Identification of protease inhibitors by a fast fluorimetric assay

Anomalous protease activities are associated with many diseases. Great efforts are paid for selecting specific protease modulators for therapeutic approaches. We have selected new modulators of enzyme activity by an homogeneous assay based on a doubly labeled small peptide used as substrate of trypsin. The substrate incorporates the fluorophore 5-[(2-aminoethyl)amino]naphthalene-1-sulfonic acid (EDANS) at one end and an EDANS-quenching moiety (Dabcyl, (4-(4-dimethylaminophenylazo)-benzoic acid)) on the other end. Following cleavage by trypsin, the peptide-EDANS product is released interrupting the fluorescence resonance energy transfer effect and yielding bright fluorescence, which can be detected using excitation wavelengths at 335-345 nm and emission wavelengths at 485-510 nm. The method optimized, tested by detecting the strong inhibiting effect of α1-antitrypsin on trypsin activity, has been developed on 384 multi-well plates in a volume of 10 μL, using an automated platform. From the screening of a chemical library, four compounds that inhibit trypsin activity with IC50s in the micromolar range have been identified. Interestingly, the most active compound (M4) shows a chemical structure recapitulating that of other more potent inhibitors with thiourea and halogenated centers. Molecular docking studies show that M4 is a competitive inhibitor recognizing most residues within or nearby the catalytic pocket.