Fibrinogen Binding to Human Blood Platelets :

Recent evidence suggests that fibrinogen binding to platelets is mediated by the 1 2 carboxyterminal amino acid residues of the ‘y chain. Because human plasma fibrinogen _y chains differ in mol wt and carboxyterminal amino acid sequence. we examined the effect of such y chain heterogeneity on platelet-fibrinogen interactions. using two fibrinogens of distinct composition. separated by ion exchange chromatography. One fibrinogen possessed only ‘V chains of mol wt 50.000 (F’T#{176}). the predominant ‘y chain species found in plasma. The other fibrinogen possessed equal amounts of ‘y chains with mol wt 50.000 and 57.500 (F ’ ’). with the longer ‘y chain (‘y57.5) possessing an amino acid extension at the carboxyterminal end. The latter fibrinogen was 50% less effective than F7 #{176} in supporting ADP-induced platelet aggregation at concentrations of .01 to 2 mg/mL. Scatchard analysis revealed no difference in the binding affinities of the two fibrinogens to ADP-treated platelets. but the amount of F 575 that was bound to platelets at saturation was only 50% that of F . Fibrinogen receptors that remained unoccupied in the presence of saturating concentrations of F #{176}’575. however. could be occupied by fresh F . Excess unlabeled F’ #{176}