CHEMICAL BASIS OF THE ANIMAL BODY. 59 



Pepsin. 



This is the characteristic proteolytic enzyme of gastric juice. It 

 was first separated out in an approximately pure form by Briicke 1 . 



His method was as follows. The mucous membrane of the stomach is separated 

 from the muscular coats, finely chopped and digested with a large volume of 5 p. c. 

 phosphoric acid. The fluid thus obtained is strained off through linen, and filtered, 

 and lime-water is added until the reaction is just not quite neutral ; by this means a 

 precipitate of calcium phosphate is obtained to which all the pepsin is adherent. 

 The precipitate is now filtered off, dissolved in a minimal amount of dilute hydro- 

 chloric acid and again precipitated by the addition of lime-water; this second 

 precipitation frees the pepsin largely from the proteids which were at first carried 

 down with it. This second precipitate is now as before dissolved in dilute hydro- 

 chloric acid. From this the pepsin is separated as follows. Cholesterin is dissolved 

 in a mixture of four parts of alcohol and one of ether, and this solution is introduced 

 below the solution of pepsin by means of a long thistle-tube. As soon as the 

 cholesterin comes in contact with the water it separates out and the separation 

 is completed, as a finely granular mass, by violently shaking the vessel in which the 

 mixture is contained. The pepsin adheres now to the cholesterin, which is filtered 

 off, washed first with water faintly acidulated with acetic acid and finally with pure 

 water. On treating the mass with pure ether in a separating- funnel the cholesterin 

 goes into solution in the ether which forms an upper layer, below which is an aqueous 

 solution of pepsin, which must be shaken up several times with renewed portions of 

 ether until all the cholesterin has been extracted. The aqueous solution of the 

 enzyme thus obtained is exposed to the air until it is free from ether, and is then 

 filtered. It may be further purified by dialysis, and is now found to give none of 

 the reactions characteristic of proteids and to be precipitable only by the acetates 

 of lead. It yielded no trace of opalescence on the addition of tannic acid, though 

 this is capable of detecting one part of proteid in 100,000 of solvent 2 . 



From the reactions of the pepsin solution obtained by Briicke's 

 method, it seems justifiable to consider that the enzyme is not really a 

 proteid. The same conclusion may be deduced from the more recent 

 investigation of Sundberg 3 . No analyses of purified pepsin appear to 

 have been made as yet, so that the views as to its non-proteid nature 

 are based solely upon the reactions of its solutions as described by 

 Briicke and Sundberg, reactions which, as already pointed out, are not 

 really conclusive. 



Preparation of peptic digestive fluids. If a few drops of a glycerine 

 extract of gastric mucous membrane be added to dilute (-2 p.c.) hy- 

 drochloric acid or if the tissue be simply extracted for a short time 

 with the dilute acid and the extract be filtered a solution is obtained 



1 Sitzb. d. Wien. Akad. Bd. XLIII. (1861), S. 601. See also his Varies, iiber 

 Physiol. (sub pepsin). 



2 Hofmeister, Zt. f. physiol, Chem. Bd. n. (1878), S. 292. 



3 Zt. f. physiol. Chem. Bd. ix. (1885), S. 319. But see Low, Pfluger's Arch. Bd. 

 xxxvi. (1885), S. 170. . 



