THE KERATINIZATION PROCESS 



217 



in zone D by heating for 30 sec at 90°C or by soaking in dilute hydrochloric 

 acid. Trypsin readily digests the same layers A, B, C and D. Alkaline 

 reagents are, however, able to destroy the birefringence in the lower parts 

 of zone E (Fig. 92). Perhaps the most spectacular and instructive effects 

 follow the application of saturated urea to the base of a plucked hair 

 follicle (Fig. 93). The lower zone D swells rapidly and is dissolved; the 

 upper half E swells only, and to a diminishing degree, as the follicle is 

 ascended (Fig. 94). 



(a) 



(b) 



(c) 



Fig. 95. The location of the keratinization zone ( + SH) in three hard 

 keratins, (a) the nail, (b) the claw and (c) a horn (edge only shown). 

 Positive SH shown black. Redrawn from Giroud and Bulliard (1930). 



Thiol and Disulphide Groups during Keratinization (Fig. 97(c)) 



The most important single observation relevant to the chemistry of 

 keratinization is that the keratinizing layers give a positive reaction for 

 thiol (SH) groups and that this reaction fades as the tissues harden. In 

 their important comparative study of numerous keratinizing tissues 

 Giroud and Bulliard (1930) established the existence of an SH-positive 

 layer in every case and distinguished between the hard and soft keratins 

 by the intensity of the reaction. Fig. 95 has been redrawn from their work. 

 Giroud and Bulliard used the nitroprusside reagent for thiol groups. Since 

 that time more permanent and specific reagents have been introduced 

 (Chevremont et al., 1943; Rudall, 1952; Barrnett, 1953; Barrnett and 

 Seligman, 1952) and the use of these has confirmed the earlier work. The 

 conclusion to be drawn from these results is that the fixed SH, i.e. that 

 resistant to washing by being joined to a protein framework, disappears in 

 the course of keratinization and that therefore one of the reactions under- 

 lying keratinization is the oxidation of the SH groups to produce cystine 

 bridges: (— S— S— ) 



2— SH + = — S— S— + H 2 

 When the SH groups are first blocked by alkylation, and the tissue reduced. 



