A Fibrinogen Fragment D (D Intermediate) with Calcium Binding but Without Anticlotting Properties

Plasmin digestion of fibrinogen in the presence of Ca2+ or of EGTA leads to the formation of two sets of fragments, designated Dcate and D EGTA, respectively. Fragments Dcate, in contrast to D EGTA, bind one calcium ion and are anticlotting. Both these properties of Dcate are related to a 13-kDa carboxyl-terminal stretch of its gamma-chain, which is missing in D EGTA. The molecular weights of the gamma-chains of Dcate and D EGTA are 38000 and 25000, respectively. Now we have prepared a D-fragment, Dint, which has a gamma-chain with a molecular weight of 29000. Dint binds one calcium ion per molecule but has no anticlotting properties. Thus the Ca2+-binding site of Dint (and Dcate) is directly dependent on the 4-kDa piece of the gamma-chain that is present in Dint but not in D EGTA. As a consequence, the anticlotting properties of Dcate reside in the gamma 9-kDa stretch that is absent in Dint.