KERATIN AND MOLECULAR BIOLOGY 33 



barbs and medulla (see Fig. 30, p. 70) made by Schroeder and Kay and 

 their associates (1955) shows that the various parts differ in composition 

 and that species differences also exist. We may conclude that feather 

 keratin, like wool keratin, also has no precisely fixed composition. The 

 two keratins, wool and feather, are, however, quite distinct. In particular, 

 feather keratin reveals itself as peculiar in having 10 per cent of the amino 

 acid proline, a circumstance which must profoundly affect the configuration 

 of the polypeptides. Forty per cent of residues are the small residues 

 glycine, alanine and serine. These features of the feather polypeptides 

 have influenced the model proposed by Krimm and Schor (p. 208). 



There may be technical reasons for small variations since these analyses 

 are difficult to perform and, further, they are usually carried out on whole 

 tissues, not on isolated purified proteins. As commonly practised the 

 materials are usually simply extracted successively with aqueous and lipid 

 solvents and, without further fractionation, are submitted to various 

 degradative processes as a preliminary to analysis. Since the keratinized 

 fibrous protein itself usually forms the predominate constituent, the 

 analytical findings will give a good idea of its composition. However, 

 it would be a mistake to regard the findings as representing precisely the 

 composition of " keratin " and to draw far-reaching inferences from their 

 exact values. Ideally the several components of the tissue ought to be 

 separated, purified and individually analysed as is obligatory in the case of 

 the soluble proteins. This is a counsel of too much perfection when dealing 

 with insoluble hardened products and little effort has been made to comply 

 with it in the case of the keratins. Nevertheless, since keratin remains as 

 an insoluble residue, when the tissues containing it are simply digested with 

 trypsin, it is regrettable that analysts have not attempted at least this 

 degree of purification before beginning an elaborate and lengthy 

 investigation. 



Undoubtedly methods of solubilizing the proteins of a keratinized 

 tissue and of extracting pure individual proteins will be perfected sooner 

 or later. When these have been attained, is it possible to anticipate that 

 definite individual " keratins " will be distinguished ? It is difficult to 

 answer this affirmatively. Rather from the biological point of view a 

 variable composition might well be expected, for it is clear that the 

 demands made upon the epidermis and its derivatives are widely variable 

 and, if an adaptive response is to arise, a variation in the nature of its 

 composition might be expected. The interesting idea, discussed by 

 Tristram (1953) and more fully by Colvin et al. (1954) and Fox (1953), 

 that proteins exhibit a certain " spread " in their composition, may well 

 be applicable to structural proteins of the keratin-type even if it proves 

 untenable in other instances. The fact that the replacement of a single 

 amino acid residue may impair the function of a haemoglobin molecule 



