The Effect of Pretreatment of Human or Rabbit Platelets With Chymotrypsin on Their Responses to Human Fibrinogen and Aggregating Agents

Platelet function and the interaction of platelets with human fibrinogen have been studied following degradation of membrane glycoproteins by chymotrypsin. Washed human and rabbit platelets were treated with α-chymotrypsin (1-50 U/ml), which cleaved glycopeptides from membrane glycoproteins without causing aggregation or release of granule contents. Chymotrypsin removed a large percentage of the sialic acid and periodic acid Schiff (PAS) staining material from the platelet membrane, but glycoprotein IV (of human platelets) or III (of rabbit platelets) were not affected. Chymotrypsin-treated platelets were either less responsive or unresponsive to aggregation induced by adenosine diphosphate (ADP) and aggregation and release induced by thrombin or collagen, depending on the concentration of chymotrypsin with which the platelets had been incubated. Unlike untreated platelets, chymotrypsin-treated platelets aggregated in response to human fibrinogen. This was not an agglutination reaction, since it was prevented by EDTA or EGTA or (with rabbit platelets) by the omission of calcium from the suspending medium. ADP enhanced fibrinogen-induced aggregation of these platelets. Fibrinogen becomes associated with untreated human or rabbit platelets during ADP-induced aggregation. 125I-fibrinogen became associated with the chymotrypsin-treated platelets during fibrinogen-induced aggregation, and more 125I-fibrinogen associated with them upon the addition of ADP than became associated with untreated control platelets during ADP-induced aggregation. Normal, untreated, disc-shaped platelets do not seem to have available fibrinogen-binding sites. It may be that when they are induced to change shape by ADP, such sites are revealed so that fibrinogen can form bridges between adjacent platelets. It appears that treatment of platelets with chymotrypsin, which removes glycopeptides from membrane glycoproteins, also causes rearrangement of platelet membrane components so that fibrinogen-binding sites become available. These observations support the concept that fibrinogen has a major role in platelet aggregation.