Lipoid Thromboplastin and Trypsin *

Trypsin is known to activate certain proenzymes, including trypsinogen and chymotrypsinogen.7 It also activates prothrombin." In the case of prothrombin activation, platelets have been shown to complement the effect of trypsin. Small quantities of trypsin and platelets together exert a much greater effect than the sum of their separate effects.9 Lipoid thromboplastin has been found to complement trypsin in a similar manner."' The mechanism of this complementary effect is unknown. Therefore, it is of interest to inquire whether lipoid thromboplastin functions as a simple cofactor for trypsin in the activation of proenzymes, generally.