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A procoagulant snake venom serine protease was isolated from the venom of the nose-horned viper (<i>Vipera ammodytes ammodytes</i>). This 34 kDa glycoprotein, termed <i>Vaa</i>SP-VX, possesses five kDa N-linked carbohydrates. Amino acid sequencing showed <i>Vaa</i>SP-VX to be a chymotrypsin-like serine protease. Structurally, it is highly homologous to <i>Vaa</i>SP-6 from the same venom and to nikobin from the venom of <i>Vipera nikolskii</i>, neither of which have known functions. <i>Vaa</i>SP-VX does not affect platelets. The specific proteolysis of blood coagulation factors X and V by VaaSP-VX suggests that its blood-coagulation-inducing effect is due to its ability to activate these two blood coagulation factors, which following activation, combine to form the prothrombinase complex. <i>Vaa</i>SP-VX may thus represent the first example of a serine protease with such a dual activity, which makes it a highly suitable candidate to replace diluted Russell’s viper venom in lupus anticoagulant testing, thus achieving greater reliability of the analysis. As a blood-coagulation-promoting substance that is resistant to serpin inhibition, <i>Vaa</i>SP-VX is also interesting from the therapeutic point of view for treating patients suffering from hemophilia.