Interaction of heparin with plasminogen activators and plasminogen: effects on the activation of plasminogen.

The amidolytic plasmin activity of a mixture of tissue plasminogen activator (tPA) and plasminogen is enhanced by heparin at therapeutic concentrations. Heparin also increases the activity in mixtures of urokinase-type plasminogen activator (uPA) and plasminogen but has no effect on streptokinase or plasmin. Direct analyses of plasminogen activation by polyacrylamide gel electrophoresis demonstrate that heparin increases the activation of plasminogen by both tPA and uPA. Binding studies show that heparin binds to various components of the fibrinolytic system, with tight binding demonstrable with tPA, uPA, and Lys-plasminogen. The stimulation of tPA activity by fibrin, however, is diminished by heparin. The ability of heparin to promote plasmin generation is destroyed by incubation of the heparin with heparinase, whereas incubation with chondroitinase ABC or AC has no effect. Also, stimulation of plasmin formation is not observed with dextran sulfate or chondroitin sulfate A, B, or C. Analyses of heparin fractions after separation on columns of antithrombin III-Sepharose suggest that both the high-affinity and the low-affinity fractions, which have dramatically different anticoagulant activity, have similar activity toward the fibrinolytic components.