342 THE BIOCHEMISTRY Ut' B VITAMINS 



due to the favorable influence of the exogenous vitamin on endogenous 

 bacterial synthesis in general, rather than to actual interconversion of an 

 exogenous molecule by a bacterium to a more active derivative of the 

 same molecule. 



Finally, the chemical processes encountered in the digestive tract un- 

 doubtedly result in the destruction of some percentage of ingested B 

 vitamins; this factor is a major one in certain of the cases where paren- 

 teral vitamin administration is markedly more effective than feeding 

 per os. Biotin, for instance, is said to be five times more active parenter- 

 al^, 12 and it might be expected that the — CH— NH— CO— NH— CH— 

 structure of its ring would receive some destruction by enzymes active 

 upon peptide linkages. For the same reason, folic and pantothenic acids 

 may be hydrolyzed to some extent in the intestine, and there may be 

 some cleavage of carbon — nitrogen bonds, such as those on the ribitol in 

 riboflavin and on the thiazole moiety in thiamine. Pyridoxal and pyridox- 

 amine are also less active and may be partially decomposed when fed 

 orally, as might be anticipated from their general chemical reactivity 

 in vitro. Vitamin B 12 is readily destroyed by the digestive processes, and 

 it is essential that this vitamin be protected by conversion into a bound 

 form. Ternberg and Eakin 12a have recently shown that "intrinsic factor" 

 (p. 415), a protein material present in the gastric juices, has this ability 

 of combining with vitamin Bi 2 and protecting it from digestive destruc- 

 tion. In pernicious anemia, intrinsic factor is absent from the gastric 

 juice, and a vitamin B i2 deficiency results due to the digestive destruction 

 of the unprotected vitamin. 



Since our nutrition is oral, and the B vitamin nutritional requirement 

 is a summation of all these effects, digestive destruction is not generally 

 a matter for extreme concern. In experimentation or medication where 

 parenteral administration is used, it is important, however, to take 

 cognizance of the fact that a much larger amount of vitamin may reach 

 the animal via this route than by supplying a similar amount orally, 

 from the standpoint both of the physiological effects that may result, and 

 of exceeding the limits below which the vitamin is not toxic. As will be 

 shown later in some detail, parenteral administration is most commonly 

 practiced with the two vitamins that appear to be most toxic, thiamine 

 and nicotinic acid. In other cases, as in these, extrapolation of the oral 

 therapeutic dose to the parenteral one is dangerous, largely because of our 

 limited knowledge of the degree of destruction of the oral dose in the 

 intestine. 



Finally, it should be pointed out that despite the fact that the B vita- 

 mins are markedly soluble in water and generally just as insoluble in 

 organic solvents, they may in some cases nevertheless have to undergo 



