Mechanism of hypochlorite-mediated inactivation of proteinase inhibition by α2-Macroglobulin

The proteinase-proteinase inhibitor balance plays an important role in mediating inflammation-associated tissue destruction. alpha(2)-Macroglobulin (alpha(2)M) is a high-affinity, broad-spectrum proteinase inhibitor found abundantly in plasma and interstitial fluids. Increased levels of alpha 2(M) and proteinase-alpha(2)M complexes can be demonstrated in patients with sepsis, emphysema, peridontitis, rheumatoid arthritis, and other inflammatory diseases. Despite these increased levels, proteolysis remains a significant problem. We hypothesized that a mechanism for inactivating alpha(2)M-mediated proteinase inhibition must exist and recently demonstrated that alpha(2)M isolated from human rheumatoid arthritis synovial fluid is oxidized and has decreased functional activity. The oxidant responsible for alpha(2)M inactivation and the mechanism of such destruction were not studied. We now report that while hypochlorite and hydroxyl radical both modify amino acid residues on alpha(2)M, only hypochlorite can abolish the ability of alpha(2)M to inhibit proteinases. Hydrogen peroxide, on the other hand, has no effect on alpha(2)M structure or function. Protein unfolding with increased susceptibility to proteolytic cleavage appears to be involved in alpha(2)M inactivation by oxidation. The in vivo relevance of this mechanism is supported by the presence of multiple cleavage fragments of alpha(2)M in synovial fluid from patients with rheumatoid arthritis, where significant tissue destruction occurs, but not in patients with osteoarthritis. These results provide strong evidence that hypochlorite oxidation contributes to enhanced tissue destruction during inflammation by inactivating alpha(2)M.