Single chain urokinase. Augmentation of enzymatic activity upon binding to monocytes.

Cellular migration typically requires cell surface-associated urokinase-type plasminogen activator (u-PA) and plasminogen, both of which are present as proenzymes. Because each active enzyme can activate the other zymogen, the mechanism by which the initial proteolytic event of this two-zymogen system occurs is unclear. A mutant of single chain u-PA that could not be cleaved to the more active two-chain u-PA was used to demonstrate that (i) u-PA in its single-chain form exhibits a reactive active site serine, (ii) the enzymatic activity of this molecule is augmented 100-fold upon binding to the u-PA receptor on monocytes as compared with the enzymatic activity of the same number of molecules in the fluid phase, and (iii) the molecule thus bound and active remains in the single-chain form. This is likely an important mechanism for the initiation and control of cell surface-associated fibrinolysis.