PROTEIN STRUCTURE 



position 7 in insulin imposes constraints on the possible con- 

 figurations which are examined further below, and which do not 

 exist in the case of the smaller peptide hormones. 



This work on insulin indicates that it is possible to obtain 

 protein preparations which are genuinely homogeneous, all 

 molecules in the preparation being identical in the sense in which 

 we may say that the molecules in pure acetone or glycine are 

 identical. There is no doubt that such purity has seldom been 

 achieved in protein preparations, and increasingly delicate 

 analytical techniques — especially those employed by the physical 

 chemists — have revealed heterogeneity in preparations earlier 

 thought to be pure. On the other hand, the increasing refine- 

 ment of the techniques of fractionation has also permitted the 

 resolution of mixtures into pure components to an extent 

 unattainable earlier. Thus, two kinds of beef insulin, differing 

 only by the conversion of one carboxyl to an amide group in a 

 molecule with 51 amino acid residues, have been separated by 

 countercurrent fractionation (46,47). If there is no difference 

 of net charge — for instance, if we consider two molecules which 

 differ only by the conversion of one valyl to isoleucyl, and one 

 alanyl to threonyl, as with beef and pig insulins (Figure 2) — 

 separation by present techniques would be much more difficult. 

 Even such separations, however, now appear practicable. It 

 remains to be seen whether a given animal makes all its insulin 

 molecules just alike, or whether it can produce several similar 

 but not identical molecules, all more or less equivalent in 

 physiological function. Even if the latter alternative corresponds 

 to the actual situation, the possibility of fractionating the mixture 

 into pure components still remains. Moreover the heterogeneity 

 found in a protein preparation may be due not to natural 

 differences between the molecules but to damage done by the 

 chemist in the process of extracting the material from its natural 

 state — damage which can be avoided by gentler techniques. 

 We express confidence here — confidence based perhaps more 

 on faith than on reason, but with a faith fortified by the results 

 already achieved with insulin — that genuinely pure protein 



381 



