Release of fibrinopeptide A (FPA) from human fibrinogen by batroxobin and human thrombin was examined. The rate was measured by means of radio-immunoassay. The following results were obtained. 1. Michaelis-Menten parameters for the interaction between the enzymes and fibrinogen at pH 7.4, 37°C were determined : Batroxobin, Vmax
=3.7nmol FPA/min/BU, Km
=0.125μM : thrombin, Vmax
=83nmol FPA/min/NIH unit, Km
=3.0μM. 2. The pH-dependence of the reaction of thrombin at 4.7μM fibrinogen concentration gave a bell-shaped curve with a maximum at pH 8.5, while a sigmoid curve with flat region over pH 9 was obtained for batroxobin. The dependence of batroxobin at 0.19μM fibrinogen concentration gave a bell-shaped curve with a maximum at pH 8.5. 3. Thrombin was inhibited by diisopropyl fluorophosphate (DFP), N2
-p-tosyl-L-lysine chloromethyl ketone (TLCK), benzamidine, and hirudin, and not inhibited by Trasylol. Batroxobin was inhibited by DFP, and not inhibited by TLCK, benzamidine, hirudin and Trasylol. 4. The rates of batroxobin-catalyzed release of FPA in the plasma and in purified fibrinogen solution were almost identical during 4 h incubation at 37°C. On the other hand, the thrombin-catalyzed release was inhibited in the plasma.