Exposure of fibrinogen receptor on human platelets by proteolytic enzymes.

Human platelets incubated with a-chymotrypsin or pronase did not aggregate with ADP or thrombin and did not agglutinate with ristocetin. On the other hand, they aggregated with fibrinogen even in the absence of platelet-activating compounds. This aggregation was not inhibited by prostaglandin El, sodium azide, or formaldehyde, indicating that it was independent of platelet metabolism. Calcium or magnesium were required for aggregation. Platelets preincubated with achymotrypsin or pronase also showed enhanced agglutination with concanavalin A or Lens culinaris lectin. This was inhibited by a-D-glucose, a-D-mannose, and a-D-methylmannoside. These monosaccharides did not inhibit the fibrinogen-induced effect indicating that it differed from that of lectins. "'1-Fibrinogen bound specifically to pronaseor chymotrypsin-treated platelets but it did not bind to untreated intact platelets. The chymotrypsin-treated platelets exposed per platelet 1,860 high affinity fibrinogen binding sites (receptors) (Kd = 1.9 X M) and 88,000 low affinity binding sites (Kd = 3.8 X IO-' M). "he mean K , value for fibrinogen calculated on the basis of the rate of aggregation was 1.0 X M (S.E. = 0.3 x lo" M) for chymotrypsintreated platelets. Limited proteolytic degradation of fibrinogen to Fragment X (Stage 1) and Fragment X (Stage 2) resulted in a decrease of platelet aggregation and of affinity to fibrinogen receptor. Fragments Y and D, resulting from the more extensive degradation of fibrinogen molecule, did not have any ability to bind to platelets or to cause their aggregation. It is proposed that the fibrinogen-induced platelet aggregation depends on the occupancy of high affinity fibrinogen receptors. Aggregation of intact platelets by ADP in the presence of fibrinogen and the fibrinogen-induced aggregation of platelets altered by proteolytic enzymes may involve the same receptors on the platelet surface.