140 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



Not only moulds, but also yeasts can be employed in the separation 

 of optically active compounds as was shown by F. Ehrlich in 1906, who 

 obtained in this way 1-alanine, d-leucine, 1-valine. Further, amino 

 acids, other than those which occur in nature, can be separated by 

 moulds and yeasts into their components, e.g., n-a-aminocaproic acid, 

 methylethylaminoacetic acid. 



It was first shown by E. Fischer, in 1894, that enzymes were 

 specific in their action ; thus maltase acts only upon a-glucosides and 

 emulsin only upon /8-glucosides. Later, he found that trypsin acted 

 " asymmetrically" upon inactive polypeptides, e.g my alanyl-leucine was 

 hydrolysed in such a way that only the compound composed of d-alanine 

 and 1-leucine, the natural isomers, was split up into its constituents, 

 whereas the compound composed of 1-alanine and d-leucine was un- 

 attacked. Again, inactive leucine ester was found by Warburg to be 

 only partially hydrolysed by trypsin ; he obtained 1-leucine and d-leu- 

 cine ester. 



Kossel and Dakin, in 1904, found that d-arginine was hydrolysed by 

 the enzyme arginase into d-ornithine and urea ; and by using this enzyme 

 Riesser, in 1906, separated dl-arginine, which he had prepared by 

 heating d-arginine with sulphuric acid to 160-180 C. into 1-arginine, 

 d-ornithine and urea, the racemic compound being hydrolysed asym- 

 metrically by the enzyme. 1-Ornithine can be prepared from the 1-argin- 

 ine by hydrolysis with baryta. 



Bacteria can also produce a separation of the stereoisomers. 

 Neuberg and Karczag found that the natural isomer of valine was 

 attacked, leaving the unnatural isomer. 



The biological method thus only serves for the preparation of that 

 isomer which does not occur in nature, since the mould or yeast or 

 enzyme destroys the naturally occurring form, leaving the other isomer 

 untouched. According to Marckwald and Mackenzie both isomers are 

 attacked, the natural one more rapidly than the other. This has 

 also been shown by Pringsheim in the cases of leucine and glutamic 

 acid. 



The method therefore does not lead to the synthesis of the naturally 

 occurring amino acid. 



