α2-Macroglobulin from Rheumatoid Arthritis Synovial Fluid: Functional Analysis Defines a Role for Oxidation in Inflammation

A hallmark of inflammation is the release of oxidants, proteinases, and cytokines, all important mediators of the inflammatory cascade. α2-Macroglobulin (α2M) is a high-affinity, broad-specificity proteinase inhibitor that also binds and regulates the biological activities of a number of cytokines. We demonstrated recently that hypochlorite-oxidized α2M has decreased ability to inhibit proteinases and regulate cytokines in vitro. The role of oxidation in regulating α2M functions in vivo is largely unknown. To determine the extent and biological consequence of in vivo α2M oxidation, we measured the degree of oxidative α2M modification from rheumatoid arthritis (RA) synovial fluid and compared this with osteoarthritis (OA) as noninflammatory controls. We found that RA synovial fluid α2M is significantly more oxidized than that from OA. RA synovial fluid also contains a twofold higher median α2M level than OA, while having only half the α2M-proteinase inhibitory activity. Detailed biochemical analysis demonstrates proteolytically degraded α2M in RA greater than in OA synovial fluid. Additionally, the hypochlorite-mediated oxidation product, chlorotyrosine, is present in RA more than in OA or plasma α2M samples. Taken together, these findings confirm a role for oxidative regulation of inflammation by altering the functions of extracellular mediators such as α2M.