ABSTRACT
BACKGROUND: Factor (F) IX/IXa inactivation by plasmin has been studied; however, whether plasmin converts FIXa to a fibrinolytic enhancer is not known. OBJECTIVE: Investigate plasmin proteolysis site(s) in FIXa that inactivates and transforms it into a fibrinolytic enhancer. METHODS: NH2 -terminal sequencing, mass spectrometry analysis, and functional assays. RESULTS: Plasmin in the presence of Ca2+ /phospholipid (PL) rapidly cleaved FIXaß at Lys316↓Gly317 to yield FIXaγ followed by a slow cleavage at Lys413↓Leu414 to yield FIXaδ. FIXaγ/FIXaδ migrated indistinguishably from FIXaß in nondenaturing gel system indicating that C-terminal residues 317-415/317-413 of heavy chain remain noncovalently associated with FIXaγ/FIXaδ. However, as compared with FIXaß, FIXaγ or FIXaγ/FIXaδ (25-75 mixture, 8-hour/24-hour incubation analysis by mass spectrometry) was impaired ~ 10-fold in hydrolyzing synthetic substrate CBS 31.39 (CH3-SO2-D-Leu-Gly-Arg-pNA), ~ 30-fold (~ 5-fold higher Km , ~ 6-fold lower kcat ) in activating FX in a system containing Ca2+ /PL, and ~ 650-fold in a system containing Ca2+ /PL and FVIIIa. Further, FIXaγ or FIXaγ/FIXaδ bound FVIIIa with ~ 60-fold reduced affinity compared with FIXaß. Additionally, in ligand blots, plasminogen or diisopropylfluorophosphate-inhibited plasmin (DIP-plasmin) bound FIXaγ and FIXaδ but not FIXaß. This interaction was prevented by ε-aminocaproic acid or carboxypeptidase B treatment suggesting that plasminogen/DIP-plasmin binds to FIXaγ/FIXaδ through newly generated C-terminal Lys316 and Lys413. Importantly, FIXaγ/FIXaδ mixture but not FIXaγ enhanced tissue plasminogen activator (tPA)-mediated plasminogen activation in a concentration dependent manner. Similarly, FIXaγ/FIXaδ mixture but not FIXaγ enhanced tPA-induced clot lysis in FIX-depleted plasma. CONCLUSION: Plasmin cleavage at Lys316↓Gly317 abrogates FIXaß coagulant activity, whereas additional cleavage at Lys413↓Leu414 converts it into a fibrinolytic enhancer.