THE CHEMISTRY OF ALBUMINS 189 



tion of a watch, if it were stated to be made up of a certain 

 weight of glass, of silver, of gold, of brass and of steel, together 

 with a few precious stones. It is obviously necessary that, in 

 order to get some idea of the construction of the watch, it 

 must be taken to pieces carefully and each independent 

 portion separately described. Similarly, in order to obtain 

 even an approximate idea of the structure of the albumin 

 molecule, means must be found to take it to pieces gradually, 

 and to identify the products thus obtained. In order to 

 accomplish this two means are at our disposal, viz., in the first 

 place the action of acids, in the second place, and especially, the 

 action of so-called proteolytic enzymes, that is, enzymes which 

 are capable of breaking up protein substances. Of these the 

 two chief are pepsin and trypsin. The methods of preparation 

 of these and their characteristic modes of action may now be 

 usefully considered. 



Pepsin. This is a characteristic enzyme of the gastric 

 juice. Ordinary * liquor pepticus ' is prepared by macerating 

 the mucous membrane of the stomach of a dog or pig with 

 dilute hydrochloric acid, 0*2 per cent., and filtering the 

 solution. The filtered solution contains pepsin. 



By extraction with glycerine in absence of acid a purer 

 but less active product is obtained. The enzyme can be 

 further purified by precipitation with sodium phosphate and 

 calcium chloride, the calcium phosphate formed carrying down 

 the enzyme. The enzyme is separated from the precipitate 

 by solution in hydrochloric acid, and the mineral salts removed 

 by dialysis, the salts passing through the parchment mem- 

 brane, leaving a solution of the enzyme in the dialyser. 



Trypsin. Trypsin is the enzyme of the pancreatic juice 

 and is obtained in a similar manner to pepsin, by digesting 

 pancreatic tissue with dilute acid or glycerine at 35 to 

 40 C. The preparation of the pure enzyme is an exceedingly 

 complex process. 



