A highly sensitive chromogenic microtiter plate assay for plasminogen activators which quantitatively discriminates between the urokinase and tissue-type activators

A simple and highly sensitive chromogenic microtiter plate assay for plasminogen activators is described. The assay is based on plasmin cleavage of the synthetic tripeptide plasmin substrate H-D-norleucyl-hexahydrotyrosyllysine p-nitroaniline, which yields the yellow chromophore p-nitroanilide. Production of the latter compound is then quantitated spectrophotometrically at 405 nm on an ELISA plate reader. Linearity of the assay can be achieved over at least four orders of magnitude in a single experiment (0.01–100 milliPloug units) with appropriate incubation times. Capitalizing on tissue-type plasminogen activator's dependence on fibrin for enzymatic activity, the selective use of soluble fibrin products allows discrimination between urokinase and tissue-type activator. The utility of this aspect of the assay for the analysis of complex samples containing both types of plasminogen activitors is demonstrated.