THE BIOCHEMISTRY OF B VITAMINS 



made on the basis of any of the reactions now recognized as being cata- 

 lyzed by pyridoxal phosphate. 



Mechanism of Pyridoxal Phosphate Action. The nonenzymatic trans- 

 amination reactions in which pyridoxal and pyridoxamine participate can 

 most logically be explained on the basis of an intermediate formation of 

 a Schiff's base which can tautomerize and be hydrolytically cleaved in 

 a manner such as this: 



OH OH 



H— C— H H— C— H 



I H COOH I COOH 



N /~\_i=0+H 2 N-i-H 12! nA-Ln4-H — 



HCHOH 



k 



HCHOH 

 pyridoxal 



+H20 



K 



a-amino 

 acid 



OH OH 



H— C— H H— C— H 



I COOH +Hi0 I COOH 



N<f \— C— N=C ^7^ n/ \— C— NH 2 + 0=C 



H I - H2 ° \__/ H I 



HCHOH 



HCH 



im 



imine a-keto 



acid 



Compounds of the Schiff's base type formed from pyridoxal and amino 

 acids are believed to exist in biological materials. It has been assumed 

 that the formation of such an intermediate is the mechanism by which 

 pyridoxal phosphate activates the amino acid molecule. The energies in 

 the postulated Schiff's base are localized in such a way that any one of 

 the groups attached to the cc-carbon atom may undergo a reaction if the 

 pyridoxal phosphate is attached to the appropriate apoenzyme. 



When the substrate for a reaction is an amino acid, the aldehyde 

 phosphate would be the appropriate state of the coenzyme for the forma- 

 tion of the postulated intermediate, and pyridoxamine phosphate would 

 not be expected to be active per se as the coenzyme for decarboxylases or 

 tryptophanases ; and it is not. However, in transamination reactions 

 (wherein both amino and keto acids must be present as substrates) it is 

 postulated that the coenzyme alternates between two states, the aldehyde 



