Degradation of Human Fibrinogen by Plasma α2-Macroglobulin-Enzyme Complexes

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Abstract
This study demonstrates that human plasma α2-macroglobulin preparations possess an enzymic activity that degrades fibrinogen, resulting in the formation of products whose structure resembles that of circulating fibrinogen catabolites. The sequence of degradation is similar to that observed in plasmin-catalyzed digests, in that Aα-chain fragmentation precedes that of Bβ-chain. The addition of plasminogen activators to plasma induced an increase in the N-α-tosyl-l-arginine methyl ester HCl esterase and fibrinogenolytic activity associated with α2-macroglobulin purified from this plasma, indicating that the enzymic activity of the complex was preserved and could be increased in the presence of other plasma enzyme inhibitors. Immunochemical studies demonstrated that an α2-macroglobulin-plasmin complex had formed in urokinase-treated plasma. This α2-macroglobulin preparation manifested an esterolytic profile like that of a complex prepared from plasmin and purified α2-macroglobulin. After complex formation with α2-macroglobulin in plasma, plasmin retained less than 0.1% of its fibrinogenolytic activity. That plasmin expressed its activity while bound to α2-macroglobulin was suggested by immunoprecipitation of this activity with α2-macroglobulin antibody and by the demonstration that pancreatic trypsin inhibitor did not effectively inhibit its fibrinogenolytic or esterolytic activity. These results raise the possibility that, in addition to its activity as a major plasma proteolytic enzyme inhibitor, α2-macroglobulin may modulate enzyme-substrate interactions, such as those resulting in the formation of circulating fibrinogen catabolites, by providing a mechanism for the preservation and protection of a portion of the enzymic activity in the presence of other circulating inhibitors.