Prothrombin protects factor Xa in the prothrombinase complex from inhibition by the heparin-antithrombin complex

Heparin is a commonly used anticoagulant drug. It functions primarily by ac celerating the antithrombin inhibition of coagulation proteinases, among wh ich factor Xa and thrombin are believed to be the most important targets. T here are conflicting results as to whether anticoagulant heparins can catal yze the antithrombin Inhibition of factor Xa in the prothrombinase complex (factor Xa, negatively charged membrane surfaces, and calcium ion), which i s the physiologically relevant form of the proteinase responsible for the a ctivation of prothrombin to thrombin during the blood coagulation process. In this study, a novel assay system was developed to compare the catalytic effect of different molecular-weight heparins in the antithrombin inhibitio n of factor Xa, either in free form or assembled into the prothrombinase co mplex during the process of prothrombin activation. This assay takes advant age of the unique property of a recombinant mutant antithrombin, which, sim ilar to the wild-type antithrombin, rapidly inhibits factor Xa, but not thr ombin, in the presence of heparin. A direct prothrombinase inhibition assay , monitoring thrombin generation under near physiological concentrations of prothrombin and antithrombin in the presence of therapeutic doses of low- and high-molecular-weight heparins, indicates that factor Xa in the prothro mbinase complex is protected from inhibition by antithrombin more than 1000 times, independent of the molecular size of heparin. (Blood. 2001;97:2308- 2313) (C) 2001 by The American Society of Hematology.