284 THE BIOCHEMISTRY OF B VITAMINS 



excretion studies, it is apparent that such an approach must involve sev- 

 eral assumptions {e.g., that the excretion is a function of the intake) and 

 techniques {e.g., eventual testing of the excreted products by nonhuman 

 means) which leave much to be desired. Nevertheless, when the conditions 

 are adequately controlled, considerable valuable information may be so 

 obtained, and extended further studies of this nature seem to be urgently 

 required. The application of such human bioassay techniques to the study 

 of the availability of thiamine in yeast 98 - 99 (p. 291) and its enzymatic 

 destruction 10 ° (p. 292) , have indeed opened a valuable new approach in 

 the science of nutrition. 



The Relative Potencies of the Various Naturally Occurring Forms of 

 the B Vitamins. It seems most probable that much remains to be dis- 

 covered concerning the variety of naturally occurring substances possess- 

 ing B vitamin activity. This is largely due to the great difficulties inherent 

 in the separation of minute quantities of structurally similar compounds. 

 Recently techniques employing a combination of paper partition chroma- 

 tography 101 and plate growth of assay organisms 102 have been developed, 

 and these show promise of adding to our knowledge of nutrilite derivatives 

 in general. For the present, however, the discussion which follows must 

 be considered in terms of the probability of the existence in nature of a 

 far greater number of B vitamin isotels. 103 



The discussion of these various vitamin forms might well be undertaken 

 in a pedagogic fashion from the standpoint of their relationship to the 

 generally accepted form(s) of the vitamin, but it is certainly worthwhile 

 to point out that other rather obvious categories of isotels do exist. Thus 

 we have the immediate vitamin structures such as the B 6 "triad" and the 

 folic acid "triad." Secondly, bound forms, available and unavailable, are 

 known for most of the B vitamins. Vitamin precursors frequently show 

 vitamin activity as in the case of the precursors of choline and nicotinic 

 acid, and the functional forms, frequently coenzymes, are generally active. 

 Catabolic products of vitamin metabolism also show activity for some 

 species. Finally, there is a large group of biologically active substances 

 which seem to have little apparent structural relationship to the B vita- 

 mins themselves, although one might theorize extensively on their activity. 

 For practical purposes, however, it has been deemed advisable to consider 

 the various known analogues in relation to each of the individual vita- 

 mins, and thus in a more systematic, though less erudite fashion. 



(1) Thiamine 



Thiamine may occur in free and combined forms (p. 30), and little 

 is known of the nature of bound thiamine. It is apparent however that 

 various animal species may utilize some, but not all, of these bound 



