α₂-Macroglobulin is a binding protein for basic fibroblast growth factor

After incubation with human serum or plasma, ¹²⁵I-basic fibroblast growth factor (bFGF) (molecular mass 18.5 kDa) exhibits molecular mass forms greater than 200 kDa as determined by nonreducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by autoradiography. These high molecular mass forms of bFGF are immunoprecipitable with antiserum raised against α₂-macroglobulin (α₂M). Purified α₂M and ¹²⁵I-bFGF form a covalent complex in a specific, saturable manner. Excess unlabeled bFGF competes with ¹²⁵I-bFGF for complex formation. Complex formation is complete after 4 h and is inhibited by pretreating α₂M with dithiothreitol, iodoacetamide, iodoacetic acid, and N-ethylmaleimide. The complex is resistant to acidic conditions and denaturants such as urea. Heparin, which binds bFGF, has no effect on complex formation. Methylamine, which blocks protease binding to α₂M, increases the amount of ¹²⁵I-bFGF that can be bound 2-fold. Plasmin and trypsin treatment of α₂M has no effect on ¹²⁵I-bFGF binding. The ability of growth factors to compete for binding is specific, as aFGF and TGF-β compete for binding to α₂M, whereas platelet-derived growth factor does not. ¹²⁵I-bFGF·α₂M complexes do not bind to low affinity bFGF binding sites and bind poorly to high affinity bFGF binding sites on BHK-21 cells. In addition, ¹²⁵I-bFGF bound to α₂M has decreased ability to stimulate plasminogen activator production in bovine capillary epithelial cells.