Calcium dependency of peptidylarginine deiminases. Enzyme-linked immunosorbent assay plates were coated with 1.0 μg/ml human fibrinogen and incubated with four different recombinant human peptidylarginine deiminase (rhPAD) preparations: ModiQuest (MQ) PAD2 (8 mU/ml), rhPAD2 (42 ng/ml), rhPAD4 (1,500 ng/ml) or MQPAD4 (49 mU/ml). Dithiothreitol was added at a concentration of 1.0 mM, and CaCl2 was added at concentrations ranging from 50 μM to 10 mM. Following 3 hours of incubation, citrullination was measured using mouse anti-cFib monoclonal antibody (0.5 μg/ml). Shown is the activity as a percentage of maximal activity for each enzyme. Symbols and error bars represent means and ranges of duplicate measurements.