282 IMMUNO-CATALYSIS 



were impermeable and digestion limited to the surface, the rate of de- 

 struction should be independent of thickness. No determinations were 

 reported concerning the nature of the products of proteolytic digestion 

 of fibrin. 



2. Chemical Reactions Involving -S-S- Linkages in the 

 Conversion of Fibrinogen to Fibrin 



Analyzing the conversion of fibrinogen to fibrin from the standpoint 

 of equilibrium of chemical reactions one would find that the formation 

 of insoluble fibrin from a solution of fibrinogen is another example of 

 those reactions which proceed uninterruptedly to completion and 

 appear to be irreversible; in these reactions the initial substances are 

 exhausted. On the other hand, the reversible reactions, at equilibrium, 

 remain more or less incomplete due to the reverse action of the reaction 

 products yielding the initial substances. In the former, the reaction 

 products are either insoluble or are of gaseous nature whereby they are 

 eliminated from the field of reaction either by forming insoluble 

 precipitates or by escaping, e.g., the formation of very sparingly 

 soluble silver chloride, or the formation of hydrogen from metallic 

 sodium and water, or the formation of insoluble fibrin from soluble 

 fibrinogen. 



The formation of fibrin from fibrinogen as discussed above possesses 

 features which might seem to be comparable with the synthesis of 

 higher insoluble polypeptides from simpler water soluble ones by the 

 catalytic action of papain. After discussing various aspects of reactions 

 at equilibrium, Bergmann and Fruton (1941) described numerous 

 reactions of this type. For example, when cysteine-activated papain 

 was added, at 40°C., to a solution containing equivalent amounts 

 of acetyl-dl-phenylalanylglycine and aniline, after several minutes a 

 copious crystallization of acetyl-1-phenylalanylglycine anilide was 

 formed. The crystallization of anilide will disturb the equilibrium; 

 the reaction will continue to proceed to the right until virtually all 

 of the initial reactants are converted into anilide. 



This simple one-step synthesis of sparingly soluble polypeptides may 

 in principle appear to be analogous to the formation of fibrin from 

 fibrinogen by the action of papain as a proteolytic process. The ques- 

 tion of whether or not papain brings about this reaction as a synthetic 



