Dependence of the rate of thrombin/antithrombin III reaction upon the turnover rate of a catalytic amount of heparin.

Commercially available heparin preparations slightly enhanced the rate of thrombin/antithrombin (AT) III reaction at pH 6.05 in the absence of NaCl. However, this accelerative activity was significantly lower than that induced by heparin with high affinity for AT III (HA-heparin), probably due to the formation of the binary complexes of HA-heparin-AT III as well as that composed of thrombin and heparin with low affinity for AT III (LA-heparin). The HA-heparin-catalyzed thrombin/AT III reaction was faster in the presence of 0.1 M NaCl at pH 6.05 than that in the absence of the salt. LA-heparin and dextran sulfate (DS) were also found to accelerate the thrombin/AT III reaction rate, but neither substance catalyzed the formation of the complex in the presence of 0.1 M NaCl at pH 7.4. LA-heparin was also confirmed to compete with HA-heparin for enhancement of the thrombin/AT III reaction. Thus, it appears that AT III tends to form a ternary complex with the thrombin-DS or thrombin-LA-heparin complex, even in the presence of 0.1 M NaCl, whereas factor Xa reacts with the AT III-DS or AT III-LA-heparin complex. These results indicate that HA-heparin is the only substance having the ability to catalyze the thrombin/AT III reaction, and that its turnover rate is markedly elevated in the presence of strongly electropositive and electronegative ions because of the decreased affinity of the enzyme for heparin under such conditions.