KERATIN AND MOLECULAR BIOLOGY 29 



keratins are all of the a-type and they owe their stability and insolubility 

 primarily to the covalent cross-links formed between their polypeptide 

 chains by the disulphide bridges of the amino acid cystine. The proteins 

 in the epidermal hard parts of reptiles and birds are also stabilized by 

 disulphide bonds; on the other hand, having /^-characteristics, they are 

 of a quite different molecular structure from the mammalian forms. 

 Mammalian keratins on being stretched can also assume the |8-form as a 

 kind of stereoisomer. Thus a definition of a keratin must include both 

 a- and /3-forms. Further in some special situations, e.g. the hair cuticle 

 (p. 265), a peculiar, very insoluble and highly cross-linked amorphous 

 keratin is found. Thus a more inclusive definition might be forced to 

 place less emphasis on the molecular character of the crystalline form and 

 define a keratin simply as a protein stabilized by disulphide cross-linkages. 

 Emphasis needs to be placed on the fact that the cystine cross-linkages 

 produce insolubility as well as stabilization or such soluble proteins as 

 insulin (12-5 per cent cystine) would be included. 



Block's definition (Block and Boiling, 1950; Block, 1931; Block and 

 Vickery, 1931) sums up a traditional view in placing emphasis on the 

 insolubility: "A keratin is a protein which is resistant to digestion by 

 pepsin and trypsin, which is insoluble in dilute acids and alkalis, in 

 water and in organic solvents." He adds, however, a further criterion 

 based on the molecular ratio of the basic amino acids found by hydrolysis, 

 which is not now admissible (pp. 31 and 32). 



The routine histological tests for " keratin " assume that its presence 

 is to be inferred from the presence of protein-bound disulphide bonds. 

 They are based either on the oxidation of the cystine bridge to produce 

 the very acidic — S0 3 H group : 



P— S— S— P -> 2P— S0 3 H 



which is then detected by the increased basophilic at low pH (Pearse's 

 method, 1951 and 1953); or by reduction of the bridge to sulphydryl 

 groups: h 



P— S— S— P -* 2P— SH 



and the detection of these by the nitroprusside test, the Prussian blue 

 test (Chevremont and Frederic, 1943) or most specifically by coupling them 

 to the Bennett reagent: l-(4-chloromercuriphenylazo)-naphthol-2: 



OH 

 CI— Hg— < /_ N— N=N— 



" mercury orange," Bennett's reagent (Barrnett, 1953) 

 to yield a coloured dye. 



