Development of a highly sensitive fluorescence probe for peptidyl arginine deiminase (PAD) activity

Peptidyl arginine deiminases (PADs) catalyze the post-translational deimination of arginine residues to citrulline residues. Aberrant levels of PAD activity are associated with various diseases, such as rheumatoid arthritis, Alzheimer's disease, and multiple sclerosis, so there is a need for simple and convenient high-throughput screening systems to discover PAD inhibitors as candidate therapeutic agents. Here, we report a highly sensitive off/on-type fluorescence probe for PAD activity based on the donor-excited photoinduced electron transfer (d-PeT) mechanism, utilizing the specific cycloaddition reaction between the benzil group of the probe and the ureido group of the PAD product, citrulline, under acidic conditions. We synthesized and functionally evaluated a series of probes bearing substituents on the benzil phenyl group, and found that 4MEBz-FluME could successfully detect citrulline with higher sensitivity and broader dynamic range than our previously reported fluorescence probe, FGME. Moreover, we succeeded in establishing multiple assay systems for PAD subtypes activities, including PAD2 and PAD4, with 4MeBz-FluME thanks to its high sensitivity. We expect that our fluorescence probes will become a powerful tool for discovering PAD inhibitors of several subtypes. Thus, it should be suitable for high-throughput screening of chemical libraries for inhibitors of PADs.