Effects of divalent cations on fibrinogen and its reaction with thrombin were re-examined and correlated to improve definition of mechanisms underlying the acceleration of clot formation by the ions. The rate of release of fibrinopeptides from fibrinogen was not affected by any of the specific ions studied, but increased rates of fibrin monomer polymerization were obtained with all but magnesium ions. Maximal acceleration of monomer polymerization was observed with the divalent ions at concentration of 2.5-5 mM, and no added specific ion effects were observed with much higher levels. The acceleratory ions were found to have a corresponding effect on the solubility of fibrinogen, as judged from acceleration of its precipitation at low temperature. Although magnesium ions had no effect on fibrin monomer polymerization, they did have a low ranking effect on cryoprecipitation. These data confirm that the principal effect of divalent cations in the fibrinogen-fibrin transformation is to accelerate fibrin monomer polymerization, and suggest that the acceleration is associated with effects on the solubility of fibrinogen.

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