CHAPTER VI. 



THE ACTION OF THE ENZYMES FOUND IN THE HUMAN 

 ALIMENTARY TRACT (Continued). 



4. Acid-proteinase (pepsin). By this term is understood 

 a proteolytic ferment which acts only in the presence of 

 an acid. Acid-proteinase is found widely distributed in 

 nature in both animal and vegetable cells. It is found in 

 many regions in the human body, more especially in the 

 muscles, and, what interests us most in this connection, in 

 the mucous lining and secretion of the stomach. Acid- 

 proteinase has the power of acting upon proteins in the 

 presence of certain acids and splitting these proteins into 

 a series of simpler substances. What these simpler sub- 

 stances are will be discussed further on. 



Acid-proteinase is present in the stomach of the human 

 being from the time of birth. Neither at this time nor later 

 in life, however, do all parts of the stomach contain it in 

 the same amount. The cardiac end of the human stomach 

 contains much more than the pyloric end. 



Pure preparations of acid-proteinase are exceedingly diffi- 

 cult to obtain. Extraction of the mucosa of the pig's stomach 

 with a 0.2 percent hydrochloric acid solution or with glycerine 

 yields very active preparations. These are, however, very 

 impure. BRUCKE'S method of extraction of the mucous 

 membrane with dilute phosphoric acid and subsequent 

 neutralization with calcium hydroxide seems to yield very 

 pure acid-proteinase. The enzyme is carried down mechan- 

 ically with the precipitate of calcium phosphate, and can be 



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