304 ESSENTIALS OF PHYSIOLOGY. 



the stomach. The collagen of connective tissue is probably converted 

 first into gelatine and then into gelatoses and gelatine peptones. The 

 protein constituent of the conjugated proteins is usually converted into 

 proteose and peptone, the prosthetic group being set free. Thus in 

 the digestion of nucleo-protein by gastric juice an insoluble residue of 

 nuclein is formed ; in the digestion of mucin (gluco-protein) glucosamine 

 is found in the products. 



(2) The effect of gastric juice upon caseinogen, the phospho-proteiri 

 of milk, is peculiar in that there is a conversion of the caseinogen into 

 a comparatively insoluble substance, casein. This action of gastric 

 juice has been for many years ascribed to a separate ferment called 

 rennin, but latterly evidence has been brought forward which suggests 

 that the formation of casein from caseinogen is due to pepsin itself. 

 The matter has not been .conclusively settled, and it will be convenient 

 to retain the term rennin in the meantime when describing the effect of 

 gastric juice on caseinogen. 



The action of rennin can be demonstrated by adding a little of an 

 extract containing this enzyme to a quantity of milk and allowing the 

 mixture to stand for a time at a temperature of 37 C. In a few minutes 

 the milk becomes clotted, and after a time the clot shrinks, squeezing out 

 a clear fluid, whey, which contains all the constituents of milk except 

 caseinogen and fat. It can be shown that the fat is entangled in the 

 clot in an unaltered form, so that the coagulation is brought about by 

 the action of the rennin on the caseinogen. If a little potassium oxalate 

 is added to milk, the subsequent addition of rennin does not result in 

 the formation of a clot, but if calcium chloride be added clotting occurs. 



Three factors are necessary for the formation of the clot, namely, 

 caseinogen, rennin, and lime salts. If rennin is added to a solution 

 of pure caseinogen, and the mixture is kept for a short time at a 

 temperature of 37 C. and then boiled to kill the enzyme, the addition 

 of calcium chloride will bring about the formation of casein. Obviously 

 the enzyme has produced some change in the caseinogen, and the only 

 factor required to complete the conversion into casein is the addition of 

 lime salts. There is in the first place, therefore, a conversion of casein- 

 ogen into " soluble casein " by the action of the enzyme, and secondly, 

 soluble casein combines with lime salts with the production of insoluble 

 casein. The process may be represented thus : 



Caseinogen<-Rennin (or pepsin) 



I 

 Soluble casein Calcium salts 



I I 



I 

 Casein 



