Structural studies of fibrinolysis by electron microscopy

Fibrin is degraded by the fibrinolytic system in which a plasminogen activa tor converts plasminogen to plasmin, a serine protease that cleaves specifi c bonds in fibrin leading to solubilization. To elucidate further the bioph ysical processes involved in conversion of insoluble fibers to soluble frag ments, fibrin was treated with either plasmin or the combination of plasmin ogen and plasminogen activator, and morphologic changes were observed using scanning electron microscopy. These changes were correlated with biochemic al analysis and with characterization of released, soluble fragments by tra nsmission electron microscopy. initial changes in the fibrin matrix include d creation of many free fiber ends and gaps in the continuity of fibers. Wi th more extensive digestion, free fiber segments associated laterally, resu lting in formation of thick fiber bundles. Supernatants of digesting clots, containing soluble derivatives, were negatively contrasted and examined by transmission electron microscopy. Large, complex fragments containing port ions of multiple fibers were observed, as were pieces of individual fibers and smarter fragments previously identified. Some large fragments had sharp ly defined ends, indicating that they had been cleaved perpendicularly to t he fiber direction. Other fibers showed splayed ends or a lacy meshwork of surrounding protofibrils. Longer times generated more small fragments whose molecular composition could be inferred from their appearance. These resul ts indicate that fibrinolytic degradation results in larger pieces than pre viously identified and that plasmin digestion proceeds locally by transvers e cutting across fibers rather than by progressive cleavage uniformly aroun d the fiber. (C) 1998 by The American Society of Hematology.