Human plasma α2-macroglobulin is an inhibitor of circulating proteases that function in hemostatic and inflammatory reactions but the biochemical nature of its interaction with these enzymes is not well defined. This investigation has found that α2-macroglobulin is comprised of subunit chains of 185,000 molecular weight as analyzed by electrophoresis in polyacrylamide gels containing sodium dodecyl sulfate. Trypsin, thrombin, plasmin, and plasma kallikrein in amounts completely bound to α2-macroglobulin attacked one region in the subunit chain producing a single derivative with a molecular weight of 85,000 indicating that hydrolysis occurred at or near the center of the parent chain. The proteolytic derivative was also identified in an α2-macroglobulin preparation from plasma incubated with the plasminogen activator, urokinase. α2-macroglobulin functionally capable of binding enzyme appeared to be required both for limiting tryptic hydrolysis and for confining the concentration dependent increase in the derivative chain to the 1st min of incubation since acid-denatured α2-macroglobulin that failed to bind trypsin was extensively degraded. Three derivative chains resulted from the interaction of α2-macroglobulin with chymotrypsin demonstrating the presence of at least two chymotrypsin susceptible regions in the precursor chain. Reduction of the α2-macroglobulin-enzyme mixture was required for the identification of the derivative subunit chains establishing that these cleavage products were covalently linked to the parent molecule by disulfide bridges. Thus, α2-inacroglobulin acts as a substrate for circulating proteases, a finding which may also pertain to the mechanism of action of other plasma enzyme inhibitors.

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