50 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



was not probably quite perfect, since it is well known that there are 

 great difficulties to be surmounted in obtaining serum or plasma absol- 

 utely free from the red colour of the corpuscles. 



On account of these results with serum Abderhalden and Rona in- 

 vestigated the action of human blood serum on glycyl-1-tyrosine in 

 certain cases of disease, as also the urine. In some diseases no hydro- 

 lysis occurred, but in other diseases there was distinct hydrolysis. As 

 yet no conclusions can be drawn from these results, as they require 

 amplification both as regards the enzyme solution and the substrate. 

 In no case had the urine any action upon glycyl-1-tyrosine ; this seems 

 at variance with the presence of an urotryptic enzyme which Cathcart 

 studied in its action upon proteins. 



The hydrolysis of polypeptides by enzymes shows most conclusively 

 that the protein molecule is built up of amino acids combined together 

 in the form of acid amides, but there still remains the possibility that 

 other modes of combination are present. In the animal body, proteins 

 are acted upon firstly by pepsin, then by trypsin and then by the other 

 enzymes. Pepsin does not hydrolyse any of the polypeptides, but it 

 hydrolyses the proteins producing a mixture of some five or six proteoses 

 and peptones. Trypsin hydrolyses the majority of the polypeptides, 

 and it hydrolyses the proteins producing amino acids, together with a 

 complex polypeptide, as Fischer and Abderhalden have shown, which 

 contains all the proline and all the phenylalanine which are present in 

 the protein. This complex polypeptide occurs in a modified form 

 when a protein is hydrolysed first by pepsin and then by trypsin ; some 

 of the proline and the phenylalanine are obtained in the free state. The 

 complex polypeptide is hydrolysed by the various intracellular enzymes 

 in the organs of the body, since it is not excreted. These enzymes 

 hydrolyse polypeptides which are not attacked by trypsin ; such com- 

 binations are therefore probably contained in this complex. The 

 enzymes in the various organs are extremely diverse in their nature : 

 certain purine bases are acted upon by the enzymes of one organ but 

 not by those of another organ, the arginase of the liver hydrolyses 

 arginine (Kossel and Dakin), but not the similarly constituted creatine 

 (as Dakin has recently shown). Enzymes are characterised by their 

 highly selective nature ; they act upon one definite substance or upon 

 groups of substances, e.g., the fats and the polypeptides. Pepsin does 

 not act upon the polypeptides, but it hydrolyses the proteins ; either the 

 polypeptides are not sufficiently complex to be attacked by pepsin, or 

 pepsin acts upon another mode of combination of the amino acids. One 



