Immunological detection of myeloperoxidase in synovial fluid from patients with rheumatoid arthritis.

We have used rocket immunoelectrophoresis and immunoblotting to detect myeloperoxidase in synovial fluid from patients with rheumatoid arthritis. This protein was enzymatically inactive but its identity as myeloperoxidase was confirmed by comparing its subunit structure with that of the purified enzyme. When neutrophils were stimulated to secrete myeloperoxidase in vitro, a polypeptide with an apparent molecular mass of 62 kDa was detected extracellularly by immunoblotting. Neutrophils isolated from synovial fluid showed a reduced level of this 62 kDa polypeptide but it was detected extracellularly in synovial fluid by immunoblotting. Thus, we conclude that neutrophils in synovial fluid from patients with rheumatoid arthritis have been activated in vivo to secrete myeloperoxidase and propose that the products of this enzyme system can contribute to the tissue damage associated with this disease.