90 THE ACTION OF ENZYMES 



(6) Ammonia (or its compounds) are also generally 

 met with in the mixture of dissociation products. 

 Possibly in some instances it is a secondary product 

 derived from the decomposition of some of the amino- 

 acids. 



The relative amounts of all these substances vary 

 considerably. For example, over 22 per cent, of leucin 

 is obtained from the hydrolysis of egg - white, from 

 casein 10 per cent, while gelatine yields only about 

 2 per cent, of this substance. The amounts of glutamic 

 acid from casein is about 10 per cent, from gelatine 

 14 per cent., from serum globulin about 2 per cent 



6. Proteases or Proteolytic Enzymes. Soon after 

 food is taken into the stomach of an animal a colourless, 

 acid liquid, the gastric juice, is poured out from the gastric 

 glands. This juice contains among other substances two 

 enzymes, namely, pepsin and rennin^ and a small amount 

 (.2 to .5 per cent.) of hydrochloric acid. The complex, 

 insoluble proteins present in the food are very soon acted 

 upon by the pepsin, with the result that they are broken 

 down, step by step, into a series of soluble proteins of 

 simpler chemical constitution which can readily diffuse 

 through animal membranes and be transformed into 

 materials subsequently used in building up the body and 

 repairing its waste. Such chemical decomposition is 

 spoken of as proteolysis : the various stages of the process 

 are indicated in a general way below. 



