COENZYMES DERIVED FROM B VITAMINS 185 



presumed that their synthesis probably involves transamination reactions, 

 either directly or indirectly. Imidazole pyruvic acid has been shown to 

 replace the histidine requirements of a bacterium provided adequate 

 vitamin B 6 is supplied. 230 The amino acid could be formed directly from 

 the former compound by transamination. 



The ability of an organism to use O-amino acids in place of the natural 

 isomers has been shown to be dependent upon the availability of pyridoxal 

 or pyridoxamine. Transamination reactions in which optically inactive 

 keto acids are formed would be a convenient method for making such 

 compounds utilizable. 



The availability of pyridoxal and pyridoxamine also exerts a sparing 

 action upon the requirements for three additional amino acids — arginine, 

 phenylalanine and tyrosine. 228 In these instances, however, adequate car- 

 bon dioxide tension must be maintained over the cultures. In a carbonate 

 free system the vitamin is incapable of altering the requirements for these 

 particular amino acids. It was at first postulated that a carboxylation 

 of amines can be effectively used for the biosynthesis of these amino 

 acids. If such were the case, however, one would have to account for the 

 origin of these amines by some process other than decarboxylation of 

 amino acids. In addition, the equilibrium is such that relatively high 

 concentrations of the rather toxic amines would have to exist for the 

 reaction to be directed toward amino acid synthesis. When phenylethyl- 

 amine was supplied the microorganism, only a slight conversion to phenyl- 

 alanine was observed, even in the presence of high concentrations of 

 carbon dioxide. It seems probable, therefore, that the carbon dioxide 

 effect is an indirect one, and that vitamin B 6 catalyzes the syntheses of 

 these three amino acids by some process other than a direct amine car- 

 boxylation. 



The third type of B 6 reaction — tryptophan synthesis — has also been 

 indicated by similar studies of nutritional requirements. Either pyridoxal 

 or pyridoxamine must be supplied in greater amounts if a bacterium is to 

 use indole or anthranilic acid (an indole precursor) in place of trypto- 

 phan. 231 



In animals, the utilization of tryptophan for nicotinic acid synthesis 

 is dependent upon an adequate vitamin B 6 intake (p. 279) , and low 

 intakes of vitamin B 6 result in abnormal tryptophan catabolism and the 

 production of urinary products not normally detected (p. 427) . 



Vitamin B 6 and Fat Metabolism. Several of the earliest observations 

 upon the physiological results of vitamin B 6 deficiencies involved this 

 vitamin in fat metabolism, particularly the metabolism of the unsaturated 

 fatty acids. 232, 233 « 234, 235 No explanation for these observations can be 



