Alpha-2-Antiplasmin

Alpha-2-antiplasmin is a member of the serine protease inhibitor superfamily. It is a single chain glycoprotein with an apparent molecular weight of 70kD and acts as the primary inhibitor of plasmin in plasma. It inhibits, in a fast reaction, plasmin whereby in a fast phase the C-terminal region of alpha-2-antiplasmin binds reversibly to the lysine-binding site of plasmin and, in a second step, the serine residue of the active site of plasmin reacts with the reactive site Arg-354 - Met-355 of alpha-2-antiplasmin.

Subsequently, an 11kD C-terminal sequence is liberated but remains non-covalently bound to the lysine-binding site 1 of plasmin.

Alpha-2-antiplasmin also crosslinks with the alpha-chain of fibrin through a site close to the N-terminal. Therefore, alpha-2-antiplasmin interferes with the fibrin-binding of plasminogen to fibrin but crosslinked alpha-2-antiplasmin enhances indirectly the binding of plasminogen to fibrin.

Alpha-2-antiplasmin circulates in plasma in a concentration of approximately 1 µM equivalent to 70 µg/ml. It is synthesized in the liver and has a biological half-life of 3.3 days. During coagulation approximately 20% of alpha-2-antiplasmin is crosslinked to fibrin.