The Occurrence of Amino Acids in Nature 143 



bacterium diphtheriae [28] seems more likely to be diaminopimelic acid. Like- 

 wise the unknown amino acid [17] found by Samarina et al. [29] in acid hydro- 

 lysates of Vibrio spp. which had been extracted with hot saline was probably 

 diaminopimelic acid. Hoare & Work [30] have described the distribution among 

 bacteria of the different stereoisomers of diaminopimehc acid. The chemically 

 related dipicolinic acid occurs in the spores of Bacillus spp., but it does not 

 seem estabUshed whether or not it is chemically bound [31, 32]. A diaminosuberic 

 acid isomer has been isolated from Actinomycetales by Work [33]. 



The above seem to be all the less common amino acids which may at present 

 be regarded as occurring in proteins. However, the occurrence of D-forms of the 

 common amino acids must also be considered. It is difficult to assess the sig- 

 nificance of the occurrence of D-forms in protein hydrolysates, since the hydro- 

 lysis itself may bring about racemization. The racemic phenylalanine found by 

 Martin & Synge [34] in hydrolysates of wool and by Galaev [28] in bacterial 

 hydrolysates may exemplify this. Even actual inversion during hydrolysis may 

 occur (cf. [35]). These possibihties have helped to complicate still further the 

 long and inconclusive controversy that has raged around the alleged occurrence 

 of D-amino acids in the proteins of cancerous tissues. Nevertheless, it is now 

 clear that D-amino acids occur in protein-like components of some bacteria. The 

 classic instance is the capsular 'polypeptide' of Bacillus spp., whose chemical 

 nature was discovered by Ivanovics & Bruckner. This has in recent years been 

 studied in great detail both by its discoverers and by many other workers. It 

 seems generally agreed that this polymeric substance is the product of con- 

 densing together residues of D-glutamic acid by peptide linkages involving almost 

 exclusively the y-carboxyl groups [36, 37]. It seems also to be estabhshed that 

 diaminopimehc acid most commonly occurs in the meso form, having the 

 D-configuration at one end of the molecule and the L-configuration at the other 

 [30]. Such examples have led several workers to examine systematically the 

 configuration of amino acids obtained by the hydrolysis of bacteria. In general 

 L-amino acids predominate, as in other Hving organisms, d- Amino acids have, 

 however, been found as follows. In Bac. brevis, Konikova & Dobbert [38] found 

 substantial amounts of D-amino acids. Somewhat smaller amoimts were found 

 by Stevens, Halpern & Gigger [39], who showed that D-aspartic acid contri- 

 buted substantially to the total. Later Stevens, Gigger & Bowne [40] showed 

 that the D-amino acids were predominantly D-aspartic acid and D-phenylalanine, 

 and were in a form which could largely be extracted from the bacteria by hot 

 aqueous ethanol; this resembles more closely the antibiotic peptides produced 

 by this organism than 'true protein'. In agreement with this, Vyshepan [41] 

 found very Uttle D-amino acid in hydrolysates of cells of Bac. brevis or of Bac, 

 mycoides which had been exhaustively extracted with ethanol. Jenkins & Ciereszko 

 [42] found L-glutamic acid to predominate in cells of Bac. subtilis despite the 

 exclusive presence of D-glutamic acid in the capsular substance. Lawrence & 

 Halvorson [43] found small amounts of D-amino acids, including D-glutamic 

 acid, in vegetative cells and in spores of Bac. terminalis. Dunn et al. [44, 45] 

 found D-glutamic acid in Lactobacillus spp. Holden & Snell [46] foimd D-alanine 

 in Lactobacillus, Leuconostoc, and Streptococcus. Camien [47, 48] found D- 



