Acceleration of fibrin polymerization by calcium ions.

T HE FINAL STEPS OF THE CLOTTING PROCESS, in which fibrinogen (Factor I) is transformed to fibrin through the action of thrombin, is customanily described as taking place in three stages. Initially, thrombin, a proteolytic enzyme of high substrate specificity, cleaves four arginyl-glycine bonds in each fibrinogen molecule, so that two pairs of peptide fragments are released. During the second stage (aggregation), the cleaved fibrinogen (now called fibrin monomer) coagulates to form a visible fibrin clot; in systems contaming purified reagents this clot is soluble in 5 M urea or monochloroacetic acid. In the third step (polymerization), probably coincident with the second, a plasma transamidase, fibrin-stabilizing factor (Factor XIII), converts the ureasoluble fibnin to an insoluble “cross-linked” form. This final reaction takes place only if calcium ions are present. The need for calcium as an adjunct to the formation of insoluble fibrin is well established, but the role of this ion in the first two steps of the fibrinogento-fibrin transformation is unclear.’6 Lorand and Konishi,7 using purified bovine fibrin, have reported accelerated aggregation in the presence of low concentrations of calcium. Elias and Iyer8 showed that calcium ions were capable of counteracting the effect of certain inhibitors of fibrin monomer aggregation. Problems in separating the three stages of clot formation have made interpretation of other reported observations difficult.5’6 The experiments to be described were designed to study the formation of a clot in the presence or absence of calcium ions and to localize the effect of these ions to the first or second steps of fibrin production. The technique used to dissect the two stages was based on studies of fibnin formation in patients with dysfibrinogenemia.9 Our data indicate that calcium ions do not enhance