176 THE BIOCHEMISTRY OF B VITAMINS 



that the bacterial cells first phosphorylate the added pyridoxal before 

 it becomes active, 192 and that this phosphorylated pyridoxal is also the 

 codecarboxylase for at least six of the known amino acid decarboxylases. 

 The vitamin B 6 activity of pyridoxine-supplied bacteria had been 

 observed in many instances to depend upon an activation of this com- 

 pound occurring when it was autoclaved with the other components 

 (particularly the amino acids) of the medium. 193 An investigation into 

 the nature of the chemical changes resulting from the autoclaving led to 

 the discovery of pyridoxal and pyridoxamine (p. 186) , and to the recog- 

 nition of a nonbiological reaction (a transamination) by which these two 

 forms of the vitamin are interconverted. 194 Glutamic acid (and most other 

 common a-amino acids) can serve as an amino donor for the formation 

 of pyridoxamine from pyridoxal, and a-ketoglutaric acid is an efficient 

 acceptor for the amino group in the reverse reaction. 



NH 2 H— C=< 



HO— C— CH 2 — CH 2 — C— COOH + HO— rT ^— CH 2 OH 

 H 



-C— COOH + HO— fT^V 



KcXJ 



N 



glutamic acid pyridoxal 



O 



h— c— c- 



H 



H— C— NH 2 



HO— C— CH 2 — CH 2 — C— C— OH + HO-^ ^— CH 2 OH 



H 3 C- 



a-ketoglutaric acid pyridoxamine 



A similar intermolecular exchange of amino and carbonyl groups, cata- 

 lyzed by enzymes, had been previously recognized as occurring in the 

 tissues of animals. 195 These enzymes, classified as transaminases, catalyze 

 the interconversion of certain a-keto and a-amino acids; consequently, 

 it was suggested that a possible function for vitamin B 6 was to serve 

 as an intermediate in transamination reactions by alternating between 

 the aldehyde and amine states. 194 To test this hypothesis, tissues of 

 vitamin B 6 -deficient rats were compared to controls from normal ani- 

 mals. 196 The deficient tissues were found to be definitely inferior in their 

 ability to catalyze a transamination reaction (the glutamic acid-aspartic 

 acid system) . The apoenzymes from resolved bacterial decarboxylases 

 were shown to be reactivated by boiled extracts of transaminase con- 

 centrates (prepared from animal tissue), indicating the existence of a 



