ANTIGENS, HAPTENS, ANTIBODIES, ETC. 421 



or other dietary nitrogen compounds. It has also been 

 shown by the use of amino -acids containing isotopic 

 nitrogen as a " marker " or " label " that the proteins 

 of the body, including globulin and albumin, are con- 

 stantly being broken down and resynthesised and that 

 the amino -acids themselves undergo similar changes. 

 Pauling has suggested that antibody globulin molecules 

 do not differ from normal globulin molecules in their 

 chemical composition, and that they are sjoithesised in 

 the normal way, but that they differ in the way that the 

 peptide chains, particularly^ at the ends, are folded into 

 a stable configuration. He regards the centre part of the 

 chain of amino -acids to be held at the s;^Tithesising 

 centres in the cell whilst the ends of the chain are more 

 or less mobile, as pictured at (i) in Fig. 8. These free 



1 



'^Iaaaj^'' ^^0> 



B 



(II) (ill) Ovj 



Fig. 8, — •Synthesis of Normal Globulin. 



ends fold into their natural stable configuration and 

 remain so because held by hydrogen bonds and similar 

 intramolecular weak bonds (at ii). In the course of 

 time the molecule becomes liberated into the blood- 

 stream (as at iii) where the whole of the peptide chain 

 settles down to the stable configuration of normal 

 globulin, represented at (iv). When globulin is syn- 

 thesised in the presence of antigen, which is taken up 

 by the cell at the site of synthesis, the polypeptide 

 chain is built up as usual (i. Fig. 9), but the free ends 

 now assume a configuration which is modified by the 

 presence of the polar determinant groups on the antigen, 

 Avhich exert an attraction on groups in the polypeptide 

 chain (see ii), and cause folding of the ends of the chain 



