Comparison between enzymatic activities of peptidylarginine deiminases 2 and 4. Enzyme-linked immunosorbent assay plates were coated with 1.0 μg/ml human fibrinogen. (A) Recombinant human peptidylarginine deiminase 2 (rhPAD2) or rhPAD4 raised in-house were used for citrullination at mass concentrations ranging from 0.1 ng/ml to 15 μg/ml. (B) Commercially available ModiQuest (MQ) PAD2 or MQPAD4 enzymes were used in units ranging from 0.1 mU/ml to 100 mU/ml. Following a 3-hour citrullination period, the anti-cFib mAb (0.5 μg/ml) was used for detection of citrullinated fibrinogen. Duplicate measurements of optical density (OD) at 490 nm are shown as mean and range.