12 THE BIOCHEMISTRY OF B VITAMINS 



answer to the second question, and these will be discussed in later chap- 

 ters. p-Aminobenzoic acid occurs universally in living matter in its com- 

 bination in folic acid and without much doubt in other forms also (p. 41) . 

 It is a highly important catalytic unit and appears to constitute a unique 

 example of a "vitamin within a vitamin." 



B Vitamins and Related Principles Not Completely Characterized 

 Chemically 



In the infancy of vitamin investigations vitamin B was thought to be 

 a single entity. Since that time, one after another well defined chemical 

 substance has been found to contribute to the physiological activity which 

 resides in crude extracts of yeast, liver, etc. How much further this dis- 

 covery of new organic components possessing vitamin activity will go, 

 no one can say. 



When the time arrives that the list is complete, it will be possible to 

 feed baby chicks and young rats completely synthetic diets, raise them 

 to maturity in a healthy condition, have them reproduce normally 

 generation after generation, and raise young as they do when fed natural 

 diets. Furthermore, it will be possible to accomplish this feat when the 

 animals are kept under sterile conditions, free from the symbiotic aid 

 of microorganisms. Until this latter is possible, one cannot be sure but 

 that some essential vitamins of bacterial origin are as yet unrecognized. 

 Biotin is an example of a vitamin which might have been overlooked in 

 non-sterile feeding experiments were it not for the experimental use of 

 raw egg white in diets and the presence in it of avidin, which combines 

 quantitatively with biotin, rendering it inactive. 



Many of the B vitamins have been discovered and concentrated by 

 the use of microorganisms, but historically they were not admitted to the 

 family of B vitamins until their functioning in animal nutrition had 

 been demonstrated. The study of microbial nutrition therefore constitutes 

 an important means whereby hitherto unrecognized nutrilites may be 

 discovered. Their acceptance as vitamins may depend upon the demon- 

 stration of their functioning in the nutrition of higher animals. 



There is the point of view among some active vitamin investigators, 

 particularly those who have approached the subject from the standpoint 

 of enzyme chemistry or microbial nutrition, that catalytic substances of 

 relatively small molecular dimensions may properly be regarded as 

 vitamins whether or not they are nutritionally required by higher animals. 

 This point of view has merit and may eventually be adopted. An alterna- 

 tive suggestion has already been made, namely, that the term "vitamin" 

 retain its historical nutritional connotation and that perhaps a new, more 

 inclusive term (e.g., catalins, p. 5) will be needed to designate all 



