THE BIOCHEMICAL PROCESSES OF DIGESTION 485 



occlusion of the arteries, the acid may penetrate the cells and cause 

 digestion of the stomach Avails. Hyperacidity may on this account 

 become dangerous, as it lowers the resistance of the cell. 



The digestive action of hydrochloric acid is closely linked with that of 

 pepsin, with which it will, therefore, be considered. 



The Action of Pepsin 



It is commonly believed that before its secretion pepsin exists in the 

 cells of the gastric glands as zymogen granules. The chief evidence for 

 this belief appears to be that after considerable activity the amount of 

 zymogen granules in the gland cells is found to be decidedly dimin- 

 ished. By such an hypothesis it is easy to explain certain interesting 

 results concerning the effect of weak alkali on the activities of extracts 

 of the mucous membrane of the stomach. When the mucous membrane 

 is extracted with weak acids, the extract is very active proteolytically. 

 If this so-called pepsin solution be made faintly alkaline, or even only 

 neutralized, and again made acid, it will be found to have lost much, 

 if not all, of its activity. On the other hand, an aqueous extract may be 

 rendered slightly alkaline for a short time and still display its digestive 

 activity on subsequent acidification. The extract made with water is 

 therefore much more resistant toward alkali than that made with weak 

 acid, and the difference is explained on the supposition that the watery 

 extract contains pepsinogen, whereas the acid extract contains pepsin. 



It is believed that there are several varieties of pepsin, because the 

 optimum concentration of acid in which pepsin derived from the stomachs 

 of different animals acts is not always the same. Pepsin of the dog, for 

 example, acts, best in a hydrogen-ion concentration corresponding to 

 that of a 0.05 N. hydrochloric acid solution, whereas that of the human 

 stomach works best at a concentration of 0.03 N. Different pepsin 

 solutions also shoAv a difference with regard to the optimum tempera- 

 ture at which they act, and with regard to the nature of the protein 

 which they most readily attack. Thus,' the pepsin of a calf's stomach 

 digests casein very rapidly, but coagulated egg white only slowly, 

 whereas the pepsin of the pig's stomach acts on both these proteins at 

 about the same rate. 



It is well known that the activity of pepsin can proceed only in the 

 presence of acids, but this action of acids does not appear to depend on 

 the hydrogen-ion concentration alone, for when equal quantities of the 

 same pepsin are mixed with quantities of different acids so that the 

 hydrogen-ion concentration of the mixtures is uniform, it is found that 

 digestion proceeds most rapidly with hydrochloric acid and least rapidly 

 with sulphuric acid. The S0 4 ion seems, therefore, to be unfavorable 



