PEPSIN. 23 



a putrid odour becomes perceptible. The aqueous infusion 

 thus obtained is precipitated with acetate of lead, which causes 

 a white flocculeut deposit, containing the pepsin mixed Avith 

 much albumen ; this precipitate is diffused through the water, 

 and must be decomposed by sulphuretted hydrogen. When the 

 liquor is filtered, the solution contains pepsin and acetic acid, 

 while coagulated albumen and sulplmret of lead remain on the 

 filter. In order to obtain solid pepsin, the filtered liquid is 

 evaporated to the consistence of a syrup, at a very moderate 

 temperature (according to AYasmanu, not higher than 95°), and 

 absolute alcohol is then added to it. After some time a whitish 

 bulky precipitate is formed, which is to be dried by exposure 

 to the air ; it then constitutes a yellowish viscid mass of a pecu- 

 liar animal odour, and a disagreeable taste. Pepsin thus ob- 

 tained has an acid reaction, because it always contains a small 

 quantity of acetic acid. This is most efficaciously removed by 

 heating the pepsin for some hours in a salt-water bath ; by 

 which means a white powder, soluble in water and possessing 

 no acid reaction, is obtained. The action of a high temperature 

 injm-es the digestive power of pepsin, but does not affect its 

 chemical composition. 



From Yogel's analysisi of this substance, it appears that it 

 may be very nearly represented by the formula C^^ H^^ N^ Oj^. 

 On comparing this with Liebig's formula for protein, it appears 

 that pepsin may be formed from protein by the subtraction of 

 tw^o atoms of water, and the addition of two atoms of nitrogen. 



The most remarkable property of pepsin is the power which 

 its aqueous solution, when shghtly acidulated, possesses of dis- 

 sohing the protein-compounds. A solution containing only 

 55^ part of pepsin, and slightly acidulated, will dissolve coagu- 

 lated albumen in six or eight hours. This property is appa- 

 rently destroyed by the alkalies. 



Sulphuric, hydrochloric, and nitric acids, Avhen added in very 

 small quantity to a solution of pepsin, throw down white flocculi, 

 which redissolve in an excess of the test : on the addition of 

 still more acid the precipitate again occurs. 



Acetic acid throws down a precipitate Avhich redissolves in an 

 excess of the test ; no second precipitate is thrown down by the 

 addition of more acetic acid. 



' See Appendix I, Note 12. 



