Activators of purified prothrombin may be divided into two groups: those which have thromboplastin activity, and those which have accelerator or thromboplastin co-factor activity. The two kinds of activators work together to convert prothrombin rapidly. Alone they are not very effective.
Experimentally, either thromboplastin or accelerator activity can be made the limiting factor in thrombin formation. Then, by increasing the concentration of the essential activator, the rate of prothrombin conversion is increased. The thrombin yield is at first increased, but later it is independent of activator concentration. This basic relationship describing rate and quantity of thrombin yield applies to thromboplastin, plasma Ac-globulin, serum Ac-globulin and very probably to platelet accelerator.
When both activators are present in limited quantity the rate of prothrombin conversion can be increased by adding either thromboplastin or accelerator. Under those conditions they are quantitatively interchangeable.