520 DIGESTION 



not fibrin but elastin. By such a method it has, for example, been shown that there 

 is some pepsin in the intestinal .contents, which indicates 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 peptic 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 

 a 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 that 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 as pancreatin and erepsin, operate, but this does not 

 imply that the exact grouping that is split apart by the hydrolytic proc- 

 ess is the same for each of these enzymes. Indeed, there is considerable 

 evidence that pepsin does not, like the other enzymes, break up the long 

 chain of amino acids that are linked together to compose the polypep- 

 tides, but that it only splits the big molecule of albumin or globulin 

 into several large groups, each of which is composed of long amino-acid 

 chains. Its action appears to be analogous with that of amylase on 

 starch, by which, it will be remembered, the big polysaccharide mole- 

 cule is split into smaller polysaccharide molecules, which then become 

 attacked by the dextrinase and split into disaccharide molecules (see 

 p<)<i'e 689). The evidence in support of this view is: (1) that pepsin is 

 unable to digest polypeptides, and (2) that it is able to digest certain 

 proteins upon which erepsin (see page 524) has no action. 



The hydrolytic splitting of large into smaller protein molecules, like 

 that by which the chains of ammo acids in the polypeptides are subso- 



