Prothrombin Activation by an Activator from the Venom of Oxyuranus Scutellatus (Taipan Snake)

The prothrombin activator from the venom of Oxyuranus scutellatus (Taipan snake) was purified by gel filtration on Sephadex G-200 and ion-exchange chromatography on QAE-Sephadex. The activator is a large protein with a molecular weight of approximately 300,000, which is composed of subunits of Mr 110,000 and 80,000 and two disulfide-linked polypeptides of Mr 30,000. One or both of these Mr 30,000 subunits contain the active site. The venom activator readily converts Factor Xa-specific chromogenic substrates and is also able to activate prothrombin (Km = 166 microM, Vmax = 2.5 mumol of prothrombin activated per min/mg of venom). Gel electrophoretic analysis of prothrombin activation indicates that the venom activator randomly cleaves the Arg274-Thr275 and Arg323-Ile324 bonds of prothrombin since both thrombin and meizothrombin are formed as reaction products. Venom-catalyzed prothrombin activation is not affected by bovine Factor Va but is greatly stimulated by phospholipids plus Ca2+ ions. This stimulatory effect is explained by a decrease of the Km for prothrombin. In the presence of 50 microM phospholipid vesicles (25% phosphatidylserine/75% phosphatidylcholine; mole/mole), the Km is 0.34 microM and the Vmax is 7.1 mumol of prothrombin activated per min/mg of venom. The purified venom activator contains gamma-carboxyglutamic acid residues which presumably function in the interaction between the venom activator and phospholipids. Treatment of the activator with 0.8 M NaSCN strongly reduces its ability to activate prothrombin but has no effect on its amidolytic activity. The prothrombin-converting activity of the NaSCN-treated activator can be restored with bovine Factor Va. During prolonged gradient gel electrophoresis, the Mr 300,000 activator dissociates into smaller subunits. This causes a loss of the prothrombin-converting activity, while the amidolytic activity is recovered in a protein with an apparent molecular weight of 57,000. This protein can, however, rapidly activate prothrombin in the presence of Factor Va or in the presence of a protein component of Mr 220,000 that also migrates on the gel. These results suggest that the prothrombin activator from the O. scutellatus venom is a multimeric protein complex consisting of a Factor Xa-like enzyme and a Factor Va-like cofactor.