CHEMISTRY OF DIGESTION AXD NUTRITION. 291 



pepsin is in a moist condition, in the total destruction of the enzyme. Pepsin 

 has never been isolated in sufficient purity for satisfactory analysis. It may be 

 extracted, however, 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. Laboratory preparations are 

 usually made by mincing thoroughly the mucous membrane and then extract- 

 ing 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 precipitated by the addition of lime- 

 water, and the pepsin is carried down in the flocculent precipitate. This pre- 

 cipitate, after being washed, is carried into solution by dilute hydrochloric 

 acid, and a solution of cholesterin in alcohol and ether is added. The choles- 

 terin 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 the 

 purest form of pepsin, but also in that it illustrates one of the properties of 

 this enzyme — namely, the readiness with which it adheres to precipitates occur- 

 ring in its solutions. Pepsin illustrates very well two of the general properties 

 of enzymes that have been described (p. 281): first, its action is incomplete, the 

 accumulation of the products of digestion inhibiting further activity at a certain 

 stage; and, secondly, a small amount of the pepsin, if given sufficient time and 

 the proper conditions, will digest a very large amount of proteid. 



Artificial Gastric Juice. — In studying peptic digestion it is not necessary 

 for all purposes to establish a gastric fistula to get the normal secretion. The 

 active agents of the normal juice are pepsin and acid of a proper strength ; and, 

 as the pepsin can be extracted and preserved in various ways, and the 1 1 CI can 

 easily be made of the proper strength, an artificial juice can be obtained at any 

 time which may be used in place of the normal secretion for many purposes. In 

 laboratory experiments it is customary to employ a glycerin extract of the gasl ric 

 mucous membrane, and to add a small portion of this extract to a large bulk of 

 0.2 per cent. 1 101. The artificial juice thus made, when kept at a temperature of 

 from 37° to 40° C, will digest proteids rapidly if the preparation of pepsin is a 

 good one. While the strength of the acid employed is generally from 0.2 to 0.3 

 per cent., digestion will take place in solutions of greater or less acidity. Too 

 great or too small an acidity, however, will retard the process; that is, there is 

 for the action of the pepsin an optimum acidity which lies somewhere between 

 0.2 and 0.5 percent. Other acids may be used in place of the IIC1 — for example, 

 nitric, phosphoric, or lactic — although they are not so effective, and the opti- 

 mum acidity is different for each; fur phosphoric acid it is given as 2 percent. 



Action of Pepsin-Hydrochloric Acid on Proteids. — It has been knovvn 

 for a long time that solid proteids, such as boiled egg-, when exposed to the 



