PANCREATIC DIGESTION 1 93 



Filter off the precipitate reserving the filtrate and wash the precipitate on the 

 filter paper thoroughly with several small portions of water. 



To small portions of the precipitate apply the Hopkins-Cole, xanthoproteic, 

 and Millon tests. Tryptophane gives a positive reaction with the first two of 

 these tests being responsible for the Hopkins-Cole reaction as applied to protein 

 (see Chapter V). 



Test portions of the filtrate from the mercuric precipitate by the Hopkins- 

 Cole, xanthoproteic, and Millon reactions. Tyrosine responds to the latter two 

 tests. 



To the remainder of the filtrate add a few drops of ammonia 1 and evaporate 

 to a volume of 10 to 20 c.c. using at first a free flame and completing the evapora- 

 tion on a water-bath. Transfer to a beaker and allow to stand for i or 2 days. 

 Examine microscopically the crystals which separate out. Tyrosine crystallizes 

 in sheaves of needles (see Fig. 25). Leucine forms small rosettes. Apply 

 Morner's reaction for tyrosine (see p. 85). 



GENERAL EXPERIMENTS ON PANCREATIC DIGESTION 



EXPERIMENTS ON TRYPSIN 2 



1. The Most Favorable Reaction for Tryptic Digestion. Prepare seven tubes 

 as follows : 



(a) 2-3 c.c. of neutral pancreatic extract + 2-3 c.c. of water. 



(b) 2-3 c.c. of neutral pancreatic extract + 2-3 c.c. of i per cent sodium car- 

 bonate. 



(c) 2-3 c.c. of neutral pancreatic extract -f 2-3 c.c. of 0.5 per cent sodium 

 carbonate. 



(d) 2-3 c.c. of neutral pancreatic extract + 2-3 c.c. of 0.2 per cent hydro- 

 chloric acid. 



(e) 2-3 c.c. of neutral pancreatic extract + 2-3 c.c. of 0.2 per cent combined 

 hydrochloric acid. 



(f) 2-3 c.c. of neutral pancreatic extract -f 2-3 c.c. of 0.4 per cent boric acid. 



(g) 2-3 c.c. of neutral pancreatic extract + 2-3 c.c. of 0.4 per cent acetic acid. 

 Add a small piece of fibrin 3 to the contents of each tube and keep them at 4OC. 



noting the progress of digestion. In which tube do we find the most satisfactory 

 digestion, and why? How do the indications of the digestion of fibrin by trypsin 

 differ from the indications of the digestion of fibrin by pepsin? 



2. The Most Favorable Temperature. (For this and the following series of 

 experiments under tryptic digestion use the neutral extract plus an equal volume 

 of 0.5 per cent sodium carbonate.) In each of four tubes place 5 c.c. of alkaline 

 pancreatic extract. Immerse one tube in cold water from the faucet, keep a 

 second at room temperature and place a third in the incubator or water-bath at 

 4OC. Boil the contents of the fourth for a few moments, then cool and also keep 



1 If the solution is alkaline in reaction, while it is being concentrated, the amino acids 

 will be broken down and ammonia will be liberated. 



2 For these experiments as well as for those on the other pancreatic enzymes commer- 

 cial preparations of trypsin and pancreatin may be employed. 



3 Congo red fibrin may be used in this and the following tests on tryptic digestion. If 

 used the experiments should be made at room temperature. For preparation of this 

 fibrin see Chapter I. 



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