Summary
In its dependence upon prothromboplastic phosphatide and calcium ions, in the conversion
of prothrombin to thrombin, Autoprothrombin C is a thromboplastic enzyme. The only other demonstrated accessory factor is V, and any needs for VII, X, XI,
or XII are ruled out by the present experiments, as were VIII or IX formerly. The
lipid requirement may be satisfied by either phosphatidylserine (PS) or phosphatidylethanolamine
(PE), purified from human blood platelets. Comparisons are made with Russell’s viper
venom (“stypven”), and with trypsin. The trypsin potentiation of factors VII and X
is prevented by pancreatic inhibitor (PI). However, when PI is added after this maximal potentiation, it is no longer inhibitory. The present status of knowledge
concerning enzymes in relation to blood coagulation is discussed.