306 - Multicellular Animals, Especially Man 



standing characteristic of the gastric juice is 

 its extreme acidity (pH = 1.2 to 0.3); and a 

 complete analysis of the juice shows the 

 average composition given in Table 16-2. 

 Functions of the Gastric Juice. The digestive 

 functions of the gastric juice are all related 

 to its enzymes, of which pepsin is the most 

 important. This very potent protease is 

 especially active in the initial stages of the 

 hydrolysis of proteins. Pepsin hydrolv/es each 

 large protein molecule into a number of 

 smaller molecules, mainly long-chain pep- 

 tides; and this reaction, as catalyzed by pep- 

 sin, may be written: 



1 mol protein 



+ 

 X mol water 



HCI 



X mol peptides 



(mainly long-chain) 



Specifically, pepsin is a powerful endo- 

 peptidase, which achieves a rupturing of 

 peptide bonds, not at the ends of the protein 

 chain, but at definite points along its length. 

 In fact, it is only the linkages adjacent to two 

 specific amino acids, namely, tyrosine and 

 phenylalanine (p. 102), which arc susceptible 

 to the hydrolytic action of pepsin. But the 

 net result of the action of pepsin in the stom- 

 ach is the conversion of the various complex 

 and frequently insoluble protein components 

 of the ingested food into much simpler 

 soluble compounds. Thus when a mass of 

 coagulated egg albumen is incubated at body 

 temperature in a test tube containing gastric 

 juice, the material dissolves within about 

 half an hour, and suitable tests prove thai 

 each of the huge molecules of albumen has 

 decomposed into about 100 smaller mole- 

 cules, which can be identified mainly as long- 

 chain peptides. 



The presence of the acid is essential to the 



activity of pepsin; neutralized gastric juice 

 displays very little peptic activity. The en- 

 zyme pepsin, as secreted by the cells of the 

 gastric glands, is in an inactive form, called 

 pepsinogen; but pepsinogen is converted 

 into pepsin when it comes into contact with 

 an acid medium. The hydrochloric acid also 

 augments the activity of the other enzymes 

 of the gastric juice; and perhaps the germi- 

 cidal properties of this acid are important in 

 counteracting infective bacteria ingested with 

 the food. 



In Mammalia, the gastric juice contains 

 an enzyme, rennin, which acts solely upon 

 caseinogen, the main protein present in milk. 

 When milk, which is the main diet of all 

 young mammals, comes into contact with the 

 gastric juice, the milk is curded immediately. 

 Essentially curding represents a chemical re- 

 action whereby the soluble protein caseino- 

 gen is converted to an insoluble protein, 

 casein, which comes out of solution as a fine 

 flocculent precipitate. This action is impor- 

 tant, because curding delays the passage of 

 the milk through the stomach, allowing time 

 for pepsin to digest the casein to the same 

 extent as other proteins. Were the milk to 

 remain in its native fluid stale, it would be 

 evacuated too quickly from the stomach, 

 since fluids such as water are passed on to 

 the small intestine within a few minutes after 

 they are drunk. Since ancient times, rennin 

 extracts from calves' stomachs have been 

 used in making certain cheeses; and the ac- 

 tive component of modern "junket" prepara- 

 tions is rennin. 



The presence of lipase in the gastric juice 

 is debatable; at best the gastric lipase has a 

 very weak fat-splitting action, eltective 

 mainly upon finely emulsified fats, such as 

 milk and cream. In adults, il adequate pre- 

 cautions are taken against a contamination 

 ol the gastric juice by juices regurgitated in- 

 to the stomach from the small intestine, little 

 il any lipase activity can be delected in the 

 gastric juice. 



Control of the Flow of Gastric Juice. The total 

 quantity of gastric juice produced during 



