Heparin facilitates dissociation of complexes between thrombin and a reactive site mutant (L444R) of heparin cofactor II

Heparin Cofactor II (HCII) inhibits Thrombin by forming a stable 1:1 complex. Heparin and dermatan sulfate increase the rate of complex formation >/=1000-fold. Mutation of leucine 444 to arginine at the P1 position of recombinant HCII (rHCII) increases the rate of inhibition of Thrombin approximately 100-fold in the absence of a glycosaminoglycan (Derechin, V. M., Blinder, M. A., and Tollefsen, D. M. (1990) J. Biol. Chem. 265, 5623-5628). We now report that heparin facilitates dissociation of the thrombin-rHCII(L444R) complex. In the presence of heparin, Thrombin is inhibited rapidly and completely by a 35-fold molar excess of rHCII(L444R), but subsequently approximately 50% of the Thrombin activity reappears with a t1/2 of approximately 20 min. At higher ratios of rHCII(L444R) to Thrombin, the reappearance of Thrombin activity is delayed and the final plateau of activity is decreased. Electrophoretic analysis indicates that proteolysis of excess rHCII(L444R) precedes the reappearance of Thrombin activity. Addition of heparin at longer intervals after formation of the thrombin-rHCII(L444R) complex causes a progressive decrease in the Thrombin plateau, suggesting that in the absence of heparin the complex is slowly converted to a non-dissociable form. By contrast to heparin, dermatan sulfate does not facilitate dissociation of the thrombin-rHCII(L444R) complex. Our findings indicate that the P1 residue of HCII affects not only the rate of inhibition of Thrombin but also the stability of the resulting complex.