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

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Abstract
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 ocurs with a comparable affinity (Km 0.40–0.77 μM), while the catalytic rate constant (kcat) is comparable for Glu-plasminogen (0.0022 s−1) and low-Mr plasminogen (0.0034s−1), but is somewhat higher for Lys-plasminogen (0.0106s−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 histidinerich 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.