234 THE BIOCHEMISTRY OF B VITAMINS 



of glycine (from serine, p. 201) which is needed as the source of the 

 metabolic unit from which carbon atoms 4 and 5 (and probably nitrogen 

 atom 7) of the purine nucleus originate. 41 In the case of fowls and rep- 

 tiles which excrete most of their metabolic nitrogen in the form of uric 

 acid, the extensive purine synthesis which must be accomplished is re- 

 flected in the unusually high glycine requirement. 42 



The initial step in the catabolism of purines is the deamination of 

 adenine and guanine (no vitamin requirement) producing hypoxanthine 

 and xanthine, which are then oxidized by the flavoprotein, xanthine 

 oxidase, to yield uric acid. No B vitamin has been implicated in the 

 further degradations which uric acid has been found to undergo. 



Biosynthesis of the B Vitamins. The biological origin of the individual 

 vitamins has been taken up elsewhere (Chapter VA). However, it is 

 logical at this point to indicate that the synthesis of one vitamin prob- 

 ably often depends upon the presence of another B vitamin. In the 

 intestinal tract the biosynthesis of one B vitamin by bacteria may be 

 influenced by the dietary level of other vitamins which are essential for 

 the growth of the intestinal flora accomplishing the synthesis. A more 

 direct interrelation exists when an enzymatic reaction necessary for the 

 synthesis of one B vitamin requires another B vitamin as a coenzyme. 

 No such case has been unequivocally demonstrated, but when the indi- 

 vidual steps in the biosyntheses of vitamins have been better defined, it 

 will undoubtedly be found that many of the reactions involved are of the 

 types which require B vitamin coenzymes. For example, pyridoxal phos- 

 phate probably catalyzes the decarboxylation of aspartic acid to form 

 the /^-alanine required for the synthesis of pantothenic acid; and it can 

 be anticipated that the pyrimidine portion of the pterin (folic acid) and 

 isoalloxanine (riboflavin) molecules will be formed by processes utilizing 

 single carbon units in a manner analogous to that observed in purine 

 synthesis, and will be mediated by a p-aminobenzoic acid coenzyme. 



Choline. The methylation of ethanolamine, forming choline, is depend- 

 ent upon an adequate dietary source of substances containing available 

 methyl groups 43 (methionine being the most important) or the capacity 

 of the organism for producing them from other metabolic processes. 



In microorganisms it has now been established that the coenzymes in- 

 volved in the metabolism of the single carbon unit (p-aminobenzoic acid, 

 folic acid, and vitamin B i2 ) likewise function in the conversion of homo- 

 cysteine to methionine. 39 Also, on the basis of studies using isotopically 

 labelled compounds, it is known that the methyl groups of methionine 

 and choline can serve as sources of "formate" (p. 197). Hence, it is indi- 

 cated that the single carbon unit will be found to be one of the precursors 

 of the available methyl groups. That mammals may possess to a limited 



