Experiments on the Nature of the Prothrombin‐Converting Principle: Alteration of Proaccelerin Thrombin

An incubation mixture of activated Stuart factor, calcium, phospholipid and proaccelerin caused the generation of thrombin from prothrombin. The activity of this prothrombin‐ converting principle was measured by the rate at which it converted prothrombin to thrombin. In the absence of thrombin in the incubation mixture, a lag phase, following addition of the prothrombin‐converting principle to prothrombin, was observed before thrombin formation occurred. But when a trace of thrombin was added to the incubation mixture, it converted the principle into an active form, causing the immediate formation of thrombin from prothrombin without any delay. It was demonstrated that thrombin had this action upon the prothrombin‐converting principle by changing proaccelerin into a more active form.