The determination of prothrombin using synthetic chromogenic substrates: Choice of a suitable activator

Normal prothrombin (factor II) was determined in whole [human] plasma by activation with staphylocoagulase, Taipan snake venom (TSV) or human factor Xa. In a 2nd reaction, the amount of activated prothrombin (factor IIa) was assayed by the amidolysis of synthetic chromogenic substrates. A convenient preparation of sufficiently pure human factor Xa was described. A linear relation was found between the prothrombin concentration and the amount of p-nitroanilides generated per unit of time. In normal plasma staphylocoagulase and the factor Xa preparation gave similar results. As staphylocoagulase coestimates decarboxyprothrombin, it could not be used to assess prothrombin during oral anticoagulation or vitamin K deficiency. Taipan snake venom and Echis carinatus venom behaved in a similar way as staphylocoagulase does. TSV was inhibited by phospholipids. Ca2+ had no effect on the activation of prothrombin by staphylocoagulase or by Taipan snake venom.