GASTRIC DIGESTION 145 



The original protein has been digested and the solution now contains the 

 products of peptic proteolysis, i.e., acid metaprotein (acid albuminate), proteoses, 

 peptones, etc. The insoluble residue may include nuclein and other substances. 

 Filter the digestion mixture and after testing for free hydrochloric acid neutralize 

 the filtrate with potassium hydroxide solution. If any of the acid metaprotein 

 (acid albuminate) is still untransformed into proteoses it will precipitate upon 

 neutralization. If any precipitate forms heat the mixture to boiling, and filter. 

 If no precipitate forms proceed without filtering. 



We now have a solution containing a mixture consisting principally of pro- 

 teoses and peptones. Separate and identify the proteoses and peptones ac- 

 cording to the directions given on pages 118 and 119. 



GENERAL EXPERIMENTS ON GASTRIC DIGESTION 



1. Conditions Essential for the Action of Pepsin. Prepare four test-tubes as 

 follows : 



(a) Five c.c. of pepsin solution. 



(b) Five c.c. of 0.4 per cent hydrochloric acid. 



(c) Five c.c. of pepsin-hydrochloric acid solution. 



(d) Two or 3 c.c. of pepsin solution and 2-3 c.c. of 0.5 per cent sodium car- 

 bonate solution. 



Introduce into each tube a small piece of fibrin and place them in the incu- 

 bator or water-bath at 4OC. for one-half hour, carefully noting any changes which 

 occur. 1 (Carmine-fibrin may be used to advantage in this and the following tests 

 under Gastric Digestion. In this case, however, the experiments should be con- 

 ducted at room temperature. For directions as to the preparation of carmine- 

 fibrin see Chapter I.) Now combine the contents of tubes (a) and (b) and 

 see if any further change occurs after standing at 4OC. for 15-20 minutes. Ex- 

 plain the results obtained from these five experiments. 



2. Influence of Different Temperatures. In each of four test-tubes place 5 

 c.c. of pepsin-hydrochloric acid solution. Immerse one tube in cold water from 

 the faucet, keep a second tube at room temperature and place a third in the 

 incubator or water-bath at 4OC. Boil the contents of the fourth tube for a 

 few moments, then cool and also keep it at 4OC. Into each tube introduce a 

 small piece of fibrin and note the progress of digestion. In which tube does the 

 most rapid digestion occur? Explain this. 



3. The Most Favorable Acidity. Prepare three tubes as follows : 



(a) Five c.c. of 0.2 per cent pepsin-hydrochloric acid solution. 



(b) Two or 3 c.c. of 0.2 per cent hydrochloric acid + i c.c. of concentrated 

 hydrochloric acid + 5 c.c. pepsin solution. 



(c) One c.c. of 0.2 per cent pepsin-hydrochloric acid solution -f 5 c.c. of 

 water. 



Introduce a small piece of fibrin into each tube, keep them at 4oC., and note 



1 Digestion of fibrin in a pepsin-hydrochloric acid solution is indicated first by a swelling 

 of the protein due to the action of the acid, and later by a disintegration and dissolving of 

 the fibrin due to the action of the pepsin-hydrochloric acid. If uncertain at any time 

 whether digestion has taken place, the solution under examination may be filtered and the 

 biuret test applied to the filtrate. A positive reaction will signify the presence of acid 

 metaprotein (acid albuminate), proteoses (albumoses), or peptones, the presence of any 

 one of which would indicate that digestion has taken place. 



