Studies of the Anti-Xa Activity of Human Plasma and Purified Antithrombin III
When samples of purified antithrombin (At III) were compared to plasma at the same At III concentration, in the absence of heparin, the anti-Xa activity of plasma was considerably higher. In the presence of heparin the anti-Xa activity of purified At III was much greater than plasma. This was shown to be due to an inhibitory effect on the heparin. At III-Factor Xa interaction in plasma which could be removed by absorption with aluminium hydroxide [Al(OH)3]. This inhibition was dependent on the molecular weight of the heparin; low molecular weight heparin was inhibited less than high molecular weight heparin, and this probably accounts for the apparently high anti-Xa activity of low molecular weight heparin.A1(OH)3 absorption of plasma also increased its anti-Xa activity in the absence of heparin. Addition of Factor IX concentrate to the absorbed plasma reduced its anti-Xa activity to that of normal plasma, and studies with purified proteins showed that this effect was due to the prothrombin in the concentrate. The addition of Factor IX concentrate or prothrombin to purified At III did not affect its anti-Xa activity.These results suggest that, in addition to At III, there is another Xa-inhibitor in plasma which competes with prothrombin for binding of Factor Xa.