Neutralization of heparin by prothrombin activation products

The neutralization of heparin by active site blocked meizothrombin and thrombin, prothrombin fragment 1.2, fragment 1 and fragment 2 was probed by the heparin-dependent factor Xa inactivation by antithrombin III (AT III). Meizothrombin had no effect on the inactivation of factor Xa, whereas thrombin had an inhibitory effect (IC50 = 700 nM). After factor Xa catalyzed cleavage of meizothrombin, the resulting products, prothrombin fragment 1.2 plus thrombin, did not show any heparin neutralizing properties. However, after isolation of the reaction products, both thrombin and prothrombin fragment 1.2 exhibited heparin neutralizing properties in the factor Xa inactivation reaction. The IC50-values were 700 nM and 100 nM, respectively. Prothrombin fragment 1, when present at 125 nM, caused a 50% reduction of the heparin-dependent rate of inactivation of factor Xa and prothrombin fragment 2 had no effect at all. From this we conclude that, in addition to the thrombin part of the prothrombin molecule, the fragment 1 region also exhibits a rather high affinity for heparin.