1. A method is described for the preparation and titration of prothrombin and thrombin.

2. Confirming the views of Morawitz, Howell (1916–17, 1925), and Bordet, thrombin cannot be regarded as an artificial by-product of coagulation (Wooldridge, Nolf (both quoted from Morawitz)). Calcium, a platelet factor, and a plasma factor (prothrombin) interact to form thrombin, and this then acts upon fibrinogen to form fibrin. The amount and rate of thrombin formation in the first reaction are independent of the presence or absence of fibrinogen. After a variable latent period, thrombin suddenly appears in large quantities, coincident with or immediately preceding the deposition of fibrin if fibrinogen is present.

3. The amount of thrombin formed in a mixture of prothrombin, Ca and platelets is independent of the platelet or Ca concentration, and depends primarily upon the amount of prothrombin used. The platelets (or cephalin) enormously accelerate the transformation of prothrombin to thrombin, and this acceleration seems to be their physiological rôle in the coagulation process.

4. Contrary to previous reports, platelets have not been demonstrated to contain significant quantities of prothrombin.

5. The available data do not allow any definite decision as to whether the platelet factor actually combines with prothrombin to form thrombin, or merely catalyzes the transformation. The very slow formation of thrombin in the complete absence of platelets may be due to dissolved traces of platelet material released during the physical manipulation of the plasma (centrifuging, Berkefeld filtration).

6. There was no evidence for a species-specific activity of platelets in the transformation of prothrombin to thrombin.

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