REACTION OF ALPHA-2-MACROGLOBULIN WITH PLASMIN INCREASES BINDING OF TRANSFORMING GROWTH FACTOR-BETA-1 AND FACTOR-BETA-2

The binding of I-125-transforming growth factors-beta-1 and beta-2 (TGF-beta-1 and TGF-beta-2) to alpha-2-macroglobulin (alpha-2M) was studied before and after reaction with plasmin, thrombin, trypsin, or methylamine. Complex formation between TGF-beta and native or reacted forms of alpha-2M was demonstrated by non-denaturing polyacrylamide gel electrophoresis and autoradiography. Reaction of native alpha-2M with plasmin or methylamine markedly increased the binding of I-125-TGF-beta-1 and I-125-TGF-beta-2 to alpha-2M. The alpha-2M-plasmin/TGF-beta complexes were minimally dissociated by heparin. Reaction of alpha-2M with thrombin or trypsin reduced the binding of I-125-TGF-beta-1 and I-125-TGF-beta-2; the resulting complexes were readily dissociated by heparin. Complexes between TGF-beta-2 and native or reacted forms of alpha-2M were less dissociable by heparin than the equivalent complexes with TGF-beta-1. These studies demonstrate that the TGF-beta-binding activity of alpha-2M is significantly affected by plasmin, thrombin, trypsin and methylamine. Observations that alpha-2M-plasmin preferentially binds TGFs-beta suggest a mechanism by which alpha-2M may regulate availability of TGFs-beta to target cells in vivo.