Studies on the Mechanism of Action of Inhibitors of the Fibrinolysin System*

With increasing awareness of fibrinolytic phenomena, improved diagnostic methods and the availability of antifibrinolytic fibrinolytic hemorrhage is progressing from the obscure register of rare syndromes to an important emergency condition.From a long list of potential inhibitors of the fibrinolysin system, three have reached the stage of clinical trials.Epsilon-aminocaproic acid (EACA) was introduced by Okamoto and associate^^,^ as was 4-aminomethyl cyclohexane carboxylic acid (AMCHA) .5,6The latter compound was originally studied as the naturally occurring mixture of stereoisomers.Later, it was found that only the trans isomer is a~tive.~-lOLaskowski and Laskowskill reviewed the literature on trypsin inhibitors isolated from organs and blood.The pancreatic inhibitor of Kunitz and Northrop12 was later isolated also from bovine salivary glands and lungs and was shown to inhibit the kallikrein-kinin system and the fibrinolysin system as well.13-15This agent is presently available for investigation under the trade name TrasyloP.The purpose of this study was to investigate the mechanism of action of these inhibitors of the fibrinolysin system.Several authors proposed that EACA and AMCHA are primarily inhibitors of the activation of plasminogen, plasmin itself being inhibited only at much higher concentrations.1°J6-18Others reported that these compounds act chiefly by inhibiting plasmin or that the inhibitory concentrations are equal for plasmin and for the activation of p l a s m i n ~g e n .~J ~-~~ The present study suggests that additional mechanisms are also involved. Materials and MethodsPlasminogen was prepared from human plasma fraction IIIt according to the method of K l i r ~e .~~ Purified streptokinaset (SK) (1,000 Christensen U/mg) and purified urokinase (UK) 8 (5,000 Ploug U/mg) were used as activators.In addition, spontaneously activated human plasmin in glycerolq and chloroformactivated bovine plasmini were used.The following proteolytic enzymes were employed: twice crystallized trypsin,]] crystallized bromelain,