Anti-Thrombin III (Last Updated - 2/25/2005)

-aka. Heparin Cofactor
-polypeptide serine protease inhibitor
-synthesized in the liver
-rapidly (1/2-Life < 0.1 s) inhibits thrombin (IIa) only in the presence of Heparin
-Heparin catalyzes Thrombin (IIa) / Anti-Thrombin III-complex formation (then Heparin is released)
-also inhibits Factors XIa, Xa & IXa (and even has a small inhibitory effect on Factor VIIa too)
-but Factors Xa and Thrombin (IIa) that are already clot-bound are protected from inhibition by anti-thrombin
-Heparin can prevent clots from forming (but if the clot has already formed then Heparin is not thrombolytic --> it will not dissolve the clot) but will prevent it from enlarging

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References:

Goodman & Gilman's: The Pharmacologic Basis Of Therapeutics - 10th Edition - 2001. Chapter 55.