Why α2-antiplasmin must be converted to a derivative form for optimal function

Abstract

Background: Human α2-antiplasmin (α2 AP), the primary inhibitor of fibrinolysis, is secreted from the liver into plasma as a 464-residue protein with Met as the N-terminus. An R6W polymorphism has been suggested to affect fibrinolytic rate. Within circulating blood, antiplasmin-cleaving enzyme (APCE) cleaves Met-α2 AP(R6) faster than Met-α2 AP(W6) at the Pro12-Asn13 bond to yield Asn-α2 AP. Objectives: To compare Met-α2 AP(R6), Met-α2 AP(W6) and Asn-α2 AP for crosslinking with fibrin and the ability to protect fibrin from digestion by plasmin. Methods and results: Asn-α2 AP utilizes Gln2 (Gln14 in Met-α2 AP) to become crosslinked to fibrin approximately twelvefold faster than Met-α2 AP(R6) or Met-α2 AP(W6), and this enhances the resistance of fibrin to plasmin. All three forms of α2 AP inhibit plasmin at identical rates. The N-terminal 12-residue peptide of Met-α2 AP slows crosslinking of Met-α2 AP(R6) or Met-α2 AP(W6) by limiting access of factor XIIIa to Gln14 rather than shifting crosslinking to other Gln residues. Edman sequencing and mass analyses of tryptic peptides from each α2 AP crosslinked with 5-(biotinamido)pentylamine showed Gln14 as the only major crosslinking site. Residues 5-8, GRQL in Met-α2 AP(R6), and residues 1-8, MEPLGWQL in Met-α2 AP(W6), slow fibrin crosslinking. Conclusion: Gln14 in both Met-α2 AP(R6) and Met-α2 AP(W6) is sheltered by the N-terminal 12-residue peptide, which, when cleaved, yields Asn-α2 AP, which is rapidly crosslinked to fibrin and maximally protects it from plasmin. The R6W polymorphism in Met-α2 AP does not affect its crosslinking to fibrin, but it does slow cleavage by APCE and reduces the amount of Asn-α2 AP available for rapid crosslinking to fibrin. © 2007 International Society on Thrombosis and Haemostasis.