THE PANCREATIC JUICE 791 



such molecules as are present in the naturally occurring proteins. 

 Thus under the action of trypsin the following polypeptides undergo 

 hydrolytic dissociation : alanyl glycine, alanyl alanine, alanyl 

 leucine A ; while the closely similar polypeptides glycyl alanine, 

 glycyl glycine, alanyl leucine B are left untouched. 



CONDITIONS OF TRYPTIC ACTIVITY 



Since the pancreatic juice is strongly alkaline it might be expected 

 that trypsin would be most effective in an alkaline medium. It 

 must be remembered, however, that the alkaline juice when secreted 

 meets the correspondingly acid contents discharged from the stomach, 

 and that the resulting mixture is practically neutral. This neutrality 

 exists throughout the small intestine, the reaction of the contents 

 of the gut being similar to that of a fluid containing alkali which 

 has been saturated by the passage of carbonic acid, viz. alkaline 

 to such indicators as methyl orange, and acid to such indicators as 

 phenylphthalein. On investigating the action of trypsin outside 

 the body, it is found that, at any rate as concerns its earlier stages, 

 this ferment is more active in the presence of sodium carbonate. It 

 is usual to make up an artificial digestive mixture by dissolving 

 commercial trypsin in 0-2 to 0-3 per cent, sodium carbonate. The 

 optimum amount of sodium carbonate depends on the strength of 

 the solution in trypsin : the more trypsin present the higher is the 

 optimum amount of sodium carbonate. It is stated that although 

 an alkaline reaction is more advantageous for the earlier stages of 

 tryptic activity, the later stages take place best in a neutral medium. 

 This result is probably due to the fact that trypsin in alkaline medium 

 is extremely unstable, so that when prolonged digestions are carried 

 out the trypsin would be rapidly destroyed if the medium were 

 strongly alkaline. The destructibility of trypsin, as well as its 

 action, is largely affected by the presence of proteins or their diges- 

 tion products in solution. Bayliss has adduced evidence to show 

 that when trypsin acts upon protein it enters into some form of 

 combination with the protein molecule. This combination protects 

 the trypsin from the destructive action of alkali. The velocity of 

 the reaction, which takes place under the influence of trypsin, gradually 

 diminishes, owing probably to a combination of the trypsin with the 

 products of digestion, e.g. with the peptones or amino-acids, and its 

 consequent removal from the sphere of action. If by any means 

 the amino-acids be removed the action of the trypsin is renewed. 

 This destruction of ferment occurs in the intestine itself. If the 

 intestinal contents be collected by means of a fistula at the lower 

 end of the ileum, they show little or no proteolytic activity. The 

 trypsin is therefore an extremely active ferment which carries out 



