Sensitive method to identify and characterize proteinases in situ after SDS-PAGE

Cells and body fluids contain numerous, different proteinases; to identify and characterize them are both important and difficult tasks. Especially difficult to identify and characterize are highly specific proteinases. Here, we present an extremely sensitive and quantitative method to characterize proteinases fractionated by SDS-PAGE that cleave specific rhodamine-based fluorogenic substrates. To test the sensitivity of the technique, we used trypsin as our model system. Filter paper impregnated with rhodamine-based fluorogenic substrates was placed on a gel, and bands of fluorescence originating from specific proteinases were visualized in real time. The method is very sensitive; picogram amounts of trypsin can be detected. The method should be very general, in that even proteinases whose substrates require amino acids C-terminal to the cleavage site may be identified and characterized. The results allow one to obtain not only information on the substrate specificity of a specific enzyme but also information about its molecular weight.