62 NITROGEN METABOLISM 



and Lb. arahinosus [36a] and also with animal tissues [26]. 

 Unequivocal proof that such results are due to transamina- 

 tion awaits the isolation of the appropriate enzymes, and, 

 indeed, a glutamic-tyrosine transaminase and a glutamic- 

 phenylalanine transaminase have been isolated from Esch. 

 coli and shown to contain prosthetic groups of pyridoxal 

 phosphate [14]. All known transaminases are specific for the 

 L-isomers of the amino-acids. 



Our conceptions of the mechanism operative in biological 

 transamination are based on analogy with in mtro systems. 

 Transamination is a true transfer process and there is no 

 evidence at all that the amino group becomes free as NH3 at 

 any stage in the reaction. Herbst has proposed that a type 

 of Schiif's base is formed by condensation between the 

 amino and keto groups of the two substrates and that this 

 is followed by a molecular rearrangement involving the 

 alpha hydrogen atom of the amino donor, after which the 

 base is ruptured by hydrolysis [25]. Incubation of alanine 

 containing deuterium in the alpha position with a-keto- 

 glutarate and a mammalian glutamic-alanine transaminase 

 resulted in the rapid appearance of deuterium in the water 

 of the experimental system and is evidence in favour of 

 such a mechanism: 

 Ri R2 Ri R2 



I I I I ■ 

 CH(NH,)+CO ^HC— N = C ^ 



I " I II 

 COOH COOH COOH COOH 



Ri R2 Ri R2 



I I I 1 



C = N— CH ^ CO + CH(NH,) 



I I I I " 



COOH COOH COOH COOH 



It is to be remembered that pyridoxal phosphate itself con- 

 tains a carbonyl group and Snell, on the basis of non- 

 enzymic chemical experiments, has made the following 

 proposal [42]: 



amino-acid A+pyridoxal phosphate ^ 



keto acid A+pyridoxamine phosphate 

 pyridoxamine phosphate + keto acid B ^^ 



amino-acid B+pyridoxal phosphate 



t 



