Effect of protamine sulfate on antithrombin III activity

When protamine sulfate was added to heparinized plasma in vitro for neutralization of heparin, the activities on both thrombin and Xa known as heparin cofactor in antithrombin action were completely abolished. However, progressive activities on thrombin and Xa both recovered within 30 minutes after protamine sulfate addition. When equivalent heparin was again added, heparin cofactor activity was immediately restored. Based on the fact that protamine sulfate did not show any direct action on the antithrombin III molecule, the presence of AT III with progressive activity was considered to play an important role in the rebound phenomenon of heparin after heparin neutralization with protamine sulfate.