[The second cofactor of heparin].

Heparin cofactor II is a plasma protein that inhibits thrombin rapidity in the presence of heparin. It is definitely distinct from antithrombin III by immunological and physicochemical criteria, protease specificity and glycosaminoglycan specificity. HC II inhibits thrombin (but not the other proteases of the coagulation or fibrinolysis), chymotrypsin and chymotrypsin-like enzymes. Dermatan sulfate and pentosan sulfate but not heparin sulfate increase the rate of thrombin inhibition by HC II. The physiological role of HC II is presently unknown. II is likely that its antithrombin activity in vivo is restricted to the areas rich in dermatan sulfate. An additional role may be related to the inhibition of various chymotrypsin-like proteases involved in protein activation or degradation in the tissues. So far, there is no clinical evidence for a physiological role of HC II. Vascular thrombosis associated to constitutional deficiency in HC II have been reported in several instance but epidemiological data are lacking. The metabolism of HC II is very similar to that of AT III. In contrast, the anticoagulant effect of dermatan sulfate is strictly dependent on HC II. As this glycosaminoglycan is effective in an experimental model of thrombosis, HC II appears to be a potential target for new antithrombotic drugs.