Studies on α2-Macroglobulin in Bovine Plasma*: III. Its Actions on Bovine Plasma Kallikrein, Plasmin and Thrombin

Abstract

The interactions of bovine α₂-macroglobulin with bovine plasma kallikrein [EC 3.4.4.21], plasmin [EC 3.4.4.14] and thrombin [EC 3.4.4.13] were investigated. Bovine plasma kallikrein did not interact with bovine α₂-macroglobulin, and its kinin releasing and TAME^*** esterase activities were not affected by the presence of excess α₂-macroglobulin. α₂-Macroglobulin rapidly formed a complex with plasmin in a molar binding ratio of 1: 1, inhibiting the caseinolytic, fibrinolytic and kinin releasing activities, without affecting its esterase activity. Unlike the esterase activity of plasmin, that of the complex was not inhibited by inhibitors of protein nature such as trypsin inhibitors from soy bean and lima bean. However, it was inhibited by specific inhibitors of active sites, such as DFP and TLCK, and a low molecular weight poly-peptide inhibitor, trasylol. These results indicated that the serine and histidine residues in the active site of bovine plasmin are not involved in binding of plasmin to α₂-macroglobulin. The interaction of α₂-macroglobulin and thrombin was a time-dependent and dissociable reaction, and the clotting activity of thrombin was only slightly inhibited by aj-macroglobulin. TAME esterase activity of thrombin was not affected by α₂-macroglobulin.