Interrelationship of protamine and platelet factor 4 in the neutralization of heparin

To determine the interaction of platelet factor 4 (PF4) and protamine sulfate in the neutralization of heparin in plasma in vitro studies were carried out using a tritium-labeled heparin and a PF4 tagged with 14C. Plasmas treated with various combinations of PF4, protamine and heparin were chromatographed on Sephadex G200 and the fractions were tested for both radioactivity and antithrombin activity. PF4 was comparable to protamine in its ability to neutralize heparin, but the complexes formed with heparin were different. In contrast to protamine, when heparinized plasma was treated with an excess of PF4, no large PF4-heparin complexes were formed and none of the PF4-heparin complexes which did form were able to activate antithrombin III (ATIII). Also, incubation of PF4-neutralized, heparinized plasma at 37 degrees C did not result in liberation of heparin and prolongation of the thrombin clotting time as was found with protamine-neutralized plasma. The action of protamine and PF4 is complimentary. When half the neutralizing dose of each was added together to heparinized plasma, no immediate antithrombin activity remained. When a neutralizing dose of protamine was added to PF4-neutralized, heparinized plasma, the protamine displaced the PF4 from its complexes with heparin. The large protamine-heparin complexes which formed also contained PF4 but could not activate fresh ATIII as has been demonstrated with protamine-heparin complexes without PF4. On incubation of the protamine-PF4-neutralized, heparinized plasmas for 5 hours at 37 degrees C, the large complexes were broken down but no active heparin appeared. The results of these experiments may have some bearing on the amount of protamine needed for the neutralization of heparin following extracorporeal bypass procedures, when large amounts of PF4 may have been released from activated or disrupted platelets.