11.  Summary
 

A monoclonal IgG from a patient with myeloma binds to residues in exosite II and alters the specificity of thrombin toward synthetic peptide substrates.

 

The monoclonal IgG, at concentrations below 1 µM, prolongs the thrombin time of normal plasma but does not affect clotting of purified fibrinogen.

 

Prolongation of the thrombin time is primarily due to a 2.7-fold increase in the rate of inhibition of thrombin by antithrombin in the presence of the IgG.  Based on the rate constants given in panel 6, the half-time for inhibition of thrombin is estimated to decrease from ~75 s to ~30 s in plasma containing 2.5 µM antithrombin.

 

In addition to stimulating the thrombin-antithrombin reaction, the monoclonal IgG interferes with the ability of thrombin to activate factor VIII.  The IgG probably competes directly with factor VIII for binding to thrombin, since mutations in exosite II markedly reduce the ability of thrombin to activate factor VIII (Esmon and Lollar, J. Biol. Chem. 271: 13882, 1996).

 

Prolongation of the aPTT in plasma containing the IgG does not depend on the presence of antithrombin and may result from impaired feedback activation of factor VIII by thrombin.

 

Our data suggest that ligand binding to exosite II can induce an allosteric change in the catalytic site of thrombin that may be physiologically important.  Exosite II could be a suitable target for novel anticoagulant agents.