- A substance present in tissues, platelets, and leukocytes necessary for the coagulation of blood; in the presence of calcium ions t. is necessary for the conversion of prothrombin to thrombin, an important step in coagulation of blood. It is now generally believed that t. activity may be developed through blood (intrinsic) or tissue (extrinsic) systems. Tissue t. (factor III) interacts with factor VII and calcium to activate factor X; active factor X combines with factor V in the presence of calcium and phospholipid to produce t. activity (also commonly called t.). SYN: platelet tissue factor, thrombokinase, thrombozyme, tissue factor, zymoplastic substance.
* * *throm·bo·plas·tin .thräm-bō-'plas-tən n a complex enzyme that is found in brain, lung, and other tissues and esp. in blood platelets and that functions in the conversion of prothrombin to thrombin in the clotting of blood called also thrombokinase
* * *n.a substance formed during the earlier stages of blood coagulation. It acts as an enzyme, converting the inactive substance prothrombin to the enzyme thrombin.
* * *throm·bo·plas·tin (throm″bo-plasґtin) factor III; see under coagulation factors, at factor.
Medical dictionary. 2011.