The mechanism of activation of human prothrombin by an activator isolated from Dispholidus typus venom.

Purified human prothrombin was activated, both in the absence and in the presence of thrombin inhibitors (diisopropylfluorophosphate or hirudin), by a coagulant principle isolated from Dispholidus typus venom. The process of activation was monitored by sodium dodecyl sulfate polyacrylamide gel electrophoresis. In the absence of thrombin inhibitor, prolonged incubation of prothrombin with the purified venom yielded thrombin, fragment 1 (F 1) and fragment 2 (F 2). In the presence of diisopropylfluorophosphate, which in the experimental conditions used inhibited only partially the thrombin generated activity, products obtained upon activation of prothrombin by venom were F 1 and a two-chain, disulfide-bridged protein of 58 000 daltons called meizothrombin (des F 1). In the presence of hirudin, which fully inhibited thrombin generated activity, prothrombin activation by the venom did not liberate any fragment, but prothrombin was converted to a derivative composed of two disulfide-bridged polypeptide chains of 48 000 and 37 000 daltons, called meizothrombin. These results are similar to those reported by others when studying the process of prothrombin activation by Echis carinatus venom and allow to conclude that Dispholidus typus venom cleaves a bond linking the A and B chains of thrombin, converting prothrombin into meizothrombin. This enzyme is then responsible for the cleavage of the bond linking F 1 and F 2 and the bond linking F2 the A chain of thrombin.