Evidence that Platelet Accelerator (Platelet Factor 1) Is Adsorbed Plasma Proaccelerin

1. Human platelets possess an accelerator activity equal to about six per cent of the proaccelerin activity of normal citrated plasma. This accelerator activity is only slightly reduced by 10 washings.

2. Thrombin increases platelet accelerator activity as much as ten fold. This means that platelet accelerator activity behaves like proaccelerin (plasma Acglobulin) rather than accelerin (serum Ac-globulin).

3. Platelets from a patient with parahemophilia (congenital proaccelerin deficiency) possess only a trace of accelerator activity. They acquire a normal amount of accelerator activity after contact with normal platelet-poor plasma.

4. Trypsin destroys 90 per cent or more of the platelet accelerator activity of normal platelets without altering their appearance. Trypsinized platelets regain normal accelerator activity upon incubation with normal platelet-poor plasma.

5. These findings strongly suggest that the platelet accelerator (platelet factor 1) is adsorbed plasma proaccelerin.