TRANSAMINATION 95 



D-alanine are found to syntliesise D-alanine. D-alanine cannot 

 be the precursor of pyridoxin but it is probable that pyridoxin 

 mediates the synthesis of the unnatural isomer of alanine. 

 This finding has led to the discovery of a new enzyme called 

 "racemase" which produces DL-alanine from L-alanine. 

 D-alanine is essential for growth and is synthesised by trans- 

 aminase followed by racemase, both enzymes having pyridoxal 

 phosphate as prosthetic group. It appears improbable that 

 transaminase is concerned with the general synthesis of 

 amino-acids, as first suggested, but it is always possible that 

 the cell-free preparations and the studies carried out with them 

 represent only a part of a more complex system within the 

 living cell. 

 The transamination reactions: 



Glutamic acid + oxalacetic acid -> a-ketoglutaric acid + 



aspartic acid, 



Glutamic acid + pyruvic acid -^ a-ketoglutaric acid -|- alanine, 



have now been demonstrated for many bacteria including 

 species of Escherichia, Shigella, Eherthella, Proteus, Pseudo- 

 monas, Azotohacter, Staphylococcus, Streptococcus, and Pneu- 

 mococcus. A cell-free transaminase has been prepared from 

 S. faecalis and resolved into specific protein and a prosthetic 

 group replaceable by pyridoxal phosphate. 



The biosynthesis of other amino-acids has been elucidated 

 by the application of a new technique which is peculiar to 

 microbiology. It has been mentioned above (see p. 60) 

 that the formation of enzymes in the cell is determined by the 

 presence of the controUing gene and that alteration of that 

 gene results in the loss of that enzyme. The rate of alteration 

 or mutation of genes can be accelerated by irradiation with 

 X-rays and this method has been used by Tatum, Bonner, 

 Beadle, and their co-workers to produce a very large number of 

 artificial mutants of the bread mould Neurospora crassa. The 

 same method was later applied to other moulds, such as species 

 of Penicillium and Aspergillus, and to bacteria such as 



