304 IMMUNO-CATALYSIS 



pepsin is inactivated very rapidly (Sumner and Somers, 1947). Rennin 

 is inactivated by strong acids. Pepsin is most active at pH 1.5 to 2.0. 



One part of rennin has been reported to clot 4.5 million parts of 

 milk at pH 6.2 and 37°C. (Tauber and Kleiner, 1932, 1934), and 72 

 million parts of fresh raw^ skimmed milk at pH 5.8 and 40°C. in 10 

 minutes (Hankinson and Palmer, 1942). 



Rennin has been isolated in the form of needle shaped crystals 

 (Hankinson, 1942, 1943), and in the form of flat plates (Berridge, 

 1943). 



b. Mechanism of Milk Clotting. According to some investigators, 

 rennin acting on casein splits it yielding acidic and basic groups in 

 the molecule. Calcium ion in the milk combines with these groups 

 producing a gel of calcium-phosphocasein (casein contains 0.85 per 

 cent phosphorus). The resultant gel is a polymer of casein or para- 

 casein (see Nord and Weidenhager, 1940). According to another inter- 

 pretation rennin splits casein through hydrolytic cleavage into soluble 

 paracasein and peptone-like products. Calcium ion in the milk com- 

 bines with paracasein forming the insoluble calcium-paracasein com- 

 plex. The question of which of these, or any other, interpretations 

 correspond to the true mechanism of milk clotting cannot as yet be 

 definitely stated. 



c. Milk and Plasma Clots Compared. The clotting of milk by 

 rennin and that of plasma by thrombin yield products which, in cer- 

 tain respects, seem to lend to comparison. These are: 



(1) Formation of insoluble elastic products from soluble protein 

 substrates. At certain moderate temperatures milk clot yields 

 strings with increasing elasticity. 



(2) Calcium ion is needed in both clotting processes. In neither 

 case does clotting occur following pretreatment wdth oxalate 

 or citrate ions which bind calcium ion. 



(3) Both clots show syneresis. In plasma clot, non-fibrinogen 

 plasma components, and in milk clot, a clear fluid called whey, 

 separate out. 



d. Antibody Against Animal Rennin. In order to find a relation- 

 ship between toxin-antitoxin and enzyme-antienzyme reactions Mor- 

 genroth (1899) undertook the following investigation in Ehrlich's 

 laboratory. He immunized goats with rennin and the immune serum 

 was shown to inhibit the milk clotting activity of rennin. Since 



