SINGLE-CHAIN UROKINASE-TYPE PLASMINOGEN-ACTIVATOR BOUND TO ITS RECEPTOR IS RELATIVELY RESISTANT TO PLASMINOGEN-ACTIVATOR INHIBITOR TYPE-1

Urokinase-type plasminogen activator (UPA) is synthesized as single-ch ain protein (scuPA) with little intrinsic activity, scuPA is activated when it is converted to two-chain urokinase (tcuPA) by plasmin or whe n it binds as a single-chain molecule to its cellular receptor (uPAR). Previous data indicate that complexes between scuPA and its receptor have somewhat higher affinity for plasminogen than does tcuPA, The cur rent study indicates that plasminogen activator activity of scuPA boun d to recombinant, soluble uPAR (suPAR) is also fivefold less sensitive to inhibition by plasminogen activator type 1 (PAI-1) than is soluble or receptor-bound tcuPA, Binding of PAI-1 to suPAR/scuPA complexes is totally reversible and can be overcome by increasing the concentratio n of plasminogen, suggesting a competitive mechanism of inhibition (K- i = 18 nmol/L). Binding of scuPA to suPAR also retards its cleavage by plasmin. These results indicates that binding of single-chain urokina se to its receptor promotes its activity, retards its inhibition, and protects it from conversion to a two-chain form of the enzyme, a step that may precede its inactivation and clearance from cell surfaces, Th ese results are consistent with a physiologic role for receptor-bound single-chain urokinase as a cellular plasminogen activator. (C) 1996 b y The American Society of Hematology.