Direct detection of cysteine peptidases for MALDI-TOF MS analysis using fluorogenic substrates.

A method is described for the direct detection of unstable cysteine peptidase activity in polyacrylamide gels after native electrophoresis using new selective fluorogenic peptide substrates, pyroglutamyl-phenylalanyl-alanyl-4-amino-7-methylcoumaride (Glp-Phe-Ala-AMC) and pyroglutamyl-phenylalanyl-alanyl-4-amino-7-trifluoromethyl-coumaride (Glp-Phe-Ala-AFC). The detection limit of the model enzyme papain was 17 pmol (0.29 µg) for Glp-Phe-Ala-AMC and 43 pmol (0.74 µg) for Glp-Phe-Ala-AFC, with increased sensitivity and selectivity compared to the traditional method of protein determination with Coomassie G-250 staining or detection of activity using chromogenic substrates. Using this method, we easily identified the target digestive peptidases of Tenebrio molitor larvae by matrix assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS) analysis. The method offers simplicity, high sensitivity, and selectivity compared to traditional methods for improved identification of unstable cysteine peptidases in multi-component biological samples.