Conjugated Polymer-Based Real-Time Fluorescence Caspase Assays

We have developed conjugated polyelectrolyte-based fluorescence turn-on assays for caspase 3 and 8. These assays are composed of a cationic polyphenylene ethynylene polymer PPE4+ and p-nitroaniline modified caspase peptide substrate. The fluorescence of the assay is initially turned-off because of the efficient quenching of the polymer by p-nitroaniline moiety on anionic peptide substrates. A turn-on effect is observed due to the cleavage of the peptide by the enzyme and formation of the neutral p-nitroaniline unit which has no quenching on the polymer. We validated this assay design and obtained kinetic parameters of caspase 3 and caspase 8. These assays demonstrated good sensitivity as in pmol/L (0.1 units/mL) for caspase 3 and nmol/L (0.2 units/mL) for caspase 8. This method also showed high specificity by using caspase 3 assay as a model system and the results demonstrated that other proteases including caspase 8, papain, pepsin, and trypsin did not show observable fluorescence turn-on effect. The dose–response curve of a caspase inhibitor Z-VAD-FMK was evaluated by caspase 3 assay, by which the IC50 value was determined to be 0.73 μM and was in a good agreement with the literature reported value at 0.62 μM. This design could be applied into the in vitro screening of small molecular inhibitors for drug discovery.