Kinetic analysis of the effects of glycosaminoglycans and lipoproteins on urokinase-mediated plasminogen activation.

The glycosaminoglycans (GAGs) heparin, heparan sulphate and chondroitin 6-sulphate stimulate the rate of urokinase activation of human plasminogen. Kinetic analysis of plasminogen activation demonstrates that heparin, heparan sulphate and chondroitin 6-sulphate increased the catalytic rate (Kcat) by 5.3-, 3.5- and 2.5-fold respectively. These stimulatory GAGs had no effect on the affinity of urokinase for plasminogen, since the Km of the reaction is unaltered by the GAGs. The GAGs may enhance the rate of plasminogen activation through an interaction with the catalytic domain of the urokinase, with dissociation constants of approx. 30 nM. Additionally, the lipoproteins, lipoprotein (a) [Lp(a)] and low-density lipoprotein (LDL) inhibit heparin and heparan sulphate stimulation of plasmin formation. Lp(a) is a competitive inhibitor (Kic 20 nM) and LDL is a mixed inhibitor of heparin-enhanced urokinase-mediated plasminogen activation (Kic 24 nM and Kiu 60 nM). These inhibition constants correlate with physiological concentrations of these lipoproteins. These data suggest that these GAGs and lipoproteins may play an important role in vivo in regulating urokinase-mediated plasmin formation.