Molecular weight dependency of the heparin potentiated inhibition of thrombin and activated factor X. Effect of heparin neutralization in plasma

Heparin purified by affinity chromatography on antithrombin III-Sepharose was fractionated according to molecular size by gel filtration. The various fractions obtained were studied with respect to their ability to potentiate the inhibition of thrombin and of Factor Xa in plasma and in a purified system containing antithrombin III. Clotting assays and chromogenic peptide substrate assays were used in parallel. The inhibition of thrombin showed a dependency on the molecular size of the heparin that was similar to that shown earlier for the APTT assay. The inhibition of Factor Xa showed a different dependency, indicating that the mechanisms for heparin-potentiated inhibition of thrombin and Factor Xa by antithrombin III are not entirely the same. The inhibition of thrombin and Factor Xa in plasma differed from that in the pure system and this difference was shown to be dependent on heparin neutralization effects.