VITAMINS 243 



which is 2-methyl-3-hydroxy-4,5-di- (hydroxymethyl) -pyridine. 



Two closely related substances, pyridoxal and pyridoxamine, are repre- 

 sented by the following formulas: 



CHO CH2NH2 



I I 



c c 



HO— C-^ "==0— CH2OH HO— C^ ^C— CH2OH 



HaC-C^^^CH HsC-G^^^CH 



Pyridoxal, CsHgOsN Pyridoxamine, C8H12O2N2 



These substances have about the same vitamin Be activity for animals and 

 for yeast cells as pyridoxine does, but are several thousand times more 

 effective for certain bacteria. A phosphorylated derivative, pyridoxal 

 phosphate, functions as a coenzyme for enzyme systems present in many 

 bacteria, which break down amino acids into the corresponding amines 

 by removing carbon dioxide from the carboxyl group of the amino acid 

 (p. 321) . It is therefore called a codecarboxylase. Both pyridoxal phos- 

 phate and pyridoxamine phosphate function as coenzymes in certain trans- 

 amination reactions (p. 343) and may, therefore, be called cotransami- 

 nases. 



CHO CH2NH2 



I I 



HO— C^ ^C— CH2O-PO3H2 HO— C^ >C— CH2O— PO3H2 



I II I II 



H3C— C^^.^H H3C— C*^^,^CH 



Pyridoxal phosphate Pyridoxamine phosphate 



Pyridoxal phosphate also serves as a coenzyme for the enzyme system 

 involved in the synthesis of tryptophan by a certain mold species {Neuro- 

 spora crassa) . It is, therefore, quite clear that the Be vitamins play 

 important roles in both the decomposition, inter conversion, and synthesis 

 of amino acids in living cells. 



Pyridoxine is stable to heat, alkalies, and strong acids, but is rather 

 easily attacked by oxidizing agents. As yet little work has been done 

 on its destruction during the cooking of food. No reliable figure for the 

 human requirement is available, but a tentative value of 1.5 mg. per 

 day has been suggested. 



Occurrence 



Pyridoxine is present in yeast, bran and embryo of cereal grains, meats, 

 milk, and leafy vegetables. The amount of this vitamin in a number 

 of common foods is given in Table 9-5. 



