Anticoagulant and calcium-binding properties of high molecular weight derivatives of human fibrinogen (plasmin fragments Y).

The present study was undertaken as a step to delineate further the localization of the calcium-binding sites in fibrinogen and to assess the anticlotting properties of fibrinogen degradation products. To this purpose, fragments Y were prepared by plasmin digestion of human fibrinogen in the presence of added Ca2+, and purified. We found that, on a molar basis, fragments Y exhibit twice as much anticlotting activity as fragments X. They possess two calcium-binding sites with Kd = 1.9 . 10(-5) M. Their predominant amino-terminal amino acids are alanine and tyrosine. It is known that one binding site in fragment Y is related to its D moiety. We conclude that the other calcium-binding site may be located in the central domain of the molecule.