Thrombin Inhibits the Anti-Myeloperoxidase and Ferroxidase Functions of Ceruloplasmin: Relevance in Rheumatoid Arthritis

Human ceruloplasmin (CP) is a multifunctional copper-binding protein produced in the liver. CP oxidizes Fe2+ to Fe3+, decreasing the concentration of Fe2+ available for generating harmful oxidant species. CP is also a potent inhibitor of leukocyte myeloperoxidase (MPO) (Kd=130nM), a major source of oxidants in vivo. Rheumatoid arthritis (RA) is an inflammatory autoimmune disease affecting flexible joints and characterized by activation of both inflammatory and coagulation processes. Indeed, the levels of CP, MPO and thrombin are markedly increased in the synovial fluid of RA patients. Here we show that thrombin cleaves CP in vitro at 481Arg-Ser482 and 887Lys-Val888 bonds, generating a nicked species that retains the native-like fold and the ferroxidase activity of the intact protein, whereas the MPO inhibitory function of CP is abrogated. Analysis of the synovial fluid of 24 RA patients reveals that CP is proteolytically degraded to a variable extent, with a fragmentation pattern similar to that observed with thrombin in vitro, and that proteolysis is blocked by hirudin, a highly potent and specific thrombin inhibitor. Using independent biophysical techniques, we show that thrombin has intrinsic affinity for CP (Kd = 60-270nM), independently of proteolysis, and inhibits CP ferroxidase activity (KI = 220±20nM). Mapping of thrombin binding sites with specific exosite-directed ligands (i.e. hirugen, fibrinogen ′-peptide) and thrombin analogues having the exosites variably compromised (i.e. prothrombin, prethrombin-2, T-thrombin), reveals that the positively charged exosite-II of thrombin binds to the negatively charged upper region of CP, while the protease active site and exosite-I remain accessible. These results suggest that thrombin can exacerbate inflammation in RA by impairing via proteolysis the MPO inhibitory function of CP and by competitively inhibiting CP ferroxidase activity.