FUNCTION 



of S. faecalis R as a source of tyrosine apodecarboxylase, G. H. 

 Sloane-Stanley ^^<* confirmed that pyridoxal acetal-3-phosphate had 

 no codecarboxylase activity, whereas p5^idoxal-5-phosphate in the ^ 

 form of its calcium salt was as active as the natural coenzyme, and 

 could be used as a standard in assays of codecarboxylase in animal 

 tissues. 



Pyridoxine may also function as a catalyst for the synthesis of 

 amino acids by a reaction which is the reverse of that just discussed. 

 For, when pyridoxine was added to a culture of L. arabinosus or S. 

 faecalis R deficient in phenylalanine, tyrosine, arginine or aspartic 

 acid, the growth of the organisms was impaired in the absence of 

 carbon dioxide ; growth was restored when this was passed into the 

 culture. ^^ It appears, therefore, that carbon dioxide is necessary for 

 specific amino acid synthesis and that pyridoxine is involved in the 

 enzyme system that carries out the synthesis. 



Transamination 



The second biological reaction for which pyridoxine and its de- 

 rivatives appear to be essential is the transfer of an amino group 

 from an amino acid to a keto acid, forming a different amino acid, a 

 reaction which is reversible, for example : 



HOOC . CH2 . CH2 . CH . COOH -f- HOOC . CH2 . CO . COOH ^ 



NH2 



glutamic acid oxaloacetic acid 



HOOC . CH2 . CH2 . CO . COOH + HOOC . CH2 . CH . COOH 



NH2 



a-ketoglutaric acid aspartic acid 



F. Schlenk and E, E. SnelP'^ observed that muscle extract pre- 

 pared from normal rats had a greater transaminase activity than a 

 similar muscle preparation from vitamin Bg-deficient animals and 

 they therefore suggested that a derivative of pyridoxine was an essential 

 component of the transaminase system. This seemed a particularly 

 attractive hypothesis in view of the transformation of pyridoxal to 

 pyridoxamine by heating with an amino acid (see page 300). 



P. P. Cohen and H. C. Lichstein,^^liowever, found that suspensions 

 of Streptococcus faecalis R were able to catalyse the transamination 

 reaction, whether the organism was grown on a medium with a high 

 or a low pyridoxine content, whereas the ability of the cells to catalyse 

 the decarboxylation of tyrosine was markedly greater when the 

 medium contained pyridoxine than when pyridoxine was absent. 



On the other hand, evidence was obtained that a phosphate of 

 pyridoxal was the prosthetic group in transaminase preparations 



333 



