B VITAMINS: WHAT THEY ARE 11 



itself is an essential cell constituent and must be furnished in the food 

 or built up by animals. Just what is required for this building up process 

 is not known. If ethanolamine is the simplest substance which in conjunc- 

 tion with methionine can overcome the lack of choline in the diet, it 

 instead of choline should be designated as the vitamin, according to our 

 previous discussion. From the practical nutritional standpoint, however, 

 choline is more important than ethanolamine, but its status as a vitamin 

 is complicated by the existence of other natural food substances which at 

 least partially replace it. 



From the standpoint of universality in cells, choline appears to be like 

 the typical B vitamins. Its catalytic functions have not been demonstrated 

 and its relative abundance and the high "requirements" of animals may 

 be cited against this possibility. On the other hand, it may (like inositol?) 

 be both a catalytic unit and a constituent of certain phospholipides. 

 There is some resistance to designating choline as a vitamin because it 

 was known to be a common constituent of natural foods (combined in 

 phospholipides) long before the typical vitamins were discovered. 



p-Aminobenzoic acid is unique among the vitamins or vitamin-like 

 substances so far considered in that it makes up an integral part of one 

 of the typical B vitamins, namely folic acid. Its nutritional functioning 



COOH 



i 



/ \ 

 HC CH 



h£ in 



NH 2 



p-Aminobenzoic acid 



has been demonstrated, but not in connection with a diet already con- 

 taining an adequate supply of folic acid. Furthermore, relatively large 

 quantities have been used. It is possible, since p-aminobenzoic acid 

 makes up a part of the folic acid molecule (p. 9), that its nutritional 

 value is dependent upon its use as a building unit out of which folic 

 acid may be made by intestinal bacteria or otherwise. Even before the 

 presence of the p-aminobenzoic acid residue in folic acid was established, 

 there was evidence that feeding p-aminobenzoic acid to chicks promoted 

 intestinal production of folic acid. 25 



The crucial questions which have not been conclusively answered at 

 the present writing are whether p-aminobenzoic acid is required nutri- 

 tionally in addition to folic acid, and whether it has catalytic functions 

 independent of folic acid. There are strong indications of a positive 



