THE CHEMISTRY OF THE DIGESTIVE JUICES 355 



In the artificial compounds of two or more ammo-acids which have 

 been synthesized by Fischer and named by him polypeptides (p. 2), 

 the carboxyl group of one amino-acid is linked with the amino group 

 of another. For example, a molecule of alanin and a molecule of glycin 

 form, with loss of a molecule of water, a molecule of alanyl-glycin, 

 according to the equation 



. 

 C 



H 2 O=NH 2 .CH 2 .CO.NH.CH.CH 3 . 

 OOH COOH 



Glycin. Alanin. Glycyl-alanin. 



Two or more molecules of the same amino-acid can be linked in the 

 same way; e.g., two molecules of glycin yield a molecule of glycyl-glycin, 

 and so on. It has been proved that polypeptides identical witn some of 

 these synthetic bodies are present in the peptone mixtures derived from 

 the native proteins, so that it must be assumed that one of the ways 

 at least in which amino-acids are linked in the protein molecule is that 

 described. 



It has been suggested that the early appearance of some of these 

 amino-acids in pancreatic digestion is not really due to trypsin, but 

 to other ferments, peptases, which act upon the peptones formed by 

 the trypsin. There is, however, no clear evidence of the existence 

 of a separate peptone-splitting enzyme in pancreatic juice, like the 

 erepsin of intestinal juice, and it is therefore most natural to suppose 

 that under the influence of trypsin the protein molecule breaks at 

 different points from those at which it ruptures under the influence 

 of pepsin. 



After the most prolonged artificial digestion with trypsin, a 

 residue of the protein remains unconverted into these relatively 

 simple substances. But even this small portion of the original 

 protein has undergone a great change, for it no longer gives the 

 biuret reaction. It can be split into amino-acids, etc., by heating 

 with acid, and also by the action of the erepsin of the intestinal juice, 

 and then yields mainly prolin and phenylalanin, substances which 

 are generally not to be detected among the decomposition products 

 of protein after digestion with pancreatic juice. This illustrates the 

 important fact that some of the ' building stones ' of the protein 

 molecule can be separated from it with far greater ease than others. 

 Tyrosin, tryptophane, and cystin appear very early in the digestive 

 fluid, and tyrosin, as shown in the following example from Abder- 

 halden, may be completely liberated at a time when glutaminic acid 

 is scarcely more than beginning to appear. 



The plant protein edestin, obtained from cottonseed, was digested 

 with pancreatic juice or an extract containing trypsin. The quan- 

 tities of tyrosin and glutaminic acid liberated at different periods 

 of the experiment are expressed as percentages of the total amounts 

 of these substances contained in the edestin. 



