Kinetic analysis of the effects of heparin and lipoproteins on tissue plasminogen activator mediated plasminogen activation

Heparin sulfate and the less sulfated glycosaminoglycan heparan sulfate enhance human plasminogen (Pg) conversion to plasmin by tissue-type plasminogen activator (t-PA). Kinetic studies indicate that both heparin and heparan increase the kcat of t-PA-mediated Pg activation by 25- and 3.5-fold, respectively. The Km of plasmin formation is unaltered by the presence of either heparin or heparan. Both heparin and heparan stimulate the activity of t-PA by interacting with the finger domain of t-PA, with association constants of 1 microM and 200 nM, respectively. Additionally, the lipoproteins lipoprotein(a) [Lp(a)] and low-density lipoprotein (LDL) inhibit the heparin enhancement of Pg activation. Lp(a) is a competitive inhibitor and LDL is a mixed inhibitor of t-PA-mediated Pg activation, with inhibition constants of 30 and 70 nM, respectively. The inhibition constants correspond to physiologic concentrations of these lipoproteins. These data suggest that heparin, heparan, and lipoproteins may play an important in vivo role in regulating cell surface associated activation of the fibrinolytic system.