GASTRIC DIGESTION 141 



or with organic acids. On the other hand, it is possible that hydro- 

 chloric acid is set free from combination with some weak base as 

 ammonia 1 or that free phosphoric acid valences may be set free by the 

 enzymic hydrolysis of organic phosphoric acid esters. 2 We cannot go 

 into a discussion of these various theories. That the hydrochloric 

 acid arises from the chlorides of the blood appears to be well established 

 but the same cannot be said with regard to the immediate or ultimate 

 origin of the hydrogen ions involved in the reaction. 



The most important of the enzymes of the gastric juice is the pro- 

 teolytic enzyme pepsin. The pepsin does not originate as such in the 

 gastric cells but is formed from its precursor, the zymogen or mother- 

 substance pepsinogen, which is apparently produced by the parietal 

 cells of the fundus as well as by the chief cells of the fundus and pyloric 

 glands. Pepsinogen may be differentiated from pepsin from the fact 

 that it is more resistant to alkali. 3 Upon coming into contact with the 

 hydrochloric acid of the secretion this pepsinogen is immediately trans- 

 formed into pepsin. Pepsin is not active in alkaline or neutral solutions, 

 but requires at least a faint acidity before it can exert its power to dis- 

 solve and digest proteins. The percentage of hydrochloric acid facili- 

 tating the most rapid peptic action varies with the character of the 

 protein acted upon, e.g., 0.08 per cent to o.i per cent for the digestion of 

 fibrin and 0.25 per cent for the digestion of coagulated egg-white. 

 While hydrochloric acid is the acid usually employed to promote arti- 

 ficial peptic proteolysis, other acids, organic and inorganic, will serve the 

 same purpose. Acidity of the liquid is necessary to promote the 

 activity of the pepsin, but the acidity need not necessarily be confined 

 to hydrochloric acid. 



In common with many other enzymes pepsin acts best at about 

 38-4oC. and its digestive power decreases as the temperature is low- 

 ered, the enzyme being only slightly active at oC. Its power is only 

 temporarily inhibited by the application of such low temperatures, 

 however, and the enzyme regains its full proteolytic power upon rais- 

 ing the temperature to 4oC. As the temperature of a digestive mix- 

 ture is raised above 40 C. the pepsin gradually loses its activity 

 until at about 8o-iooC. its proteolytic power is permanently 

 destroyed. 



Our ideas regarding the nature of the products formed in the course 

 of peptic proteolysis have undergone considerable revision in recent 

 years. The former view that these products included only acid albu- 

 minate (acid metaprotein), proteoses, peptones and pep tides is no 



1 Mathews: Physiological Chemistry, New York, 1915, p. 374. 

 2 Bergeim: Proc. Soc. Exp. Biol. and Med., 12, 21, 1914. 

 3 Langley: Jour, of PhysioL, 3, 246. 



