234 ORIGIN OF STRUCTURES AND FUNCTIONS 



features of their structure responsible for their biologically 

 important functions. Only after this shall we be in a position 

 to reconstruct for ourselves the ways by which there arose, 

 during the process of the development of matter, those struc- 

 tural peculiarities of the primaeval polymers of amino acids 

 which are required for the vital processes. In discussing the 

 chemical structure of proteins we must first make clear to 

 what extent these * working mechanisms ' of protoplasm 

 which have been isolated from living organisms (various en- 

 zymes, hormones, toxins, etc.) exist at the molecular level and 

 to what extent they appear as chemically definable substances, 

 in connection with which the concept of a molecule is the 

 same as for other organic compounds. As early as 1940 N. W. 

 Pirie" expressed doubts as to the validity of this approach 

 and to some extent these doubts still appear in the scientific 

 literature on proteins.^*' ^^ 



In fact, many proteins which were earlier thought to be 

 individual substances have been shown, by more refined 

 methods of separation, to be mixtures. For example, egg 

 albumin has been shown to be a mixture, notwithstanding 

 the fact that it forms beautiful crystals. ^^ The same is true 

 of serum globulins. ^^ For many years purified casein was 

 considered as a single protein. This seemed to be proved by 

 the good agreement of the analytical results obtained by 

 scientists in different countries. However, it has now been 

 established that pure casein consists of a mixture of at least 

 three proteins which have been separated from one another.^" 



In his detailed paper dealing with the isolation of proteins 

 J. F. Taylor^^ points out what a complicated matter it is 

 to obtain individual proteins from naturally occurring mix- 

 tures of them. At the end of his paper he gives a list of those 

 proteins which are now recognised as chemically homogene- 

 ous compounds. We cannot be certain, however, that even 

 these proteins are completely uniform. 



In connection with the lack of molecular homogeneity of 

 casein, G. R. Tristram^^ has also pointed out that ^-lacto- 

 globulin^' is not a single substance either, and rightly poses 

 the question as to whether the proteins which are now held 

 to be individual substances are not really mixtures of related 

 compounds, among which even the amino acid composition 



