The protease specificity of heparin cofactor II. Inhibition of thrombin generated during coagulation

¹²⁵I-labeled heparin cofactor II (HCII) was mixed with plasma and coagulation was initiated by addition of CaCl₂, phospholipids, and kaolin or tissue factor. In the presence of 67 μg/ml of dermatan sulfate, radioactivity was detected in a band which corresponded to the thrombin-HCII complex (M(r) = 96,000) upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis. No other complexes were observed. The thrombin-HCII complex was undetectable when 5 units/ml of heparin was present or when prothrombin-deficient plasma was used. In experiments with purified proteases, HCII did not significantly inhibit coagulation factors VIIa, IXa, Xa, XIa, XIIa, kallikrein, activated protein C, plasmin, urokinase, tissue plasminogen activator, leukocyte elastase, the γ-subunit of nerve growth factor, and the epidermal growth factor-binding protein. HCII inhibited leukocyte cathepsin G slowly, with a rate constant of 8 x 10⁴ M^-1 in the presence of dermatan sulfate. These results indicate that the protease specificity of HCII is more restricted than that of other plasma protease inhibitors and suggest that the anticoagulant effect of dermatan sulfate is due solely to inhibition of thrombin by HCII.