CHAP. II.] METHODS OF PREPARING PEPSIN. 87 



latter, which is shaken again and again with ether, until all traces of 

 cholesterin are removed. The aqueous solution is then found to be 

 slightly turbid, but on filtration may be obtained perfectly clear. 

 The filtered liquid, when acidulated, possesses proteolytic activity. 

 When allowed to evaporate spontaneously, it leaves a greyish, 

 amorphous, n on- hygroscopic, nitrogenous body, soluble with some 

 difficulty in water, but more readily soluble in dilute acids. 



There is no reason to suppose that the substance obtained by 

 this method is a definite chemical individual. It has been alleged in 

 its favour that it yields the digestive enzyme pepsin in a purer con- 

 dition than that in which it could be obtained by older methods. 

 The process of preparation is however tedious in the extreme and 

 very costly, and it has made way for other methods which furnish us 

 with much more active solutions of pepsin, though still not of a pepsin 

 which we can consider to represent the pure enzyme. This method 

 has not in the hands of later observers given satisfactory results. 



An aqueous solution of Brlicke's pepsin is not precipitable by 

 platinum tetrachloride, by mercuric chloride, by lead acetate, neutral 

 or basic, by tannic acid, by iodine, or by concentrated nitric acid. 

 The solution only exhibits in a faint manner the xanthoproteic re- 

 action. According to Briicke it is precipitated by neutral and basic 

 lead acetate and made cloudy by solution of platinum tetrachloride ; 

 though Krasilinikow, in Briicke's laboratory, by dialysing the solu- 

 tion, got rid of the platinum reaction, but not of the lead acetate 

 reaction. 



v. Wittich's Pepsin, as was discovered by v. Wittich 1 , shares 

 method of pre- the property possessed by the majority of enzymes, of 

 paring a solu- being soluble in glycerin. In order to prepare a gly- 

 tagiyc f erin PSl cer in extract of pepsin, the finely divided and cleansed 

 mucous membrane of the fundus of the stomach may 

 be placed for eight or ten days in concentrated glycerin. On subse- 

 quently straining and filtering, a glycerin solution of pepsin of 

 considerable activity is obtained. 



A better way of proceeding is to place the mucous membrane 

 for twenty-four hours in water, and then to dehydrate the finely 

 divided mucous membrane by placing it for twenty-four hours in an 

 excess of 80 per cent, alcohol, filtering, driving off the alcohol which 

 adheres to the tissue by evaporation in air, comminuting the dried 

 residue still further, and then adding to it its own weight of glycerin. 

 After some days the glycerin is strained off and replaced by a 

 fresh quantity, the process being repeated several times. 



From the glycerin extract of pepsin the impure ferment may be 

 obtained in a solid form by adding a large excess of absolute alcohol, 

 which precipitates it. This impure pepsin may be further precipi- 

 tated by having recourse to the method described in the next 



1 v. Wittich, Pfliiger's Archir, Vol. n. p. 193, and Vol. in. p. 193. 



