KERATIN AND MOLECULAR BIOLOGY 21 



theless in almost all animal and plant groups deposits of calcium salts may 

 be found intracellularly as well as extracellularly or on the cell membrane. 

 The salts appear usually in association with a protein matrix and it is 

 supposed that some spacial relationship exists between the salt molecules 

 (or ions) and sites on the surface of the protein molecules. In the verte- 

 brates the greater part of the calcium found in bone is always associated 

 with collagen (Fig. 12) and mucopolysaccharides. In this case, the 

 macromolecular form of the collagen is also important since, of the 

 several arrangements of the collagen molecules (see p. 128) which can be 

 formed in vitro, only that having the naturally occurring spacing of 

 640 A appears able to initiate salt deposition. The fact that cells con- 

 taining keratin seem rarely to accumulate calcium may also mean that 

 the special arrangements of surface groupings required is lacking in the 

 keratin molecule (Bachra et al., 1959). 



A third method of hardening proteins is keratinization which resembles 

 tanning in that covalent cross-links are established between protein chains; 

 but these are of a special type, the sulphur bridges made possible by the 

 linking of cysteine residues in adjacent chains. Keratinized proteins are 

 also in part stabilized by crystallite formation as is shown in fact by the 

 existence of the a X-ray pattern. 



~~r ~r 



s s 



I I 



s s 

 ! 1 



— CH 2 — S— S— CH 2 — 

 the cystine " bridge " or disulphide link 



All these methods of insolubilizing and toughening natural polymers 

 have their analogies in the chemistry of artificial polymers and much 

 understanding of the natural process has come from a study of these. 



Valuable information concerning the stability of a fibrous system, which 

 is particularly relevant to our present interest in stabilized, protective 

 proteins, can be obtained directly by X-ray means simply by taking a 

 photograph after a tissue has been subjected to some disorienting influence 

 such as heating in water. If the structure has been disrupted by the 

 treatment, the fibre-type pattern of distinct spots or arcs is replaced by a 

 pattern of diffuse, circular haloes. A loss of birefringence may also be 

 detected by means of a polarizing microscope (Fig. 109, p. 213). The X-ray 

 pattern may also change its character. An a-keratin may frequently be 

 converted into a /?-type keratin by heating in water (Rudall, 1946) and 



