IMPORTANCE OF FACTOR-XA IN DETERMINING THE PROCOAGULANT ACTIVITY OF WHOLE-BLOOD CLOTS

The binding of thrombin to fibrin is thought to be an important mechan ism by which thrombi exhibit procoagulant activity; however, the exten t to which other procoagulants are associated with thrombi has not bee n previously defined. This study was designed to determine whether clo tting factors other than thrombin are bound to whole-blood clots and c an thereby con tribute to significant procoagulant activity. Clots for med in vitro from human blood exhibited minimal thrombin activity when incubated in plasma depleted of vitamin K-dependent factors by barium -citrate adsorption, as indicated by increases in the concentration of fibrinopeptide A (FPA), a marker of fibrin formation, to 72 nM after 30 min. Incubation of clots in barium-absorbed plasma repleted with 0. 9 muM human prothrombin under the same conditions resulted in marked i ncreases in the concentration of FPA (> 1,000 nM) and clotting by 30 m in. The increases in FPA were attributable to activation of the added prothrombin by clot-associated Factor Xa, judging from concomitant inc reases in the concentration of prothrombin fragment 1.2. Similar resul ts were obtained with thrombi induced in the axillary arteries of dogs by vascular injury and incubated with plasma in vitro. Activation of prothrombin was inhibited in a dose-dependent manner by tick anticoagu lant peptide, a direct inhibitor of Factor Xa, at concentrations of 0. 5-5.0 muM. Clot-associated Factor Xa activity was resistant to inhibit ion by anti-thrombin III, judging from the lack of inhibition of proth rombin activation during incubation of clots in plasma containing hepa rin pentasaccharide, an anti-thrombin III-mediated inhibitor of Factor Xa. Thus, the activity of Factor Xa appears to be an important determ inant of the procoagulant activity of whole-blood clots and arterial t hrombi, and is resistant to inhibition by anti-thrombin III-dependent inhibitors.