gamma and alpha chains of human fibrinogen possess sites reactive with human platelet receptors.

Abstract

Fibrinogen, a clottable plasma protein, agglutinates both prokaryotic cells (e.g., staphylococci) and eukaryotic cell fragments (e.g., platelets) through interaction with specific receptors. To identify the region of the fibrinogen molecule responsible for its interaction with human platelets, polypeptide chain subunits (.alpha., .beta., and .gamma.) of human fibrinogen were prepared by reduction and carboxymethylation. A mixture of the chains induced aggregation (clumping) of human platelets separated from plasma proteins and treated with ADP. When individual chains of fibrinogen were tested, .gamma.-chain multimers caused platelet aggregation at a molar concentration comparable with that of intact human fibrinogen. The .beta. chain remained inactive, and the .alpha. chain was 1/4-1/5 as reactive as the .gamma. chain. Monospecific antibody fragments against the .gamma. chain inhibited binding of 125I-labeled fibrinogen to the human platelet receptor and blocked aggregation of platelets induced by ADP in the presence of fibrinogen or .gamma.-chain multimers. The .gamma. chain of human fibrinogen apparently bears the main site for interaction with the platelet receptor.