Fibrin degradation products. A review of structures found in vitro and in vivo.

This review attempts to relate subunit structures of fibrin degradation products (FnDP) made in vitro with structures found in vivo. The domainally directed fragmentation in vitro of both fibrinogen and fibrin is emphasized, all fragments being various associations of the two core fibrin fragments D and E. The digestion of fibrinogen by plasmin in vivo is rare, and the crosslinking of fibrin in vivo takes place at a very early stage in the clotting/polymerization process. The notion that as fibrin forms in vivo it orchestrates its own destruction is developed. Plasmas from patients suffering from disseminated intravascular coagulation demonstrate very large crosslinked FnDP fragments in their plasmas which seem to contain not alone fibrinopeptide A but also subunits with intact alpha chains. This is interpreted to mean that many of the large soluble fragments found in vivo, and heretofore known as FnDP, are in reality long fibrin polymers, random parts of whose structures have been converted to FnDP by lysis of the carboxy terminal regions of the alpha chains in the polymer. The ratio of intact fibrin to FnDP in these large soluble structures may be a useful clinical marker; however, such data can only be relied upon when blood samples are taken into an anticoagulant mix that contains a fibrinolytic inhibitor. Some of the biological effects of FnDP structures in vivo (fibrinogen synthesis, vasoactivity) are still quite ambiguous.