ELEMENTAKY PHYSIOLOGICAL CHEMISTKY 223 



Nearly pure 'pepsin' can also be prepared by saturating gastric juice 

 with ammonium sulphate, which precipitates it. Whether the actual 

 ferment pepsin is what we obtain by these methods, or whether it is 

 simply proteid with the ferment adherent to it, cannot as yet be 

 decided. 



The Action of the Gastric Juice. To study this we employ an 

 artificial gastric juice prepared by macerating the mucosa of the stomach 

 for several days with twenty times its weight of glycerine containing 

 0-1-0-2 per cent, hydrochloric acid. The acid changes the pepsinogen (the 

 mother substance or zymogen of pepsin) present in the gland cells into 

 pepsin, and this then dissolves in the glycerine. 



We may employ any proteid for the investigation, the most con- 

 venient one being blood fibrin, which has been very thoroughly washed 

 with boiling acidulated water so as to remove all impurities. 



EXPERIMENT III. Label six test tubes A, B, (7, D, E, F, and place a 

 piece of fibrin in each. Half fill A with water, B with 0-2 per 

 cent. HC1, G with water and a few drops of the peptic extract, D with 

 0-2 per cent. HC1 and a few drops of peptic extract, E same as D but 

 boil the mixture, and F with 1 per cent, sodium carbonate and a few 

 drops of the peptic extract. 



Place all these in a water-bath kept constantly at body temperature 

 (40 C.). 1 Observe (1) that in A the piece of fibrin remains unchanged, 

 whereas in B, D arid E, which all contain 0*2 per cent. HC1, it becomes 

 swollen and transparent. In F y which contains alkali, it does not 

 swell. 



EXPERIMENT IV. After about half an hour, remove a sample of the 

 contents of any of the tubes containing acid, colour it with some litmus, 

 and then carefully neutralise with weak sodium carbonate solution (1 

 part 1 per cent. Na 2 C0 3 + 2 parts water). A precipitate of acid albumen 

 or syntonin is produced (for reactions see proteids, p. 177). 



The first stage in gastric digestion of proteids consists, therefore, in 

 the production of acid albumen by the -2 per cent. HC1. As we shall 

 see later, this preliminary change is necessary before pepsin can further 

 hydrolyse the proteid. 



EXPERIMENT V. (2) Eemove a sample of the contents of D and 

 apply the following tests : (a) The Biuret reaction rose-pink colour, 

 (b) Add HN0 3 (con.) white precipitate, which clears up on heating and 

 returns on cooling. These two results show us that proteoses have 

 been produced. 



1 This can be approximately determined by dipping a finger into the water; if 

 the water be at the same temperature as the skin it will feel neither cold nor hot. 



