AUTOPROTEOLYSIS OR PLASMIN-MEDIATED CLEAVAGE OF FACTOR XA-ALPHA EXPOSES A PLASMINOGEN BINDING-SITE AND INHIBITS COAGULATION

Blood coagulation factor Xa (FXa) has recently been shown to function as a plasminogen receptor in the presence of procoagulant phospholipid (phosphatidylserine; PS) and Ca2+. In the current work, the possible effect of autoproteolytic and plasmin-mediated cleavage of FXa on comp lex formation with plasminogen was investigated. I-125-plasminogen bin ding to derivatives of FXa electrotransferred to polyvinylidene difluo ride revealed that the autoproteolytic conversion of FXa alpha to FXa beta was required for the expression of a plasminogen binding site. In the presence of PS and Ca2+, plasmin was shown to convert FXa alpha t o a FXa beta-like species at least 3 orders of magnitude faster than t he autoproteolytic mechanism. This also resulted in the exposure of a plasminogen binding site, Further processing by plasmin generated a fr agment (33 kDa) due to cleavage at Gly(331) in, the FXa heavy chain. P roduction of this species enhanced apparent plasminogen binding compar ed with FXa beta and resulted in the loss of FXa amidolytic and clotti ng activity. In the absence of either PS or Ca2+, the plasmin mediated fragmentation of FXa alpha was altered to include a FXa beta-like mol ecule and a species (40 kDa) with intact beta-heavy chain disulfide li nked to a COOH-terminal fragment of the light chain starting at Tyr(44 ). Neither of these products was observed to interact with plasminogen . The 40 kDa species had amidolytic activity comparable with FXa alpha but inhibited clotting activity. Cumulatively the data provide the fi rst evidence for a functional difference between the FXa subforms and suggest a mechanism where autoproteolysis and plasmin-mediated cleavag e modulate the function of FXa alpha from a procoagulant enzyme to a p rofibrinolytic plasminogen receptor.