162 IMMUNO-CATALYSIS 



tate basic substances, namely, tungstic, phosphotungstic, flavianic, 

 picric, and picrolonic acids. The inhibitor is not precipitable with 

 trichloracetic acid. 



The inhibition of pepsin by this inhibitor is demonstrated by the 

 rennet method (decrease in milk clotting activity of pepsin) which is 

 carried out at pH 5.8. 



The pepsin inhibitor has no demonstrable effect on the activity of 

 crystalline trypsin, on the milk clotting activity of crystalline chymo- 

 trypsin or commercial rennet. Conversely, the crystalline trypsin in- 

 hibitor has no effect on the milk clotting action of pepsin. This 

 indicates a high degree of specificity, that usually is associated with en- 

 zymes, also exists among some inhibitors of enzymes. Bovine pepsin 

 was inhibited to the same degree as swine pepsin, but chicken pepsin 

 was not inhibited at all. On the other hand, a crude inhibitor solution 

 prepared from chicken pepsinogen inhibited both swine and bovine 

 pepsin, but had no effect on the chicken pepsin. 



According to Bourdillon (1945) the action of pepsin (and other 

 proteolytic enzymes) on antitoxin (diphtheria) pseudoglobulin yields 

 in addition to a heat labile and low molecular weight nitrogenous 

 material, a new protein of reduced molecular weight (60 per cent of 

 the native substance) which still has all the characters of a native sub- 

 stance. Split antitoxin is only slowly hydrolyzed by pepsin in moder- 

 ately acid medium and is thus able to form reversible compounds of 

 appreciable stability with pepsin. This complex contains from two to 

 three molecules of pepsin to one of antitoxin. The split antitoxin-pepsin 

 complex is markedly similar to the edestin-pepsin combination. In both 

 cases, maximum precipitation occurs at about pH 4.0 and the two pro- 

 teins combine with pepsin in approximately equal amounts. Both com- 

 plexes are richer in pepsin when formed in the presence of excess 

 pepsin. 



2. Trypsin Inhibitor of Pancreatic Extracts 



Kunitz and Northrop (1936) isolated the trypsin inhibitor from 

 trypsinogen crystals. The inhibitor is believed to play a very important 

 part in regulating the activation of trypsinogen, and in partly activated 

 pancreatic extracts more or less active trypsin occurs in the form of an 

 inactive compound with the inhibitor. Like pepsin inhibitor, the 



