352 THE BIOCHEMISTRY OF B VITAMINS 



nized forms of the B vitamins to their respective functional forms, only 

 in the cases of nicotinic acid and choline are the animals apparently able 

 to synthesize the vitamin from some precursors. Special consideration 

 must be given to the synthesis of these vitamins, therefore, and also to 

 the breakdown of nicotinic acid, concerning which we have considerable 

 knowledge. The transformation of the other B vitamins to their excretory 

 products has been little studied and few data are available in their regard. 

 Coenzyme Synthesis. Thiamine, riboflavin, pyridoxal, nicotinamide, 

 and pantothenic acid are known to function in the specific coenzymes 

 cocarboxylase, flavin mononucleotide and dinucleotide, codecarboxylase, 

 coenzymes I and II, and coenzyme A, respectively, all of which contain 

 phosphate. In the case of cocarboxylase, flavin mononucleotide, and 

 codecarboxylase, the vitamin phosphate apparently constitutes the entire 

 coenzyme, and the synthesis of the coenzymes by phosphorylases is ap- 

 parently performed within the various cells of the animal in a rather 

 direct manner. This is certainly so in the case of thiamine, since prepara- 

 tions from liver and kidney, and to a lesser extent muscle and brain 

 actually convert it to cocarboxylase. 67 Rat kidney extracts in phosphate 

 buffer at pH 8.4 and containing a trace of arsenite are particularly 

 effective in this respect, 68 and it has been shown that some energy- 

 yielding system is necessary to perform the phosphorylation, 69 as would 

 be expected. It is unclear, however, just how the vitamin B 6 triad func- 

 tions in maintaining the codecarboxylase concentration, although it seem.s 

 likely that pyridoxine is first oxidized and then phosphorylated to 

 pyridoxal phosphate; this then functions in a reversible transamination 

 system with pyridoxamine phosphate in those processes which activate 

 the a carbon atom of a-amino acids, such as transamination, decarboxyla- 

 tion, and trytophan decomposition. Present evidence indicates at least 

 that pyridoxamine requires the presence of a keto acid for its conversion 

 to pyridoxal 69 (p. 176) . It further seems likely, but is not proved, that 

 riboflavin is first phosphorylated and then coupled with adenylic acid, 

 since adenylic acid and riboflavin-5'-phosphoric acid are formed on the 

 enzymatic hydrolysis of the "fiavin-adenine-dinucleotide." 70 In this 

 regard, riboflavin must be considered to be a step more evolved toward 

 the coenzyme than nicotinic acid, animals apparently not being able 

 to couple ribitol to the 6,7-dimethyl-isoalloxazine nucleus, as they are 

 able to couple ribose to nicotinamide. The steps involved in the con- 

 version of nicotinamide to coenzymes I and II are not known, but by 

 analogy with riboflavin one might expect the process to involve com- 

 binations with ribose, phosphorylation, and coupling with adenylic acid 

 successively. It has been shown that rabbit brain contains a DPN 



