DEPOLYMERIZED HOLOTHURIAN GLYCOSAMINOGLYCAN WITH NOVEL ANTICOAGULANT ACTIONS - ANTITHROMBIN-III-INDEPENDENT AND HEPARIN-COFACTOR-II-INDEPENDENT INHIBITION OF FACTOR-X ACTIVATION BY FACTOR-IXA FACTOR-VIIIA COMPLEX AND HEPARIN-COFACTOR-II-DEPENDENT INHIBITION OF THROMBIN

The inhibition mechanism of a polysaccharide anticoagulant, depolymeri zed holothurian glycosaminoglycan (DHG), was examined by analyzing its effects on the clotting time of human plasma depleted of antithrombin III (ATIII), of heparin cofactor II (HCII), or of both heparin cofact ors. The effect exerted by this agent on the activation of prothrombin and factor X in purified human components were also examined and all effects were compared with those of other glycosaminoglycans (GAGs). T he capacity of DHG to prolong activated partial thromboplastin time wa s not reduced in ATIII-depleted, HCII-depleted, or ATIII- and HCII-dep leted plasma, whereas its capacity to prolong prothrombin time and thr ombin clotting time was reduced in HCII-depleted plasma. DHG inhibited the amidolytic activity of thrombin in the presence of HCII with a se cond order rate constant of 1.2 x 10(8) (mol/L)(-1) min(-1). These res ults indicated that DHG has two different inhibitory activities, one b eing an HCII-dependent thrombin inhibition and the other an ATIII- and HCII-independent inhibition of the coagulation cascade. The heparin c ofactors-independent inhibitory activity of DHG was investigated in th e activation of prothrombin by factor Xa and in the activation of fact or X by tissue factor-factor VIIa complex or by factor IXa. DHG signif icantly inhibited the activation of factor X by factor IXa in the pres ence of factor VIIIa, but not in the absence of factor VIIIa. The inte raction between DHG and factors IXa, VIIIa, and X was investigated wit h a DHG-cellulofine column, on which DHG had strong affinity for facto rs IXa and Villa. These findings show that the heparin cofactors-indep endent inhibition exhibited by DHG was caused by inhibition of the int eraction of factor X with the intrinsic factor Xase complex, probably by binding to the factor IXa-factor Villa complex. (C) 1995 by The Ame rican Society of Hematology.