406 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



The test tubes are placed in a bath at 40. 



Pepsin will only act in the presence of acid ; consequently digestion 

 or solution of the fibrin will only take place in the third test tube, 

 where, in about half an hour, the fibrin will have disappeared. In 

 order to show that it is not the acid which has this effect, the second 

 tube, containing no pepsin but only acid, was used as a control. In 

 this tube the fibrin will have become swollen, but not dissolved. The 

 first, fourth and fifth tubes will be seen to be unaltered They both 

 contained pepsin and they show that pepsin will not act in a neutral 

 or alkaline medium. Hence pepsin only acts in the presence of acid. 



Action of Alkali on Pepsin. 



Pepsin is destroyed by the action of dilute alkaline solutions, such 

 as are found in the intestine, where the action of pepsin ceases for this 

 reason ; thus 2 cc. of dilute sodium carbonate solution are added to 

 5 cc. of pepsin solution and it is put in a water- bath at 40 for at 

 least half an hour. It is neutralised with 0*4 per cent hydrochloric 

 acid, an equal volume of 0*4 per cent, hydrochloric acid and a piece 

 of fibrin are added and it is again kept at 40. Digestion will not 

 occur. 



The Products of the Action of Pepsin. 



Proteins are hydrolysed by pepsin and converted into metaproteins, 

 proteoses and peptones. In a very prolonged digestion amino acids 

 may be formed in small quantities : they are most probably formed by 

 the action of other proteoclastic enzymes the autolytic enzymes 

 which have been extracted from the cells of the mucous membrane 

 together with pepsin. 



Several grams (2-5) of fibrin or egg-white are placed in *iN 

 hydrochloric acid solution and 5-10 c.c. of pepsin solution are added. 

 The fibrin dissolves in the course of half to one hour. 



The presence of 



(1) Metaprotein is shown by neutralising and filtering. 



(2) Proteoses by boiling and acidifying the filtrate and testing a 

 portion with concentrated nitric acid. The precipitate which is 

 formed dissolves on heating and reappears on cooling. They are re- 

 moved by saturating the solution with ammonium sulphate. 



(3) Peptone by testing the filtrate from the ammonium sulphate 

 precipitate by the biuret reaction in the presence of excess of caustic 

 soda (p. 373). 



