Cofibrins and Fibrin-Intermediates as Indicators of Thrombin Activity in Vivo

IDENTIFICATION of fibrinous or fibrinogenous deposits in vascular tissue suggests that the material resulted from conversion of fibrinogen to fibrin. However, fibrinogen is one of the most easily precipitable proteins of plasma, and its deposition could result from coprecipitation or formation of insoluble complexes with polyelectrolytes. Because the methods used have not differentiated between fibrinogen and fibrin in the deposits, another approach to identification of thrombin activity is needed. Measurement of the by-products and intermediates in the conversion of fibrinogen to fibrin is the subject of the present communication. Enzymatic conversion of fibrinogen to fibrin by thrombin involves release of several peptides, designated cofibrins.' Because of the stoichiometric relationship between peptide release and fibrin formation, recovery of cofibrins from plasma and urine should provide a measure of conversion. Recovery of cofibrins from urine appeared to be feasible, because Christensen, Lynch, and Powers had shown that peptides from partially hydrolyzed casein were rapidly excreted in urine. Bettelheim, Blomback and Vestermark, and Blomback and Laurent showed that cofibrins of bovine fibrinogen are released at different rates, and that fibrin-intermediates, consisting of fibrinogen that has lost only a portion of the cofibrins, are formed during conversion to fibrin. However, the interme-