Higher levels of protein carbonyls and 3-chlorotyrosine were detected in synovial fluid, and other types of RA samples, along with increased activity of myeloperoxidase, which catalyzes the reaction between hydrogen peroxide (H2O2) and chloride anion (Cl-) producing hypochlorous acid (HOCl) that can cause oxidative damage in the host tissue. Here, MPO is linked to rheumatoid arthritis.