428 ANIMAL BIOCHEMISTRY 



hydrochloric acid by the parietal cells, and pepsinogen by the chief 

 cells. Both mucin formation and pepsinogen formation (converted 

 to pepsin) appear to be primarily problems of protein synthesis, and 

 little is known of these two cases. Secretion of hydrochloric acid is 

 stimulated by histamine formed by decarboxylation of histidine. The 

 mode of action is unknown. Likewise the mechanism of hydrochloric 

 acid formation is still in doubt in spite of much study and the pro- 

 posal of several theories. An ample supply of chloride ion is available 

 from the blood, but the large-scale concentrating mechanism needed 

 for the hydrogen ions has not been identified. 



Gastric digestion follows the action of salivary amylase mentioned 

 earlier and begins when secretion of gastric juice and muscular activity 

 combine to lower pH below 4. At this acidity the salivary amylase 

 ceases to function and pepsin becomes active. Thereafter, gastric 

 digestion is proteolytic in nature and is catalyzed at the low pH by 

 pepsin, which shows its maximal activity at pH 1.5, still functions, 

 though much more slowly, at pH 3, but is inactive for practical pur- 

 poses above pH 4. 



It is probable that proteins must first be denatured before they are 

 attacked by pepsin, but most proteins are readily denatured at low 

 pH. Thus the normal environment in the stomach prepares ingested 

 proteins for peptic hydrolysis. The hydrolysis is never complete for 

 two reasons. Even under optimal equilibrium conditions, pepsin 

 cannot split all the peptide bonds of any protein. Furthermore, the 

 stomach contents are not retained long enough to reach even the 

 equilibrium level of hydrolysis. Instead the denatured proteins are 

 broken into large water-soluble fragments composed of several amino 

 acid units. These polypeptides are sometimes called peptones and 

 are digested further in the intestinal tract. Limited amounts of small 

 peptides and free amino acids appear with the peptones but probably 

 comprise only a small fraction of the material potentially available. 

 Operation of the pyloric sphincter allows the digesting slurry of 

 stomach contents to pass slowly from the stomach for the subsequent 

 operations of intestinal digestion. 



A dilemma is readily apparent in contrasting the action of pepsin 

 on the ingested proteins with the apparent lack of hydrolysis of the 

 stomach itself. Certainly the stomach of one animal is digestible 

 when eaten by another individual. Therefore, a unique structure 

 of the protein is not involved. Perhaps the cells of the living stomach 

 somehow resist denaturation by the hydrochloric acid and thus are 

 not susceptible to pepsin. Conceivably the gastric mucin might offer 

 protection to the surface. In any event, whatever the unknown protec- 



