The activation of human [Glu1]plasminogen by human single-chain urokinase

The activation of human [Glu1]plasminogen ([Glu1]Pg) by single-chain human urokinase (SCUKase) displays a substantial lag phase at physiological levels of [Glu1]Pg. Employing a monoclonal antibody that exhibits a high level of specificity for SCUKase, as compared to two-chain urokinase (TCUKase), we have demonstrated conclusively that during this lag phase a progressive loss of SCUKase occurs, most likely resulting from its conversion to TCUKase, in a reaction catalyzed by plasmin (HPm). The overall activation of [Glu1]Pg by SCUKase is inhibited by physiological levels of Cl- and stimulated by epsilon-amino caproic acid. Kinetic studies demonstrate that both these effects are based on first, the reaction of [Glu1]Pg with the TCUKase that is formed during the activation, and, second, the concomitant rate at which HPm is provided for the conversion of SCUKase to TCUKase. The results indicate that at physiological levels of [Glu1]Pg, its activation in the presence of SCUKase is regulated in one manner by the rate at which SCUKase is converted to TCUKase, in a process that is strongly influenced by physiological levels of Cl-. Finally, and importantly, we show that SCUKase possesses very little, if any, inherent ability to activate [Glu1]Pg at a rate that influences the kinetics of HPm generation under physiological conditions of [Glu1]Pg and Cl- concentrations.