138 PHYSIOLOGICAL CHEMISTRY 



tion of egg white bears a relationship to the digestive power of 

 the solution. The length of the column of egg white digested 

 is proportional to the square root of the amount of the enzyme. 

 The products produced by the digestion of the protein inhibit 

 the digestive action of the pepsin, so the method is not an ideal 

 one, but it is useful for obtaining comparative data. 



Products of Peptic Digestion. Pepsin-hydrochloric acid at- 

 tacks the proteins and breaks them up into fragments. The acid 

 metaprotein first breaks into proteoses of varying degrees of 

 complexity. These are again broken into products which are not 

 precipitated by saturation with ammonium sulphate, the pep- 

 tones. It is probable in fact that peptones also are produced 

 directly from the protein molecule in the first stages of hydro- 

 lysis. There has been much discussion as to whether or not 

 amino acids are split off by pepsin. Weight of evidence seems 

 to indicate that pepsin does not split off amino acids, and that 

 the presence of these substances in mixtures digested by an ex- 

 tract of the stomach wall was due to another enzyme, called 

 erepsin, or to still other agencies. The proteoses and peptones 

 produced by the action of pepsin vary greatly in their composi- 

 tion and properties, as might well be expected on considering the 

 extremely complex nature of the protein from which they are 

 produced. 



The ultimate fate of pepsin is a matter of interest, of which 

 little is known. It probably is largely destroyed in the intestine, 

 but a portion undoubtedly is absorbed into the blood. What 

 happens to this portion is not known, but a part at least is ex- 

 creted in the urine. Perhaps the remainder is destroyed by 

 agents in the blood. 



Rennin. Gastric juice, or the water extract of the stomach 

 mucosa of a young animal contains an enzyme rennin, which 

 is instrumental in clotting milk. For this clotting, calcium also 

 is necessary. The process is believed to consist in the transforma- 

 tion of the soluble protein caseinogen into a derivative called 

 paracasein. In this process an "albumose" or "whey protein" 

 is split off. The calcium salt of caseinogen is soluble, but that of 



