The Occurrence of Amino Acids 

 in Nature 



R. L. M. SYNGE 



The Rowett Research Institute, Bucksburn, Aberdeenshire, Scotland 



These are the first two sentences from The Specificity of Serological Reactions 

 by Karl Landsteiner [i]. 'The morphological characteristics of plant and animal 

 species form the chief subject of the descriptive natural sciences and are the 

 criteria for their classification. But not until recendy has it been recognized that 

 in living organisms, as in the reahn of crystals, chemical differences parallel the 

 variation in structure.' 



Since this was written, we have seen great advances in our understanding of 

 proteins and amino acids, and especially in the range of the Hving organisms which 

 have been studied chemically in this respect. My purpose is to try to extract 

 from the mass of scattered observations, made with a variety of motives, such 

 generalizations as appear relevant to the subject of the Symposiimi. 



It is convenient to consider amino acids as occurring in hving organisms : (a) 

 chemically combined in proteins; (b) chemically combined in peptides and other 

 compounds of relatively low molecular weight; (c) free. 



The distinction between (a) and (b) is somewhat arbitrary. It has been con- 

 venient to draw the line at about mol. wt. 10,000, but the less well-defined 

 materials have often been placed in one class or the other on the basis of solubihty 

 in hot water, in aqueous trichloroacetic acid or of some other easily observed 

 property. Compounds having a specially simple amino acid composition have 

 often been placed in class (b) while having obviously high molecular weight. 



Despite the arbitrary and empirical nature of this distinction, it is neverthe- 

 less possible to make a generahzation about class (a), the true proteins. These 

 are often found to be 'conjugated' with 'prosthetic groups' varying very widely 

 in chemical nature. However, the amino acid residues are nearly always the 

 L-isomers of the following: glycine, alanine, valine, leucine, isoleucine, serine, 

 threonine, cyst(e)ine, methionine, aspartic acid, asparagine, glutamic acid, glut- 

 amine, proline, phenylalanine, tyrosine, tryptophan, lysine, arginine and histidine. 

 It is not twenty years since the last of these twenty amino acids, threonine, was 

 proved to be a protein constituent. Since then, despite intensive search in pro- 

 teins from a wide variety of Hving organisms, with use of methods of analysis 

 much better than those previously available, no new amino acid has been found 

 that is generally distributed and a number of the better-studied protein prepa- 

 rations have had all their components accounted for quantitatively from among 

 these twenty common amino acids. Moreover, it is unusual to find more than 



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