Factor Xa acting in the prothrombinase complex is profoundly protected from inhibition by antithrombin III, both in the presence and absence of heparin.
We have determined the rate constants of inactivation of factor Xa and thrombin by antithrombin III/heparin during the process of prothrombin activation. The second-order rate constant of inhibition of factor Xa alone by antithrombin III as determined by using the synthetic peptide substrate S-2337 was found to be 1.1 X 10(6) M-1 min-1. Factor Xa in prothrombin activation mixtures that contained prothrombin, and either saturating amounts of factor Va or phospholipid (20 mol % dioleoylphosphatidylserine/80 mol % dioleoylphosphatidylcholine, 10 microM), was inhibited by antithrombin III with a second-order rate constant that was essentially the same: 1.2 X 10(6) M-1 min-1. When both factor Va and phospholipid were present during prothrombin activation, factor Xa inhibition by antithrombin III was reduced about 10-fold, with a second-order rate constant of 1.3 X 10(5) M-1 min-1. Factor Xa in the prothrombin activation mixture that contained both factor Va and phospholipid was even more protected from inhibition by the antithrombin III-heparin complex. The first-order rate constants of these reactions at 200 nM antithrombin III and normalized to heparin at 1 microgram/mL were 0.33 and 9.5 min-1 in the presence and absence of factor Va and phospholipid, respectively. When the prothrombin concentration was varied widely around the Km for prothrombin, this had no effect on the first-order rate constants of inhibition. It is our conclusion that factor Xa when acting in prothrombinase on prothrombin is profoundly protected from inhibition by antithrombin III in the absence as well as in the presence of heparin.(ABSTRACT TRUNCATED AT 250 WORDS)
Lindhout et al. (Tue,) studied this question.