The gene for peptidylarginine deiminase 4 (PADI4) has been found to be closely associated with rheumatoid arthritis (RA). Peptidylarginine deiminase (PADI) catalyses the post-translational modification of peptidylarginine to citrulline, a reaction known as citrullination. PADI extracted from rabbit muscle has been reported to citrullinate antithrombin, a principal plasma inhibitor of thrombin. Thrombin is known to induce angiogenesis, fibrin formation and inflammation, the primary events of the RA joint. Here, we investigate whether human PADI4 can inhibit antithrombin by catalysing antithrombin citrullination and how the enzyme is involved in RA pathogenesis. Antithrombin was incubated with recombinant PADI4 protein, and the inactivation of antithrombin was determined by reduction of its thrombin-inhibiting activity. Citrullination of antithrombin was detected by western blotting and enzyme-linked immunosorbent assay (ELISA). In addition, the citrullination level, activity and concentration of antithrombin in RA plasma were investigated by sandwich ELISA. Incubation of antithrombin with PADI4 resulted in loss of thrombin-inhibitory activity and in citrullination of antithrombin. RA plasma showed higher levels of citrullinated antithrombin than controls with non-arthritis disease and healthy individuals. The results indicate that PADI4 could inactivate antithrombin through citrullination. The abnormal expression or activation of PADI4 in RA synovium is suggested to be responsible for the high level of citrullinated antithrombin in RA plasma. Local inhibition of antithrombin activity in RA synovium might lead to the excessive angiogenesis, fibrin deposition and inflammation of the tissue.
