A snip enhances α₂-antiplasmin

HEMOSTASIS, THROMBOSIS, AND VASCULAR BIOLOGY

α₂-Antiplasmin (α₂-AP, also called α₂-plasmin inhibitor) is only slowly revealing its full range of subtleties. α₂-AP is a member of the serpin family and the principal physiologic inhibitor of plasmin, which it rapidly inactivates. But there is more to α₂-AP than its reactive center, because the inhibitor can also bind plasminogen and can be cross-linked to fibrin, both affecting its local action. In plasma, the inhibitor occurs in different forms, reflecting proteolytic cleavage. The inhibitor has a 50-residue C-terminal extension relative to the other serpins. Cleavage in the middle of this extension, by an unknown protease, removes the plasminogen-binding capacity; at any one time the nonbinding form normally constitutes about one third of plasma α₂-AP.

In this issue, Lee and colleagues (page 3783) clarify the other and less well-understood variation in α₂-AP, which is at the amino-terminus. Originally, α₂-AP was thought to have an N-terminal Asn and a 39-residue signal peptide,¹  but later work suggested that the mature protein had an additional 12 residues at the amino-terminus, such that the signal peptide was 27 residues. Human plasma contained both the longer (Met-α₂-AP) and the shorter (Asn-α₂-AP) form, the latter predominating.²  Thus, questions about α₂-AP synthesis and processing were raised, and these are answered satisfactorily by Lee and colleagues, who identify the protease that processes α₂-AP, antiplasmin-clearing enzyme (APCE). The cleavage between Pro12-Asn13 reveals the cross-linking site (Gln14) in the inhibitor, a site that is essentially cryptic in the mature protein, where the additional 12 residues mask it.

The cross-linking of α₂-AP to fibrin by Factor XIIIa is crucial to its function in regulating fibrinolysis. Indeed the abnormality in α₂-AP–deficient plasma was illustrated only in fibrin clots prepared in the presence of calcium, which is required for cross-linking. Fibrin from an α₂-AP–deficient patient lysed rapidly, even when immersed in normal plasma, proving the efficacy of cross-linked α₂-AP.³  Antibodies specific for cross-linked α₂-AP greatly stimulated lysis of fibrin.⁴  The importance of cross-linked α₂-AP is sometimes ignored because of emphasis on the relative protection from α₂-AP of plasmin formed on the fibrin surface. These apparent contradictions can be reconciled if we consider that the separate effects have been demonstrated in isolation and that the actual physiologic situation will reflect some balance between them. Thus enough local α₂-AP will overcome plasmin and vice versa. There are still several questions about the physical interaction between cross-linked α₂-AP and fibrin-bound plasmin, but there is no doubt that cross-linking of α₂-AP to fibrin decreases fibrinolysis markedly.

The identification of a protease that is closely related to seprase as the α₂-AP cleaving enzyme now leads to several important questions. Is the APCE activity expressed in liver cells, where α₂-AP is synthesized, or on endothelial cells, where it might act on circulating α₂-AP? Is its synthesis regulated in inflammatory or other conditions, such that it might have impact on the local stability of fibrin in the vessel wall? APCE is present in plasma at only trace concentrations and its isolation from plasma is an impressive feat. What is the source of this plasma APCE? The breakthrough by Lee and colleagues in identifying APCE reminds us of the many substrates in the hemostatic system that may be influenced by the emerging families of transmembrane serine proteases⁵  and the interesting subtleties in regulation that may yet be discovered.