts autocatalytically to intermediate ECV, liberating the N-terminal fragment, and active intermediate ECV generates alpha-thrombin, releasing the inner fragment. Thus, only a single peptide bond cleavage along the polypeptide chain of prothrombin is associated with activation by the venom activator...