Monoclonal antibodies to discrete regions in alpha 2-plasmin inhibitor.

Three monoclonal antibodies to alpha 2-plasmin inhibitor (alpha 2PI) were characterized. The first, JTPI-1, was directed against the reative site of alpha 2PI and inhibited antiplasmin activity by interfering with the formation of alpha 2PI-plasmin complexes. The avidity of JTPI-1 to the preformed alpha 2PI-plasmin complex was markedly lower than that to free alpha 2PI, which made this antibody useful for measuring the free alpha 2PI in plasma. The second, JTPI-2, recognized an epitope in the C-terminal fragment of alpha 2PI (11,000 daltons [11 K]) that was cleaved from alpha 2PI by plasmin upon complex formation but remained noncovalently attached to the complex. However, binding of JTPI-2 to alpha 2PI was not inhibited by the C-terminal 26-residue peptide containing the plasminogen-binding site and had no effect on the function of alpha 2PI. These data suggested that JTPI-2 recognized an epitope between the C-terminal 26-residue peptide and the reactive site. The third, JTPI-3, bound the alpha 2PI-plasmin complex (150 K) as well as alpha 2PI. Binding was inhibited by the N-terminal 12-residue peptide of alpha 2PI, but factor XIII-catalyzed cross-linking of alpha 2PI to fibrin was not inhibited by JTPI-3. These results suggested that the antibody recognized an epitope near the N terminus. These three monoclonal antibodies were useful for analyzing the mechanism of interaction between alpha 2PI and plasmin.