Characterization of the high-affinity interaction between human plasminogen and pro-urokinase

Activation of human Glu-plasminogen, Lys-plasminogen and low-Mr plasminogen (lacking lysine-binding sites) by pro-urokinase (pro-UK), obtained from a human lung adenocarcinoma cell line (Calu-3, ATCC), obeys Michaelis-Menten kinetics. Activation occurs with a comparable affinity (Km 0.40-0.77 microM), while the catalytic rate constant (kcat) is comparable for Glu-plasminogen (0.0022s-1) and low-Mr plasminogen (0.0034 s-1), but is somewhat higher for Lys-plasminogen (0.0106 s-1). The rate of activation of plasminogen by pro-UK is not significantly influenced by the presence of 6-aminohexanoic acid, purified fragments LBS I or LBS II or histidine-rich glycoprotein, indicating that the high affinity of pro-UK for plasminogen is not mediated via the high-affinity lysine-binding site of plasminogen located in kringles 1-3 (LBS I) nor via the low-affinity lysine-binding site comprised within kringle 4 (LBS II). The site(s) in plasminogen involved in the high-affinity interaction with pro-UK thus appear to be located within the low-Mr plasminogen moiety.