Functional domains of rabbit thrombomodulin.

Thrombomodulin isolated from rabbit lung was separated by ion-exchange chromatography on DEAE-cellulose into a retarded (acidic) and a nonretarded (nonacidic) fraction. Both fractions contained the cofactor required for the activation of protein C. In addition, the acidic fraction (but not the nonacidic fraction) prevented the clotting of fibrinogen by thrombin ("direct" anticoagulant activity) and accelerated the inhibition of thrombin by antithrombin (effect corresponding to 2-10 international units of heparin per mg of protein). Both of these activities were readily neutralized by the synthetic polycation Polybrene, which did not appreciably affect protein C activation. They were also eliminated by digestion of thrombomodulin with bacterial heparinase, which, in addition, converted the acidic form of the protein C activation cofactor to a nonacidic form. Similar conversion observed during storage of thrombomodulin was attributed to endogenous proteinase activity. Density-gradient centrifugation of the acidic form of thrombomodulin in CsCl/4M guanidinium chloride failed to separate either of the direct or antithrombin-dependent anticoagulant activities from the protein C activation cofactor, which showed a buoyant density of 1.31-1.34 g/ml. The nonacidic cofactor had a lower density, 1.26-1.28 g/ml. Unreduced thrombomodulin yielded two major fractions of protein C activation cofactor on NaDodSO4/PAGE, with apparent Mr of approximately 68,000 and 57,000, respectively. The larger component contained essentially all of the direct and antithrombin-dependent anticoagulant activities. We propose that these activities as well as the negative charge and the higher buoyant density of the acidic, Mr 68,000 form of thrombomodulin are due to a heparin-like polysaccharide and, further, that this component can be separated from the major portion of the molecule, which contains the protein C activation site, through the action of a proteinase.