title: Proteolytic activity of alpha 2-macroglobulin-enzyme complexes toward human factor VIII/von Willebrand factor.
abstract: The low level of enzymatic activity of certain alpha 2-macroglobulin-proteinase complexes could be important to the function of factor VIII/von Willebrand glycoprotein since it is especially sensitive to proteolytic cleavage. To test this possibility, complexes of alpha 2-macroglobulin with plasmin, trypsin, and thrombin were formed in at least a 2:1 molar ratio of alpha 2-macroglobulin:proteinase and tested for effects on the factor VIII procoagulant activity of the factor VIII/von Willebrand glycoprotein. Neither the alpha 2-macroglobulin-trypsin complex nor the alpha 2-macroglobulin-plasmin complex affected factor VIII procoagulant activity. The behavior of the alpha 2-macroglobulin-thrombin complex was different. When alpha 2-macroglobulin and thrombin were incubated in a mole ratio of 3:1 or less, factor VIII procoagulant activity was enhanced to about the same extent as with free thrombin. Even at a 24:1 mole ratio, the mixture could produce 45% of the increase in factor VIII activity obtained with free thrombin. The isolated alpha 2-macroglobulin-thrombin complex could also activate the factor VIII procoagulant function to about 45% of the level obtained with an identical amount of uncomplexed thrombin. Analysis of the alpha 2-macroglobulin-125I-labeled thrombin complexes by rechromatography or by polyacrylamide gel electrophoresis in sodium dodecyl sulfate indicated that this activation was not due to free thrombin. We conclude that the alpha 2-macroglobulin-thrombin complex retains sufficient proteolytic activity to activate the procoagulant function of factor VIII/von Willebrand glycoprotein despite the latter being a very large substrate, having an estimated molecular weight of 1-20 million.
mesh_terms: D000941:Antigens / Q000378:metabolism; D001779:Blood Coagulation Factors / Q000378:metabolism*; D010450:Endopeptidases / Q000097:blood*; D005169:Factor VIII / Q000276:immunology / Q000378:metabolism*; D005341:Fibrinolysin / Q000378:metabolism; D006801:Humans; D011480:Protease Inhibitors; D011485:Protein Binding; D013917:Thrombin / Q000378:metabolism; D013997:Time Factors; D014357:Trypsin / Q000097:blood; D000511:alpha-Macroglobulins / Q000378:metabolism* / Q000494:pharmacology; D014841:von Willebrand Factor / Q000378:metabolism*
chemical_list: D000941:Antigens; D001779:Blood Coagulation Factors; D011480:Protease Inhibitors; D000511:alpha-Macroglobulins; D014841:von Willebrand Factor; D005169:Factor VIII; D010450:Endopeptidases; D014357:Trypsin; D013917:Thrombin; D005341:Fibrinolysin