Clotting of fibrinogen. 2. Calorimetry of the reversal of the effect of calcium on clotting with thrombin and with Ancrod

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Abstract
When clotting is effected by thrombin in the presence of Ca, the endotherm for the D nodules of fibrinogen broadens significantly and then becomes narrow again, while increasing in size. Clotting effected by the snake venom enzyme, Ancrod, which releases only the A fibrinopeptides from the E nodule, shows only the broadening of the D endotherm. Accordingly, significant interactions of the D nodules of fibrinogen become possible only when the B fibrinopeptides of the E nodule are released on clotting. When Ca present during clotting is removed from the fibrin clot with ethylenediaminetetraacetic acid, the endotherm for the D nodules of fibrin shows nearly complete reversal if clotting was effected with Ancrod but appears to be divided into 2 endotherms if clotting was effected with thrombin. At neutral pH, new endotherms were observed for fibrinogen in the temperature range 105-140.degree. C.