712 PHYSIOLOGY OF DIGESTION AND SECRETION. 



influenced in its action by temperature, as is the case with the other 

 enzymes; low temperatures retard, and may even suspend its 

 activity, while high temperatures increase it. The optimum tem- 

 perature is stated to be from 37 to 40 C., while exposure for some 

 time to 80 C. results, when the pepsin is in a moist condition, in the 

 total destruction of the enzyme. Pepsin may be extracted from the 

 gastric mucous membrane by a variety of methods and in different 

 degrees of purity and strength. The commercial preparations of 

 pepsin consist usually of some form of extract of the gastric mucous 

 membrane to which starch or sugar of milk has been added. Labora- 

 tory preparations are made conveniently by mincing thoroughly 

 the mucous membrane and then extracting for a long time with 

 glycerin. Glycerin extracts, if not too much diluted with water or 

 blood, keep for an indefinite time. Purer preparations of pepsin 

 have been made by what is known as "Briicke's method," in which 

 the mucous membrane is minced and is then self-digested with a 5 

 per cent, solution of phosphoric acid. The phosphoric acid is pre- 

 cipitated by the addition of lime-water, and the pepsin is carried 

 down in the flocculent precipitate. This precipitate, after being 

 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 adhere to precipitates occurring in their solutions. 



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

 mined. Most authors agree that it is a protein or protein-containing 

 body. 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 nucleo- 

 proteins. 



Pepsin is supposed to be formed in the chief 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 

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



