Alpha 2-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 alpha 2-macroglobulin (alpha 2M). Purified alpha 2M 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 alpha 2M 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 alpha 2M, increases the amount of 125I-bFGF that can be bound 2-fold. Plasmin and trypsin treatment of alpha 2M has no effect on 125I-bFGF binding. The ability of growth factors to compete for binding is specific, as aFGF and TGF-beta compete for binding to alpha 2M, whereas platelet-derived growth factor does not. 125I-bFGF.alpha 2M 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 alpha 2M has decreased ability to stimulate plasminogen activator production in bovine capillary epithelial cells.