2.  Introduction

We recently isolated a monoclonal IgG from a patient with multiple myeloma, who presented with severe bleeding associated with prolongation of the thrombin time (Colwell et al., Br. J. Haematol. 97: 219, 1997).

 

We reported that the IgG binds to Arg-233 and nearby residues in thrombin exosite II.  Binding of the IgG to thrombin induces an allosteric change that alters the kinetics of hydrolysis of synthetic peptide substrates (Colwell et al., Biochemistry 37: 15057, 1998).

 

We now find that the IgG accelerates the inhibition of thrombin by antithrombin and thereby prolongs the thrombin time of plasma.  The IgG also interferes with activation of factor VIII and prolongs the aPTT by a mechanism independent of antithrombin.

Previously, we showed that substitution of alanine for Arg-233, Arg-101, or Lys-236 (l) in exosite II decreased the affinity of thrombin for the monoclonal IgG, while mutation of many other residues (l) had no effect.  Residues that were not investigated in this study are also indicated (l).  The active site of thrombin is occupied by the peptide Phe-Pro-Arg (l) in this model.  [From Biochemistry  37: 15057, 1998]