Natural heterogeneity of alpha 2-antiplasmin: functional and clinical consequences

Human alpha 2-antiplasmin (alpha 2AP, also called alpha 2-plasmin inhibitor) is the main physiological inhibitor of the fibrinolytic enzyme plasmin. alpha 2AP inhibits plasmin on the fibrin clot or in the circulation by forming plasmin-antiplasmin complexes. Severely reduced alpha 2AP levels in hereditary alpha 2AP deficiency may lead to bleeding symptoms, whereas increased alpha 2AP levels have been associated with increased thrombotic risk. alpha 2AP is a very heterogeneous protein. In the circulation, alpha 2AP undergoes both amino terminal (N-terminal) and carboxyl terminal (C-terminal) proteolytic modifications that significantly modify its activities. About 70% of alpha 2AP is cleaved at the N terminus by antiplasmin-cleaving enzyme (or soluble fibroblast activation protein), resulting in a 12-amino-acid residue shorter form. The glutamine residue that serves as a substrate for activated factor XIII becomes more efficient after removal of the N terminus, leading to faster cross-linking of alpha 2AP to fibrin and consequently prolonged clot lysis. In approximately 35% of circulating alpha 2AP, the C terminus is absent. This C terminus contains the binding site for plasmin(ogen), the key component necessary for the rapid and efficient inhibitory mechanism of alpha 2AP. Without its C terminus, alpha 2AP can no longer bind to the lysine binding sites of plasmin(ogen) and is only a kinetically slow plasmin inhibitor. Thus, proteolytic modifications of the N and C termini of alpha 2AP constitute major regulatory mechanisms for the inhibitory function of the protein and may therefore have clinical consequences. This review presents recent findings regarding the main aspects of the natural heterogeneity of alpha 2AP with particular focus on the functional and possible clinical implications