272 Essays in Biochemistry- 



how complex the structure, however, where there is homogeneity there 

 is hope. If the hope is ill-founded and proteins really are families of 

 molecules, detailed investigations of their structure are bound to reveal 

 this fact. 



In finally deciding whether or not the biochemical events directing 

 the synthesis of protein molecules are so ordered as to permit exact 

 structural duplication from one molecule to the next, it will be neces- 

 sary to decide not only whether a given protein preparation is hetero- 

 geneous, but also whether heterogeneity, if found, has been imposed 

 in the process of isolating the material from natural sources. Oppor- 

 tunities abound for introducing heterogeneity where none originally 

 existed. At the outset, the choice of source material may be crucial 

 in this connection. For years, insulins derived from the ox, the pig, 

 and the sheep were tacitly assumed to be identical by many investi- 

 gators simply because each had identical effects on blood sugar. The 

 recent amino acid analyses of Harfenist 3 and the structural studies 

 of Sanger 4 have finally proved, however, that the insulins of these 

 species are slightly different. This finding should not be too surprising 

 in view of the abundant immunological evidence demonstrating the 

 species specificity of proteins. Most investigators would agree that 

 structural work on a protein preparation derived from several animal 

 species would probably be a waste of time, but how many would agree 

 on just what constitutes a species? What degree of genetic homogene- 

 ity must one demand before molecular homogeneity can be expected? 

 Does individuality extend to the molecular level? It does not seem 

 inconceivable that in the case of species, such as man, possessing an 

 unknown and uncontrollable genetic constitution, true homogeneity of 

 some types of proteins may only be attainable in a preparation derived 

 from a single individual. 



Even granted a suitable starting material containing a mixture of 

 initially homogeneous proteins, however, the problems involved in iso- 

 lating a single molecular species without introducing alterations in the 

 molecule remain among the most intractable facing the protein chemist. 

 For a long time, virtually the only preparative procedures available 

 made use of fractional precipitation in some form. Fortunately, recent 

 years have seen the introduction of multistage techniques of high re- 

 solving power such as count ercurrent distribution, zone electrophoresis, 

 and chromatography. Chromatography in particular would appear to 

 possess several features that, in principle, should make it ideally 

 adapted to work with proteins. The method is gentle, flexible, has high 

 resolving power, and can be used for both analytical and preparative 



