Microplate immunocapture assay for plasminogen activators and their specific inhibitors

We report a convenient sensitive enzyme activity assay for urokinase and tissue-type plasminogen activators, based on a solid-phase microtitre plate method using readily available polyclonal antibodies. The sensitivities for urokinase (active and proenzyme) and tissue activator were better than 1 ng/ml. The specificity was very high, with no significant contribution of urokinase in tissue activator assays or vice versa. This method is particularly useful for the assay of urokinase proenzyme in samples containing inhibitors. We describe how this assay may also be used to measure specific inhibitors of plasminogen activators, making use of their rapid formation of stable complexes with solid-phase activator. Inhibitors may be assayed in samples containing proenzymes.