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

Human alpha(2)-antiplasmin (alpha(2)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-alpha(2)AP(R6) faster than Met-alpha(2)AP(W6) at the Pro12-Asn13 bond to yield Asn-alpha(2)AP.

To compare Met-alpha(2)AP(R6), Met-alpha(2)AP(W6) and Asn-alpha(2)AP for crosslinking with fibrin and the ability to protect fibrin from digestion by plasmin.

Asn-alpha(2)AP utilizes Gln2 (Gln14 in Met-alpha(2)AP) to become crosslinked to fibrin approximately twelvefold faster than Met-alpha(2)AP(R6) or Met-alpha(2)AP(W6), and this enhances the resistance of fibrin to plasmin. All three forms of alpha(2)AP inhibit plasmin at identical rates. The N-terminal 12-residue peptide of Met-alpha(2)AP slows crosslinking of Met-alpha(2)AP(R6) or Met-alpha(2)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 alpha(2)AP crosslinked with 5-(biotinamido)pentylamine showed Gln14 as the only major crosslinking site. Residues 5-8, GRQL in Met-alpha(2)AP(R6), and residues 1-8, MEPLGWQL in Met-alpha(2)AP(W6), slow fibrin crosslinking.

Gln14 in both Met-alpha(2)AP(R6) and Met-alpha(2)AP(W6) is sheltered by the N-terminal 12-residue peptide, which, when cleaved, yields Asn-alpha(2)AP, which is rapidly crosslinked to fibrin and maximally protects it from plasmin. The R6 W polymorphism in Met-alpha(2)AP does not affect its crosslinking to fibrin, but it does slow cleavage by APCE and reduces the amount of Asn-alpha(2)AP available for rapid crosslinking to fibrin.