The physiologically most important plasmin inhibitor is the serpin alpha2-antiplasmin It is a single chain 70 KD glycoprotein synthesized in the liver with a half-life of 3.3 days. The rapid reaction between plasmin and alpha2-antiplasmin results in the formation of an inactive complex composed of one molecule of each component. Two steps are involved in this process. First a reversible complex is formed between the lysine binding sites on plasmin and complementary sites on the COOH-terminal region of the alpha2-antiplasmin molecule. In a second step, an irreversible complex is formed in association with the cleavage of a peptide bond in the inhibitor. The plasma concentration of a-2-antiplasmin is approximately 1 μM or 70 μg/mL. Approximately 20% of alpha2-antiplasmin present in plasma is crosslinked when blood is clotted. During thrombolytic therapy alpha2-antiplasmin is consumed.

Measurement of plasmin-alpha2-antiplasmin complexes (PAP complexes) provides information concerning the extent of plasminemia and, thus, the possibility of hemorrhagic phenomena. In all diseases which are accompanied by increased fibrin formation and, thus, by an increased reactive plasminemia, one encounters increased PAP complex formation and, in certain situations, there exists a correlation between the occurrence of fibrin split pro-ducts and the occurrence of PAP complexes. Thus, the determination of these complexes serves as a good indicator of ongoing thrombotic-thrombolytic events.