The Occurrence of Amino Acids in Nature 227 



aspartic acid in organisms of the same genera. Stevens et al. [39] further found 

 significant amounts of D-amino acids in Bacillus subtilis and Torulopsis utilis. The 

 cell-wall material of Gram-positive bacteria has in recent years been shown to 

 yield on hydrolysis sugars, hexosamines and characteristic mixtures of only a 

 few amino acids; the mixtures found have proved of some value to the syste- 

 matist. These amino acids belong chiefly to the group glycine, alanine, aspartic 

 acid, glutamic acid, lysine and diaminopimelic acid (for references see [49, 50]). 

 It is particularly interesting that the greater part of the D-alanine present in 

 lactic acid bacteria was present in the cell wall [51]. By adding D-a-aminobutyric 

 acid to the media in which the bacteria were grown, it was to some extent 

 incorporated in the cell-wall material. There is thus no reason as yet for sup- 

 posing that D-amino acid residues occur in the 'typical proteins' of bacteria — 

 they have only so far been unequivocally demonstrated in 'peptide' fractions 

 soluble in organic solvents and in capsular and cell-wall materials that are not 

 in any sense 'typical proteins'. 



Thus it seems that proteins, which are found in substantial amount in every 

 Uving organism so far studied, form a remarkably compact chemical grouping. 

 When this chemical uniformity is seen in relation to the diversity of important 

 functions which are carried out by proteins in the hving organism, it seems very 

 reasonable to postulate a common evolutionary origin for all the organisms at 

 present living on Earth. The observations at least cover a wide range of animals, 

 higher plants, algae, fungi, heterotrophic bacteria and viruses. They are deficient 

 in respect of autotrophic bacteria. Dr Howard Lees tells me that the only evidence 

 of which he is aware relating to the amino acids present in such organisms is 

 for Thiohacillus thio-oxidans [52] and for Nitrosomonas [53]. In both these in- 

 stances most of the common amino acids and no unusual ones were observed. 

 Fossils have in general proved unsuitable for amino acid analysis*, but it is of 

 interest that a very incomplete amino acid analysis of haemoglobin from a 

 Crossopterygian fish [54] did not give figures much outside the range already 

 observed among Vertebrata. 



I have elsewhere engaged in speculation [55, 56] as to why these particular 

 twenty amino acids came to make up the proteins of Hving organisms, and at 

 the Symposium itself no doubt further speculation will take place. 



As concerns amino acid residues in substances of class (è), it is important to 

 remember how few substances belonging to this class have been known until 

 recent years. Such substances tend to be thrown away when proteins are pre- 

 pared by the traditional methods. Systematic searches for such substances have 

 been few, although they are becoming more frequent. The great majority of 

 the substances in this class have been found in the course of trying to purify 

 materials possessing some striking biological activity. Substances in this class 

 have been listed by Bricas & Fromageot [36]; cf [57, 58]. The number known is 

 at present increasing very rapidly. It is striking that amino acid residues or 

 related chemical groupings of novel structure or steric configuration occur in a 

 large proportion of those substances possessing specific toxic activities which 

 have been isolated from bacteria and fungi. Many of these amino acids have 



* [Note added in proof.] But see P» H. Abelson, Ann. N.Y. Acad. Sei., 69, 276, 1957. 



