4 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



This long list is sufficient evidence of the complexity of the protein 

 molecule ; and, as yet, it seems to be incomplete, for several other 

 products have been described. Of these, the presence of aminobutyric 

 acid, which would complete the series of monoaminomonocarboxylic 

 acids, was assumed by Schiitzenberger, but has not been demonstrated 

 by any of the subsequent investigators. A large number of new pro- 

 ducts were added to the list by Skraup in 1904, but he has since shown 

 that two of them were mixtures of glycine and alanine. Another 

 amino-oxy acid was described, as also caseanic and caseinic acids ; the 

 latter is apparently identical with Fischer and Abderhalden's dia- 

 minotrioxydodecanic acid C 12 H 26 N 2 O 5 . Another product, diamino- 

 oxysebacic acid, was stated by Wohlgemuth to be a constituent, and 

 a substance, C 1]L H 12 N 2 O3, termed oxytryptophane was described by 

 Abderhalden and Kempe ; Gortner mentions the presence of a body in 

 keratin which gives Millon's reaction, but is not tyrosine. Their presence 

 as well as that of those described by Skraup has not been definitely 

 proved ; they cannot therefore be regarded as units of the protein 

 molecule, and cannot be included in the above list until their presence 

 in the molecule is thoroughly established. 



3, 5-Diiodotyrosine is present in the protein contained in corals and 

 other sea animals : it is formed when proteins are treated with the 

 halogen : it is not included since it is of such rare occurrence and 

 presumably a derivative of tyrosine. 



The presence of glucosamine in the protein molecule is also a dis- 

 puted question ; there is no doubt that a carbohydrate containing nitrogen 

 is contained in the glucoproteins in their prosthetic group, but it is 

 doubtful if it be present in the protein part of the molecule, although a 

 carbohydrate has been obtained from carefully purified proteins con- 

 taining no prosthetic group, such as crystallised egg-albumin, serum- 

 albumin (Langstein). The fact that the yield of carbohydrate from 

 such a protein becomes smaller the more often it is recrystallised, 

 suggests that the presumably pure protein still contained an impurity ; 

 this impurity would be a glucoprotein, which is found in both egg white 

 and serum from which the crystallised proteins are separated, and this 

 would give rise to the carbohydrate. Glucosamine is therefore excluded 

 from the above list. 



Numerous amino acids including diamino- and oxyamino-acids 

 have also been synthesised of recent years by Neuberg and his co- 

 workers and by Sorensen. Our knowledge of these acids should render 

 the task of identifying a new unit in the protein molecule less laborious 



