Peptidylarginine deiminase 4 (PADI4) is an enzyme that plays an important role in the organism. It mainly catalyzes the conversion of arginine residues in proteins into citrulline residues. PADI4 plays a key role in various physiological processes such as cell differentiation and immune response. The abnormal expression or change in its activity is closely related to autoimmune diseases like rheumatoid arthritis and systemic lupus erythematosus, making it an important target in the research of these related diseases.The activity of recombinant human PADI4 is determined by its ability to catalyze Benzoyl-arginine ethyl ester (BAEE) to form citrulline.The reaction was performed in 50 mM Tris, 0.1 M NaCl, 10 mM CaCl2, 5 mM DTT solution (Assay Buffer, pH 7.5). A total volume of 300 μL was achieved by adding 75 μL of 100 μg/mL PADI4 to 75 μL of 10 mM BAEE. After 30-minute incubation at 37°C, the reaction mixture was acidified with color-developing solution containing ammonium iron sulfate and measured at 540 nm .