- Biochemistry. a lipoprotein in the blood that converts prothrombin to thrombin.
- Pharmacology. a commercial form of this substance, obtained from the brains of cattle, used chiefly as a local hemostatic and as a laboratory reagent in blood prothrombin tests.
- any of a group of substances that are liberated from damaged blood platelets and other tissues and convert prothrombin to thrombinAlso called: thrombokinase
- A plasma protein present in tissues, platelets, and white blood cells necessary for the coagulation of blood and, in the presence of calcium ions, necessary for the conversion of prothrombin to thrombin.factor III platelet tissue factor thrombokinase