The interaction of immobilized thrombin with human antithrombin III

Interaction of human antithrombin III (AT III) with human alpha-thrombin coupled to Sepharose 4B was investigated. Despite markedly reduced esterolytic, amidolytic and especially coagulant activity, more than 90% of immobilized thrombin formed stable complexes with purified AT III. Presence of high affinity heparin did not facilitate the inhibition to the degree seen in reactions conducted with soluble thrombin. Instead, heparin induced proteolysis of up to 66% of the inhibitor that remained in solution. This led to the isolation of a homogeneous protein fragment which migrated in SDS-gel electrophoresis as a band of 50,000 Mr, cross-reacted with antibodies to human AT III but showed no biologic activity nor sufficient affinity for heparin. Out of the three major inhibitors capable of binding soluble thrombin in human plasma, only AT III reacted with immobilized thrombin. However, Sepharose-coupled thrombin mixed with plasma in the presence of heparin produced outstanding quantities of residual immunoreactive AT III devoid of inhibitory activity. These data suggest that presence of high affinity heparin in the environment of thrombin attached to a solid support may dramatically decrease the efficiency of enzyme inhibition.