SELECTIVE-INHIBITION OF FACTOR XA IN THE PROTHROMBINASE COMPLEX BY THE CARBOXYL-TERMINAL DOMAIN OF ANTISTASIN

Studies of antistasin, a potent factor Xa inhibitor with anticoagulant properties, were performed wherein the properties of the full-length antistasin polypeptide (ATS-119) were compared with the properties of forms of antistasin truncated at residue 116 (ATS-116) and residue 112 (ATS-112), ATS-119 was 40-fold more potent than ATS-112 in prolonging the activated partial thromboplastin time (APTT), whereas ATS-119 inh ibited factor Xa 2.2-fold less avidly and about B-fold more slowly tha n did ATS-112, The decreased reactivity of ATS-119 suggests that the c arboxyl-terminal domain of ATS-119 stabilizes an ATS conformation with a reduced reactivity toward factor Xa, The observation that calcium i on increases the reactivity of ATS-119 but not that of ATS-112 suggest s that calcium ion may disrupt interactions involving the carboxyl ter minus of ATS-119. Interestingly, ATS-119 inhibited factor Xa in the pr othrombinase complex 2-6-fold more potently and 2-3-fold faster than A TS-112. These differences in affinity and reactivity might well accoun t for the greater effectiveness of ATS-119 in prolonging the APTT and suggest that the carboxyl-terminal domain of ATS-119 disrupts interact ions involving phospholipid, factor Va, and prothrombin in the prothro mbinase complex. The peptide RPKRRLIPRLS, corresponding to the carboxy l domain of ATS-119 prolonged the APTT and inhibited prothrombinase-ca talyzed processing of prothrombin, but it failed to inhibit the cataly tic activity of isolated factor Xa, Thus, this novel inhibitor appears to exert its inhibitory effects at a site removed hom the active site of factor Xa.