486 DIGESTION 



for peptic activities. The acid seems to combine with the protein before 

 the pepsin attacks the latter; for, if we first combine the protein with 

 acid ami then -wash away all traces of free acid, the protein can be 

 digested in a neutral pepsin solution without the liberation of any free 

 acid. 



There is evidence to show that pepsin itself also becomes combined 

 with the protein during the digestive process. If a piece of protein such 

 as fibrin be immersed in a solution of pepsin and then taken out and 

 washed thoroughly to get rid of all adherent pepsin, it will be found, on 

 placing it in ;i hydrochloric acid solution of the proper strength, that 

 peptic digestion proceeds. Advantage may be taken of this fact to 

 separate pepsin from a solution, but the best protein to use for this pur- 

 pose is not fibrin but elastin. By such a method it has, for example, 

 been shown that there is some pepsin in the intestinal contents, which in- 

 dicates that when the chyme passes into the intestine, the pepsin is not, as 

 used to be thought, immediately killed by the proteolytic enzyme. 



Products of Peptic Digestion 



With regard to the products of gastric digestion, little can be said 

 here. The first product is a metaprotein known as acid albumin or 

 syntonin. It is precipitated from the digestion mixture by neutraliza- 

 tion. The next product is known as primary proteose, being precipi- 

 tated by half saturation with ammonium sulphate. The third product 

 is secondary proteose, produced by complete saturation with the above 

 reagent ; and after all these bodies have been separated out, there re- 

 mains in solution the fourth product — peptone — which among other 

 things is characterized by the fact that with the biuret test it gives not 

 n violet but a rose-pink color. 



It has often been claimed that along with these products a certain 

 amount of free amino acids may also appear in a peptic digestive mix- 

 ture. This, however, may be due to the action of erepsin, which is 

 usually present in pepsin preparations. It is important to note that the 

 term proteose is a general one, and that there are probably many varieties 

 of this substance, differing from one another according to the protein 

 from which they are derived. 



The change produced by pepsin and hydrochloric acid is of the nature 

 of an hydrolysis, for it has been found thai the amount of hydrogen and 

 oxygen in the digestive products is greater than that in the original 

 protein. It is by a similar process of hydrolysis that the other proteolytic 

 enzymes, such ;is pancreatin and erepsin, operate, but this does not 

 imply that the exact grouping thai is splil aparl by the hydrolytic proc- 



