Thrombokinase is prepared from bovine plasma by a procedure involving: treatment with diatomaceous silica, adsorption on barium sulfate, flowing elution with two successive phosphate buffers, ammonium sulfate fractionation, "spontaneous" activation in concentrated solution, and isoelectric precipitation. The yield of nitrogen is 0.002 per cent, corresponding to 1.2 mg. protein per liter of plasma.

When diluted back to the volume of parent plasma, and complemented by calcium plus cephalin, the product causes appreciable activation of prothrombin in 1 minute. Thus, the quantity of thrombokinase obtainable is compatible with a physiologic role. In the more complex system used for routine assay, thrombokinase can be supplied by crude plasma at a dilution of 1/500. In parallel tests, the product appears to be more active than its parent plasma, although it contains only 0.002 per cent of the nitrogen. However, the thrombokinase of the product has been activated, whereas the thrombokinase of the plasma is probably in an inactive precursor state.

When diluted back to the volume of parent plasma, to a concentration of 0.2 microgram nitrogen per ml., thrombokinase can slowly activate prothrombin in the presence of oxalate, and without the addition of accessory factors. Activation of prothrombin in the presence of oxalate is faster with higher concentrations of thrombokinase.

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