150 THE DIGESTIVE FLUIDS. 



and here as there the action of the enzyme ultimately ceases 

 when the digestive products accumulate beyond a certain amount. 

 Impure extracts can be prepared, as has been pointed out, by 

 extracting the gland with glycerin after it has been left exposed to 

 the air. The purer forms, on the other hand, which can be obtained 

 according to Kiihne's method (see below), are insoluble in glycerin 

 and alcohol, but soluble in water. 



^ Of the chemical nature of trypsin little is known. In acid solu- 

 tion it is coagulated by heat, and, according to Kiihne, decomposed 

 into an albumin and peptone. An analysis of a fairly pure prepara- 

 tion has given the following results : carbon, 52.75 percent.; hydro- 

 gen, 7.51; nitrogen, 16.55; oxygen plus sulphur, 23.19 (Loew). 

 Its composition is thus very similar to that of the peptones, and it is 

 hence possible that, unlike the other digestive ferments which we 

 have thus far considered, trypsin may be an albuminous substance. 



Test for Trypsin. The test for trypsin resolves itself into the 

 demonstration of the presence of a proteolytic ferment which is 

 capable of digesting albumins in alkaline solution, with the ultimate 

 formation of amino-acids. To this end (if necessary) the solution in 

 question is rendered alkaline with sodium carbonate to the extent of 

 from 0.25 to 1.0 per cent. A small flake of fibrin is then added and 

 an amount of thymol sufficient to saturate the solution, so as to guard 

 against putrefactive processes. The mixture is kept at a tempera- 

 ture of 40 C. If the fibrin is then dissolved and leucin or tyrosin 

 can be demonstrated in the resulting solution, the presence of trypsin 

 may be inferred. To this end, it is only necessary to evaporate the 

 solution to a thick syrup and to examine this microscopically (see 

 tyrosin). Should the solution to be tested contain a free mineral 

 acid, or larger amounts of organic acids, no result will be obtained, 

 as the trypsin has in that case been destroyed. 



As Abderhalden has shown, trypsin in contradistinction to pepsin 

 is capable of splitting peptids containing an amino-acid which is 

 soluble with difficulty (tyrosin, cystin), such as glycyl-1-ty rosin. 



Isolation of Trypsin. Unless it is desired to obtain trypsin in as 

 pure a form as possible, alkaline solutions of the common pancreatin 

 preparations which are sold in the shops can be used for experimental 

 purposes. Otherwise the method of Kiihne, as modified by Gautier, 

 should be employed : The fresh pancreas is finely hashed and 

 washed with ice-cold water, to which 1 pro mille of salicylic acid 

 has been added. After four hours the mass is treated with a 

 large amount of a 5 pro mille solution of sodium carbonate, 

 containing an excess of thymol, and is kept for twelve hours at a 

 temperature of from 37 to 40 C. The acid and alkaline extracts 

 are then mixed, treated with 0.5 per cent, of sodium carbonate, 

 filtered, feebly acidulated with acetic acid, and saturated with 

 ammonium sulphate in substance. The precipitate which then 

 separates out contains all the trypsin. It is dissolved in water, 

 filtered, and dialyzed, so as to remove the salt which is still present, 

 as also traces of peptones and leucin. The resulting solution is 



