Clotting of fibrinogen. 1. Scanning calorimetric study of the effect of calcium

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Abstract
The denaturation temperature Td and the enthalpy of thermal denaturation .DELTA.Hd of the D nodules of fibrinogen increase 12-13.degree. C and 40%, respectively, when fibrinogen is clotted by thrombin in the presence of 10-3 M Ca2+. The rate of changeof Td and .DELTA.Hd is 1st order in thrombin concentration. In the absence of Ca, little change in Td is observed, but the increase in.DELTA.Hd still occurs. The shift in Td as a function of logarithm of Ca concentration is sigmoid, with a half-point at 2.5 .apprx. 10-5 M Ca for human and 6.0 .times. 10-5 M Ca for bovine fibrinogens, suggesting that the shift is due to binding of Ca at the high-affinity binding sites of fibrin. The T3 of the D nodule of native fibrinogen also increases, but not as much, on addition of Ca. This increase in Td is also sigmoid with log Ca, with a half-point of 1.6 .apprx. 10-3 M Ca for human and 3.2 .times. 10-3 M Ca for bovine fibrinogens, and appears to be due to binding of Ca to the low-affinity binding sites of fibrinogen. At Ca concentrations > 10-4 M, traces of factor XII in the bovine fibrinogen preparation become activated and cause cross-linking of the fibrin gel. But the changes in Td and .DELTA.Hd still occur when factor XIIIa is inactivated by iodoacetamide, and the rate of the changes is not altered by addition of large amount os factor XIIIa. Thus, the changes in Td and .DELTA.Hd are not due to cross-linked but result from intermolecular interactions of the D nodules in the fibrin clot, which are strengthened by binding of Ca at the highly-affinity binding sites of fibrin.

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