ACTIVATION OF THROMBIN-INACTIVATED SINGLE-CHAIN UROKINASE-TYPE PLASMINOGEN-ACTIVATOR BY DIPEPTIDYL PEPTIDASE-I (CATHEPSIN-C)

Single-chain urokinase-type plasminogen activator (scu-PA) is inactiva ted by thrombin, which cleaves the peptide bond between Arg156 and Phe 157. In a search for potential activators of thrombin-cleaved two-chai n urokinase-type plasminogen activator (tcu-PA/T), we found that the l ysosomal aminopeptidase dipeptidyl-peptidase I or cathepsin C efficien tly activates tcu-PA/T. Cathepsin C was as active towards tcu-PA/T as the bacterial proteinase thermolysin and about 300-times more active t han plasmin. The activation by cathepsin C proceeded in a concentratio n-dependent and time-dependent manner with a pH optimum between 5 and 7. Furthermore, the effect of cathepsin C was inhibited by cystatin an d stimulated by cysteine, typical for the action of a thiol proteinase . As no degradation of the tcu-PA/T molecule by cathepsin C was visibl e on SDS/PAGE, we suggest that activation of tcu-PA/T occurs by cleava ge between Lys158-Ile159 and removal of the two N-terminal amino acid residues (Phe157-Lys158) of the B chain of tcu-PA/T. We conclude that both thrombin and dipeptidyl-peptidases like cathepsin C might play a regulatory role in the plasminogen-plasmin system by inactivating scu- PA and activating tcu-PA/T, respectively.