KERATIN AND MOLECULAR BIOLOGY 31 



It is evident that, unless a very narrow definition based on distribution 

 is insisted on, the keratins are far from being a homogenous body of 

 proteins. Neither by morphology, by molecular structure nor by overall 

 amino acid composition can they be classed together. This is in great 

 contrast to the collagens, for example. These proteins are widely spread 

 yet always characterized by the same high angle X-ray pattern (Fig. 8), 

 usually, but not invariably, by a low-angle X-ray pattern and electron 

 microscopic appearance derived from the presence of a master period of 

 ~ 640 A, and by an amino acid composition in which one residue in three 

 is always glycine (a feature known now to be necessarily associated with 

 the molecular structure which gives rise to the X-ray pattern) and quantities 

 of the amino acid hydroxyproline are always present (see Fig. 2). 



In fact it may not be far from the truth to say that keratinization is a 

 fate which could befall any of a number of kinds of protein, provided they 

 contain enough cysteine or are mixed with a cysteine-rich accessory 

 (see p. 248) in a biochemical milieu where the cysteine can be oxidized to 

 cystine. Thus it is only the process of keratinization for which a distinction 

 is evident and the conditions for its occurrence seems to be found mainly 

 in epithelia. 



The Significance of the Variable Amino Acid Composition 



Before proceeding it is desirable to give some further consideration to 

 the question of the variable composition of these proteins as revealed by 

 the analyses quoted. How many keratins are there ? And are they unique 

 substances with a constant composition or not ? From what will be said 

 below concerning the morphology of the various epidermal tissues, it 

 will be clear that any keratinized tissue is a mixture of numerous 

 chemical species among which a variety of keratin predominates in 

 amount. The analysis of such tissue is not therefore the analysis of a 

 single definite chemical substance and we can envisage the possibility, 

 apparently confirmed by actual analysis, that the composition may vary 

 from animal to animal, and from time to time, and from site to site even in a 

 single animal. The keratinized tissues are in this respect far less constant 

 in composition than other tissues of the body. Block and Boiling (1950) 

 have shown that the total amino acid composition of most tissues is 

 remarkably constant. This finding probably reflects the constancy both of 

 the cytoplasmic apparatus and of the specialized cell products in the tissue. 

 Of the tissues studied by Block, the keratins showed by far the widest 

 spread in their amino acid pattern. 



These early analyses of Block and Boiling are not as complete as those 

 now available, nevertheless for comparative purposes they form a basis of 

 comparison, since the same procedures were followed in each case. Block 

 believes, however, that the hard keratins (eukeratins in his nomenclature) 



