Inhibition of myeloperoxidase by synovial fluid and serum

An inhibitor of myeloperoxidase has been identified in the synovial fluids and sera from patients with rheumatoid arthritis and sera from normal subjects. Initially, these fluids were found to inhibit stimulus induced degranulation of polymorphonuclear leucocytes independently of the stimulating agent. Subsequently, the fluids were shown to inhibit the released enzyme rather than the degranulation response of polymorphonuclear leucocytes. Both rheumatoid and normal serum samples contained high concentrations of the inhibitor but the concentrations were lower in rheumatoid synovial fluids. The inhibitory activity seemed to be specific for peroxidase as the fluids did not inhibit beta-glucuronidase activity. A protein of relative molecular mass (Mr) 150 kd was purified from synovial fluid by affinity chromatography on myeloperoxidase-Sepharose. It is concluded that serum and synovial fluid contain a novel myeloperoxidase inhibitor, which acts by binding to myeloperoxidase and thereby prevents myeloperoxidase releasing oxidative products in serum.