Plasmin Binding to the Plasminogen Receptor Enhances Catalytic Efficiency and Activates the Receptor for Subsequent Ligand Binding.

Specific cell surface receptors for plasminogen (Pg) are expressed by a wide variety of cell types. The colocalization of receptors for Pg and its activators restricts plasmin (Pm) activity to specific sites and serves to promote fibrinolysis and local Pg activation. These studies show that both Pg and Pm bind to cellular receptors on monocytoid U937 cells. Limited Pm pretreatment of the cells enhances total Pg binding and alters the kinetics of Pm binding. Furthermore,surface-bound Pg is converted to Pm in the absence of exogenous activators. Cell-bound Pm exhibits a 12-fold increase in catalytic efficiency (kcat/Km) relative to Pm free in solution. These studies demonstrate that Pg/Pm receptor occupancy can be regulated by Pm in the microenvironment and may play a significant regulatory role in fibrinolysis and extravascular proteolysis.