Antithrombin activity of a fucan sulfate from the brown seaweed Ecklonia kurome

The mechanism of antithrombin action of a fucan sulfate (C-II), which was isolated from the brown seaweed Ecklonia kurome, was examined by clotting method using a thrombin-fibrinogen system and by amidolytic method using a chromogenic substrate in the presence and the absence of antithrombin III (AT III) or heparin cofactor II (HC II). C-II significantly inhibited the clotting of fibrinogen by thrombin even in the absence of the protease inhibitors, and the amidolytic activity of the protein only in the presence of HC II. C-II was not adsorbed on an AT III-agarose column and its anticoagulant activity in AT III-depleted plasma was the same as that in normal one. Examination of interaction of C-II with fibrinogen by gel filtration chromatography demonstrated that C-II bound to the protein. These results indicated that the antithrombin activity of C-II was mediated by HC II and not by AT III, and that the polysaccharide bound to fibrinogen, thereby blocking thrombin action, and also that its direct thrombin inhibition was very weak.