DIGESTION AND ABSORPTION IN THE STOMACH. 785 



washed, is carried into solution by dilute hydrochloric acid, and a 

 solution of cholesterin in alcohol and ether is added: The cholesterin 

 is precipitated, and, as before, carries down with it the pepsin. This 

 precipitate is collected, carefully washed, and then treated repeatedly 

 with ether, which dissolves and removes the cholesterin, leaving the 

 pepsin in aqueous solution. This method is interesting not only 

 because it gives a pure form of pepsin, but also in that it illustrates 

 one of the properties of enzymes namely, the readiness with which 

 they are adsorbed by precipitates occurring in their solutions. 



In spite of much work, the chemical nature of pepsin is undeter- 

 mined. Pekelharing* has prepared pepsin from gastric juice by 

 dialysis, the substance precipitating as the acid is dialyzed off. 

 The precipitate may be purified by repeated resolutions in acid 

 followed by dialysis. As prepared by this method pepsin is a 

 substance of a protein nature which contains sulphur and also 

 some chlorin, but no phosphorus. It does not belong, therefore, 

 to the group of nucleoproteins. Other authors, on the contrary, 

 assert that active preparations of pepsin may be obtained which 

 give no protein reactions, although they contain nitrogen. 



Pepsin is supposed to be formed in the central cells of the gastric 

 tubules, but as in other cases it is present in the cells as a zymogen 

 or propepsin, which is not changed to the active pepsin until after 

 secretion. The propepsin may be extracted readily from the mucous 

 membrane, and, since it is known that the zymogen is converted 

 quickly to active pepsin by the action of acids, it is evident that in 

 the normal gastric juice the existence of the hydrochloric acid 

 insures that all of the pepsin shall be present in active form. There 

 has been much discussion as to the nature of the secretion of the 

 pyloric glands. Heidenhain isolated this portion of the stomach and 

 collected its secretion. He found that it was alkaline and contained 

 pepsin. Later observers, however, still continue to doubt the secre- 

 tion of a true pepsin in this portion of the stomach. Glaessnerf 

 states that propepsin can not be obtained from extracts of the pyloric 

 glands, and that the proteolytic enzyme that can be shown in this 

 portion of the stomach by self-digestion in acid or alkaline media is 

 not a true gastric pepsin. The possibility that a special secretin 

 (hormone) is formed in the pyloric mucous membrane has been 

 referred to above (p. 782). From the description of the events 

 in the stomach (p. 727) it would seem that the food material which 

 is churned and stirred by the contractions of the pyloric musculature 

 has already been charged with pepsin and hydrochloric acid by the 

 glands of the middle and fundic regions before reaching the 

 ant rum pylori. 



Artificial Gastric Juice. In studying peptic digestion it is not 



Pekelharing, "Zeitschrift f. physiol. Chemie," 35, 8, 1902. 

 t Glaessner, "Beitrage zur chem. Physiol. u. Pathol.," 1, 24, 1901. 

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