Amino-terminal fragment of urokinase-type plasminogen activator inhibits its plasminogen activation

The amino terminal fragment (ATF, Ser(1)-Lys(135)) of urokinase-type plasminogen activator (uPA) containing an epidermal growth factor-like (EGF) and kringle domain is critically involved in some important functions of uPA, such as receptor binding and chemotactic activity. In this report, the effect of ATF on single-chain uPA (sc-uPA) induced plasminogen activation was investigated. It was shown that sc-uPA-induced activation of Glu-plasminogen or Lys-plasminogen was significantly inhibited in the presence of ATF. In addition, sc-uPA activation to two-chain uPA (tc-uPA) by Lys-plasmin and plasminogen activation to plasmin by tc-uPA were both found to be inhibited by ATF. The inhibition of these activations was significantly attenuated but not diminished when ATF was pretreated with immobilized carboxypeptidase B (CPB), indicating that the C-terminal Lys(135) as well as internal Lys/Arg residue binding was involved in the mechanism. Kinetic analysis showed that sc-uPA activation by Lys-plasmin competitively inhibited by ATF and CPB pretreated ATF (CPB-ATF) with an inhibitory constant (K(i)) of 3.8+/-0.31 and 12.4 +/- 1.8 microM, respectively. In contrast to sc-uPA-induced Glu- or Lys-plasminogen activation, sc-uPA-induced mini-plasminogen activation, sc-uPA activation by mini-plasmin and mini-plasminogen activation by tc-uPA were not affected by ATF. These findings suggested that the inhibitory effects of ATF on sc-uPA activation by Lys-plasmin and Glu- or Lys-plasminogen activation by tc-uPA were related to the binding of ATF (by its C-terminal Lys(135) and internal Lys/Arg residue) with the kringle 1-4 of plasmin and plasminogen, respectively.