CHEMISTRY OF DIGESTION AND NUTRITION. 229 



ing for a Icmg 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. 219): 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 HC1 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 gastric 

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

 0.2 per cent. HC1. 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 per cent. Other acids may be used in place of the HC1 for example, 

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

 mum acidity is different for each ; for phosphoric acid it is given as 2 per cent. 



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

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

 action of a normal or an artificial gastric juice, swell up and eventually pass 

 into solution. The soluble proteid thus formed was known not to be coagu- 

 lated by heat ; it was remarkable also for being more diffusible than other 

 forms of soluble proteids, and was further characterized by certain positive 

 and negative reactions which will be described more explicitly farther on. 

 This end-product of digestion was formerly described as a soluble proteid 

 with properties fitting it for rapid absorption, and the name of peptone was 



