Studies of Heparin Affinity to Antithrombin III and Other Proteins in Vitro and in Vivo

Effect of thrombin and endotoxin on the in vivo metabolism of antithrombin III (AT III) in dogs

Effect of thrombin and endotoxin on the metabolism of I-125-labelled canine AT III was studied in mongrel dogs. Under control condition, mean total amount of intravascular AT III with standard deviation was 23.4 +/- 2.4 mg/kg, plasma half life of i.v. injected I-125-AT III was 1.7 +/- 0.2 days, and the fractional catabolic flux (j3x) was 16.3 +/- 1.6 mg/kg/day. The total amount of intra- and extra-vascular AT III was 36.0 +/- 0.34 mg/kg. Neither a 3 hour infusion of a small dose (30 units/kg/hr) of thrombin nor i.v. injection of a large amount of thrombin (5,000-15,000 units/day) with heparin significantly affected AT III metabolism except for a transient decrease in AT III concentration in the latter case, although decrease in plasma fibrinogen concentration and platelet count was observed in both cases. Two injections with 200 micrograms/kg of endotoxin resulted in an evident acceleration of AT III metabolism with significant decrease in the plasma AT III, fibrinogen concentrations and platelet count. More marked changes in AT III metabolism were induced by a single infusion with 1 mg/kg of endotoxin. Changes in hemostatic system coincided with those observed in DIC.

Fibrinolytic action of a new semi-synthetic polysaccharide sulfate, galactan polysulfate (DH 6322), in the rat

Galactan polysulfate, DH 6322, when added to rat plasma, shortened euglobulin clot lysis time through both increase in the recovery of plasminogen activator activity in euglobulin precipitates and direct acceleration of fibrinolysis. DH 6322 treatment also resulted in accelerating plasma clot lysis induced by urokinase. The effect of DH 6322 was further studied with a purified system composed of bovine fibrinogen and plasminogen. In this system, DH 6322 was found to be a potent stimulator of both fibrinolysis and fibrinogenolysis induced by urokinase. The activation process of plasminogen was not affected by DH 6322 even in the presence of fibrin or fibrinogen. The accelerating effect of DH 6322 on fibrinolysis in the purified system was scarcely affected by addition of plasma antifibrinolytic factors. DH 6322 also stimulated fibrin clot lysis induced by a low concentration of purified plasmin in place of plasminogen and urokinase, and its accelerating effect was not observed in the presence of tranexamic acid. Formation of fibrin clot susceptible to generated plasmin was observed at relatively high concentrations of DH 6322 above 80 micrograms/ml. Additional experiments indicated that DH 6322 increased the amount of plasminogen bound to fibrin. These results suggested that DH 6322 stimulated fibrinolysis through increasing the binding of plasminogen or plasmin to fibrin.

The interaction of protein-bound heparin and antithrombin III

When commercially prepared porcine mucosal heparin is added to human plasma, some of the heparin fractions form a complex with antithrombin III activating it to an immediate inhibitor of thrombin as well as of other serine proteases. Certain fractions of heparin may complex with other proteins such as alpha 2-macroglobulin, another progressive inhibitor of thrombin. Without complexing with antithrombin III, this protein-bound heparin fraction(s) still retains the capacity to activate it to an immediate inhibitor of thrombin. Protamine sulfate inactivates those heparin fractions that bind to antithrombin III but not those bound to alpha 2-macroglobulin. Activated antithrombin III may undergo a molecular change in the presence of protamine which not only changes it back to a progressive inhibitor but makes it resistant to activation by the protein-bound heparin fraction(s). However, it can still be reactivated by other heparin fractions in fresh whole heparin. The observations presented may help explain heparin "rebound" in patients believed adequately neutralized with protamine.