THE KERATINIZATION PROCESS 



245 



Table 19.* The Amino Acid Composition of Hydrolysates of Fractions 

 from Merino 64's WooL.f 



* per cent N of total-N. 



| Earland and Wiseman (1959). 



The location of the cystine residues 



The evidence, which has been reviewed up to this point, is adequate to 

 establish that during the later stages of the process of keratinization the 

 thiol groups (SH) of cysteine residues in some polypeptides are oxidized to 

 yield cystine bridges or disulphide cross-linkages ( — S — S — ). There are, 

 however, two different views as to the location of these cross-linkages. The 

 simpler view is that keratin is a single uniform protein of more-or-less 

 definite composition and that half cystine residues are distributed along 

 the component polypeptide chains in a definite manner as are the residues 

 (Fig. 101) of other proteins. In a physicochemical sense the resulting 

 cross-linked system is closely analogous to the artificial, three-dimensionally 

 cross-linked polymers, such as vulcanized rubber, to which it is often 

 likened. This picture gives an explanation of the major facts : the stability, 

 insolubility and the sensitivity to disulphide rupture. It is compatible 

 with the evidence that the sulphur enters after the establishment of the 

 fibrous structure, if we assume that the original polypeptides contain less 

 cysteine and that other residues (e.g. serine) are later converted into 

 cysteine residues by a topochemical reaction and then cross-linked. This 

 view of the location of the cystine is widely accepted and is adequate for 

 the interpretation of most physical and chemical experiments on intact 

 material. 



The second view tries to take into account a number of other obser- 

 vations which suggest that keratin is not a uniform material at the 



