Degradation of cross-linked fibrin by human leukocyte proteases.

We have examined the dissolution of plasminogen-free fibrin clots by proteases in a leukocyte lysate using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) to monitor alterations of cross-linked fibrin structure before solubilization and to characterize the structure of soluble derivatives. Progressive clot lysis was also followed by quantitation of fibrin derivatives that were present as solubilized products, as clot-associated fragments, and as residual degrading clot. An estimate of the total fibrinolytic potential of the plasminogen system relative to that of entrapped leukocytes was based on the specific fibrinolytic activities of plasmin and leukocyte lysate, and this indicated that the capacity of leukocyte-mediated fibrinolysis was only 3% of fibrin-bound plasminogen. Early during leukocyte lysate digestion the predominant soluble products were small peptides of Mr less than 20,000, whereas at later times heterogeneous groups of large fragments were present that were distinct from those produced during plasmic degradation. Electrophoresis in nondissociating conditions showed that a later leukocyte lysate digest of cross-linked fibrin contained distinct bands with mobilities indistinguishable from plasmic derivatives DD/E and DY/YD, suggesting a similar assembly in the native state of the leukocyte lysate fragments to those produced by plasmin. During degradation by leukocyte lysate, up to 70% of the degrading, insoluble clot could be solubilized in SDS, indicating that extensive early cleavage of the fibrin matrix failed to release much of the protein into solution. A market difference in the composition of fragments and polypeptide chains in the protein noncovalently bound to clot was seen in comparison with soluble derivatives. This appeared to be caused by the relative resistance to degradation of the C-terminal portions of the gamma-chain of the soluble derivatives, whereas the matrix-associated protein could be more easily cleaved in this region. The results demonstrate a distinct difference in the overall pattern of degradation compared with plasmic fibrinolysis.