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

A cold-precipitable, thrombin-coagulable protein, tentatively designated "cryoprofibrin," was separated from plasma of rabbits treated with E. coli endotoxin. It was shown to contain fibrin-intermediates, consisting of fibrinogen that has lost only a portion of the peptides liberated during the conversion of rabbit fibrinogen to fibrin. These peptides have been called cofibrins. On reaction with thrombin, fibrinogen and cryoprofibrin yielded the same amount of cofibrin B (0.9 moles per 220,000 Gm. of fibrin clot); however, cryoprofibrin yielded 30 per cent less cofibrin A than did fibrinogen. Cryoprofibrin and fibrinogen yielded 0.7 and 1.0 moles of cofibrin A per 220,000 Gm. of fibrin, respectively. The deficit of cofibrin A in preference to cofibrin B provides evidence that, cryoprofibrin consists of fibrinogen that had lost cofibrin A by limited action of thrombin, because cofibrin A is liberated from fibrinogen by thrombin more rapidly than is cofibrin B. Only a small portion of the cryoprofibrin could have been produced by action of thrombin subsequent to exsanguination, because the plasma contained only about one-third of the cofibrin A that had been liberated from fibrin-ogen during its conversion to cryoprofibrin. Accordingly, cryoprofibrin provides a valid measure of thrombin activity in vivo, at least in the endotoxin-treated rabbits. An alternative approach to measurement of thrombin activity involving recovery of cofibrin A from urine proved inapplicable, because no confibrin A was found to be excreted in urine. An average of 0.264 ± 0.038 mg. cryoprofibrin per ml. was obtained from plasma of treated rabbits, as compared with 0.044 ± 0.016 mg. of cold-precipitable fibrinogen per ml. of normal plasma. The demonstration of increased thrombin activity after endotoxin treatment, as evidenced by the cryoprofibrin, implicates thrombin in the mechanism of fibrinoid deposition. The demonstration that cryoprofibrin consists of fibrin-intermediates supports Thomas' hypothesis that fibrinoid consists of fibrin-intermediates.