272 KERATIN AND KERATINIZATION 



in nitric acid or in sulphuric acid plus hydrogen peroxide, and " keratin 

 B " which was soluble. The system was little used although it gave rise to 

 the more useful distinction between hard and soft keratins. Nevertheless, 

 it would be valuable when classifying keratins to have measures of two 

 factors, which really were at the root of Unna's ideas : (a) the amount of 

 keratinized protein present in a tissue relative to non-keratinized material, 

 and (b) the degree to which the keratin is insolubilized. From the stand- 

 point of our present-day knowledge, these could be usefully defined as 

 follows : 



D .• f i s ^ i weight of tissue soluble in 0*1 N NH 3 



Ratio of keratins to total r .... c „, - . on/ 



= after oxidizing for 24 hr in 2% peracetic 



acid 



weight 



original weight 



Ratio of easily soluble . , r . , , , n , , 



, • ^ , J , , , weight of material extracted by 8 M urea 



keratin to keratin soluble = «„»» !• i «• -it- i TT -, 



• , ,-rn lt ^ 0*2 M thioglycolhc acid adjusted to pH 7 



weight of peracetic acid oxidized material 

 soluble in 0-1 N NH 3 



In the choice of a solvent for the " softer " keratin in the second ratio, 

 there is, of course, an arbitrary element and other solvents could be used. 

 In textile circles, for example, the " alkaline solubility," is assessed, i.e. the 

 amount of fibre soluble in 0*1 N caustic soda at 65 °C in 1 hr, and this is 

 found useful as a measure of " damage", where this is defined as a deteriora- 

 tion in the insolubility or stability of the keratin. After allowing for 

 custom and the practical aspects, it would seem nevertheless, better to use 

 a solvent, such as thioglycolhc acid and urea, whose action is based on 

 differences in the degree of H-bonding and disulphide cross-linking which 

 are the chemical bases of insolubilisation. The experiments of Lees and 

 Elsworth (1955), Jones and Mecham (1943) and of Ward and Lundgren 

 (1954) on dissolving keratins show that they differ significantly when their 

 solubilities are compared by solvents such as proposed above. 



Uneven keratinization and its histological distribution 



The variations in keratinization, shown by such tests as " alkaline 

 solubility " or " urea-bisulphite " solubility or by histological stains, 

 which reveal directly the distribution of disulphide concentration (Plate 24B) 

 in a tissue, may arise in either of two ways. Firstly, since in a hard keratin 

 the consolidation process takes place after the synthesis and organization 

 of the protein into a fibrous structure and requires a certain time, it could 

 easily fail to achieve completion for accidental or systemic reasons. Secondly, 



