The Occurrence of Amino Acids in Nature 145 



been listed by Bricas & Fromageot. The conclusion seems inescapable that the 

 toxic properties are directly related to the 'abnormal' features of chemical struc- 

 ture, and the student of evolution is tempted to see here a specialized adaptation 

 which does not throw much light on the evolution of protein structure or of the 

 common amino acids. All twenty of these last have been found in compounds of 

 class (6), but of course their lower molecular weights preclude the simultaneous 

 presence of such a wide variety of amino acid residues as is usually present in 

 a protein. 



As concerns group {c) — free amino acids — all the twenty common amino 

 acids have been found free in all hving organisms where thorough analysis has 

 been made, although often the concentrations in the tissue juices may be very 

 low. There is much evidence that free amino acids are directly involved in the 

 anabohsm and catabolism of proteins. However, the common amino acids seem 

 to have many other biochemical functions. Increasingly many amino acids 

 which do not occur in proteins have been found free in Uving organisms in 

 recent years. The variety seems particularly abundant in higher plants, where 

 a particular amino acid may characterize a particular genus or natural order. 

 Lists have been given by Grobbelaar, Pollard & Steward [59], Synge [58] and 

 Virtanen [60] but the number increases almost weekly nowadays. It is particu- 

 larly great for leguminous plants. The evolutionary problem seems somewhat 

 similar to that presented by the alkaloids. 



This survey of the natural occurrence of free and combined amino acids in 

 nature serves to emphasize the chemical similarity of all Hving organisms on the 

 Earth, and thus agrees on the whole with the results of chemical and biochemical 

 studies of other classes of substance. The information will be of great interest 

 when the biochemistry of Mars and of planets in other planetary systems be- 

 comes accessible. As concerns the terrestrial origin of life, it seems rather to 

 block the progress of studies, suggesting that most of the chemical evolution of 

 proteins occurred in a remote past from which have survived only organisms 

 which had already arrived at a very standardized chemical structure. 



However, the study of amino acids and amino acid composition is only the 

 most elementary part of protein chemistry. Recent progress towards the under- 

 standing of the structure and function of proteins opens a fascinating prospect 

 for comparative terrestrial biochemistry in the not too far distant future. The 

 possibihty of observing species differences in the structural chemistry of proteins 

 was suggested by early serological work with animal and plant products [i], 

 especially if this is considered together with Landsteiner's own work on the 

 serological specificity of synthetic and natural peptides [61]. Differences in Fe 

 and S contents of haemoglobins from different species have long been known. 

 We now have a number of examples of species differences between proteins and 

 peptides which have been verified in full chemical detail. Thus, Brown, Sanger 

 & Kitai [62] have shown that sheep insulin differs from bovine insulin at only 

 one amino acid locus in the whole molecule, while pig insuhn differs from both 

 these at two, different loci. The vasopressin from the pituitary gland of the pig 

 has a lysine residue where bovine vasopressin has an arginine residue [63]. Tyro- 

 cidines A and B, produced by one strain of Bacillus brevis, are cyclic decapep- 



