384 PKACTICAL PHYSIOLOGY 



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

 the production of acid meta-protein by the weak HC1. As we shall see 

 later, this preliminary change is necessary before pepsin can further 

 hydrolyse the protein. 



EXPERIMENT IX. Remove a sample of the contents of D and apply 

 the following tests: (a) The Biuret reaction rose-pink colour; (b) Add 

 nitric acid (cone.) white precipitate, which clears up on heating and 

 returns on cooling; (c) Add a few drops of a saturated solution of 

 salicyl sulphonic acid. A white precipitate results which disappears on 

 heating and returns on cooling. These results show us that proteoses 

 have been produced (see p. 310). 



The production of proteoses constitutes the second stage of peptic 

 digestion, and it is the pepsin which produces the change. If samples 

 of any of the other test tubes than D be examined, no proteose will 

 be found, either because no pepsin has been present (as in A and B), 

 or because, though present, its action has been destroyed by heat (as 

 in E\ or there has been no acid present to produce syntonin and help 

 its action (as in C and F). 



There are two principal varieties of proteoses developed, namely 

 " primary " and " secondary " ; the former are precipitated by half 

 saturation with ammonium sulphate, the latter begin to be precipitated 

 at two-thirds saturation with this salt. 



EXPERIMENT X. Take a sample of a peptic digest of two days' 

 duration. Heat this to near boiling point, and add ammonium sulphate 

 crystals till no more will dissolve. Now change the reaction of the 

 fluid to alkaline and allow to cool. 1 Filter and test the filtrate, for 

 Peptone. 



1. By Biuret reaction (remember to add a large excess of caustic 

 alkali, so that more than is sufficient to decompose the ammonium 

 sulphate may be present in the fluid) rose-pink. 



2. By nitric acid or salicyl sulphonic acid tests no precipitate. 



In peptic digestion some of the protein is always further broken 

 down to amino acids. 



Method of Separation of Products of Gastric Digestion. Fibrin is 

 boiled first with tap water, and then with O'l per cent, hydrochloric acid to 

 purify it. It is placed for 1-2 hours in an incubator along with five times its 

 bulk of 0'2 per cent, hydrochloric acid and a sixth its bulk of commercial peptic 

 extract. (A solution of Witte's peptone may also be employed). 



The products of digestion can be separated from this digest by the following 

 process : 



1 It is only by thus saturating in the heat, both in acid and alkaline reaction, 

 that all traces of secondary proteoses are precipitated. 



