Effects of Different Heparins on the Enhancement of the Thrombin- Antithrombin Reaction

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Abstract
Kinetic analyses were made of the thrombin/antithrombin III (ATIII) reaction in the presence of catalytic amounts of three kinds of mammalian HA-heparin (bovine, pig and whale), which have the same affinities for ATIII. In the absence of NaCl, the first-order rate constant was higher with whale HA-heparin than with the other two HA-heparins. However, the strength of the interactions of thrombin with the HA-heparins was in the order, bovine > Pig > whale. Thus, the slow reactions in the case of bovine and pig HA-heparins were probably due to preferential binding of the two HA-heparins to thrombin rather than to ATIII, thus causing the HA-heparins to be inhibitory for the reaction by reducing the turnover rates of the polysachsarides as catalysts. In the presence of 0.15M NaCl, the first-order rate constants were slightly higher in the case of pig HA-heparin than of the other two HA-heparins. Since the strength of the interactions of these HA-heparins with thrombin was in the order, pig .gtoreq. bovine > whale, the faster reactions were probably due to higher associations of the enzyme with the essential HA-heparin-ATIII complex.

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