A simple but efficient electrochemical method to assay protein arginine deiminase 4

Nowadays, protein arginine deiminase 4 (PAD4) has become a potential therapeutic target for human diseases, especially in rheumatoid arthritis (RA) patients. In this paper, we have reported a simple but efficient electrochemical method to assay protein arginine deiminase 4 (PAD4) activities and screen its inhibitors. The electropositive peptide monolayer can prevent [Ru(NH3)5Cl]2+ from approaching to the electrode surface due to the strong electrostatic repulsion. After PAD4 catalyzes the citrullination of arginine within substrate peptide, the signal molecules can be much closer to the electrode surface for the reduced positive charges on the peptide monolayer, thereby leading to an obvious electrochemical response. By tracing the electrochemical response of [Ru(NH3)5Cl]2+, our method has displayed satisfactory sensitivity and specificity toward PAD4 assay with a low detection limit of 3.5 pM. Moreover, the electrochemical response has been found to decrease with the addition of the potent PAD4 inhibitor Cl-amidine, suggesting the potential application of our method for the inhibitor screening in the future.