Nature of the thrombin-inhibiting effect of incubated fibrinogen

This study investigated the effect of a number of factors on the inhibition of thrombin by the anticoagulant fraction of incubated fibrinogen (AFIF). Heating above 46°C reduces and at 55°C completely abolishes the thrombin-inhibiting activity of AFIF. The prolongation of the thrombin time of fibrinogen by AFIF is proportional to the pH of the reaction mixture, while that of plasma tends to level off beyond pH 7.5. Calcium and magnesium up to a concentration of 10 mm shorten the thrombin time of both plain fibrinogen and fibrinogen mixed with AFIF in a proportional manner. AFIF partially inhibits the thrombin time reducing property of protamine on fibrinogen. Related probably to this is the observation that AFIF and serum protect purified fibrinogen from precipitation by protamine. The inhibitory effect of AFIF on the action of thrombin on fibrinogen was found to be competitive in nature. The hydrolysis of tosylarginine methylester by thrombin was shown to be unaffected by AFIF. The properties of the anticoagulant studied did not allow its identification with heparin.