THE KERATINIZATION PROCESS 261 



shows that the keratin itself is unchanged (Mercer et al., 1956); sections 

 examined electron-microscopically show that the bundles of keratinized 

 fibrils are of normal appearance and reveal that the components which have 

 been removed are the cell membrane and intercellular cement. The loss 

 in weight is of the order of only 10% (Elod and Zahn, 1946) but it repre- 

 sents the vital links connecting the chains of cells. 



A further important characteristic of these altered membranes is that 

 their chemical character is complementary to that of the keratinized protein. 

 That is to say, chemical conditions which soften or dissolve keratin have 

 little effect on the membranes. The strength and weakness of keratin 

 itself lies largely in the disulphide bond which is peculiarly vulnerable to 

 reduction, hydrolysis and oxidation. However, the system of membranes 

 resists these actions to a far greater degree. It is found that, when keratin 

 is dissolved (by the methods described above) the insoluble residue 

 consists largely of membranes (Mercer, 1951 and 1953) (Fig. 112) (Lager- 

 malm et al., 1951). 



Considering their biological origin the chemical resistance of these 

 membranes is remarkable. They are not dissolved by the following strong 

 reagents, which include both reducing agents and hydrogen bond breakers: 

 5 N caustic soda, 8 M urea, 8 M urea containing thioglycollic acid or 

 sodium bisulphite at pH 10, concentrated formic acid, 10% sodium 

 sulphide, and aqueous peracetic acid followed by 0*1 N alkali. On the 

 other hand, when not protected by being incorporated in a solid, intact 

 tissue, they are rapidly digested by proteolytic enzymes. 



These properties show that, while a protein constituent is certainly 

 present, the resistance cannot be due entirely to hydrogen bonds or 

 disulphide bonds of the type found in keratin. It is perhaps permissible 

 to see that a certain biological advantage is gained by enclosing keratin in 

 small sacs which resist dissolution by precisely those reagents most 

 injurious to their contents. 



The little known of the composition of biological membranes (p. 37) 

 does not help to explain the changes which could convert them into the 

 singularly-insoluble form they assume in keratinized tissues. That a 

 protein moiety is present is shown by the dissolution by proteolytic 

 enzyme; the several reports (Corfield et al, 1958) of amino acids found in 

 membrane hydrolysates confirm this. Matoltsy (1957) has reported finding 

 the following amino acids in membranes from human skin : glycine, valine, 

 leucine, woleucine, serine, threonine, aspartic and glutamic acids, arginine 

 lysine, histidine and methionine. Other analyses indicating protein have 

 been made on the particularly-toughened membrane obtained from 

 Allworden sacs (p. 267) on wool fibres. Corfield, Robson and Skinner 

 (1958) carried out a complete amino acid determination of the residue 

 remaining after oxidized wool is extracted with ammonia (referred to as 



