MODIFIED AND CONJUGATED ANTIGENS 333 



shown to be capable of reacting witli very simple eiystalline sub- 

 stances such as benzoic acid, etc. (Landsteiner, Specificity of 

 Serological Reactions, 1936, p. IIS). Landsteiner has also dis- 

 covered the importance of spatial relationships and of the relative 

 position of chemical groups in determining specificity. For a 

 better understanding of the early work of Obermayer and Pick 

 and the distinction between their altered antigens and the new 

 conjugated antigens of Landsteiner and others, structural formulae 

 are employed in the following discussion : 



Altered or Modified Antigens. — Iodized Protein. — When iodine 

 Ls added to a protein it very likely replaces the hydrogen adjacent 

 to the hydroxyl (-0H) group of some aromatic nucleus; thus if 

 tyrosine is in the ]-)rotein, tlie reaction could be illustrated as 

 follows : 



H H O J H H O 



HO- 



./' 



-C— C— C— OH + T.. = HO—/ \— C— C— C— OH + HI 



H NH, H NH, 



If an additional mole of iodine is added, the other hydrogen 

 adjacent to the hydroxyl (-OH) group may be replaced to give a 

 compound of the following formula : 



H H 



HO— < >— C— C— C— OH 



\ / I I 



I H NH, 



In the case of phenylalanine it is hard to introduce iodine since 

 there is no hydroxyl or other activating group on the ring, as 

 will be seen from the following formula of phenylalanine : 



H H O 



<^ \— C— C— C— OH 



H NH, 



Iodotryptopiiane. — Tryptophane has not been extensively 

 studied, but it is regarded as iodizable, probably through the re- 

 placement of a hydrogen on the nucleus as follows : 



