METABOLISM OF THE B VITAMINS 343 



transformations, like the fat-soluble vitamins, to render them absorbable. 

 This is most certainly so in the case of some of the absorbed forms of 

 choline, and may be true of other vitamins, such as inositol. For some 

 of the lower forms of life, for which the nutrition must reach the absorp- 

 tive membrane in particulate form, and liquid media will not suffice, a 

 similar consideration may well be involved. In any case, the water 

 solubility of a substance cannot be considered a -priori to be the final 

 end to be achieved in the digestive process. 



The Absorption of B Vitamins 



The state of knowledge of the processes involved in the absorption of 

 the individual B vitamins is to a large extent a function of the time that 

 the B vitamin in question has been well recognized. The more explicit 

 information which is available concerning thiamine and riboflavin absorp- 

 tion indicates clearly that the absorption of the B vitamins cannot be 

 regarded as a simple process, even though it is frequently assumed to be 

 for the more recently discovered vitamins. Passage of a metabolite across 

 a living membrane seldom is a matter of passive transfer or simple 

 diffusion. This, is certainly a good generalization for the B vitamins, even 

 in view of the lack of much experimental data to verify the assertion. It 

 should also be reemphasized at this point that vitamin-protein complexes 

 may be absorbed in some cases, just as undigested proteins in general 

 may be absorbed to a limited degree. 



In the case of both thiamine and riboflavin, it is believed that phos- 

 phorylation occurs in the intestinal mucosa prior to absorption. 13 Through- 

 out the gamut of physiological processes, phosphorylation is a frequent 

 adjunct to the passage of metabolites of many kinds across membranes, 

 so that in the cases of thiamine and riboflavin the process is by no means 

 unique. Both thiamine and its phosphate are readily absorbed in the 

 small gut; 14 and although thiamine occurs in the blood plasma in the 

 free form 15 (part of which may be again secreted in the gastric juice), 

 duodenal phosphorylase readily phosphorylates thiamine in vitro, 1Q> 17 

 so that phosphorylation and dephosphorylation apparently are involved 

 in passage across the intestinal wall into the circulation, as in the case 

 of transfer of many other metabolites. Riboflavin, its 5'-phosphate, and 

 its adenine dinucleotide are all available to the higher animals, but it is 

 possible that the coenzymes are broken down prior to absorption. Free 

 riboflavin is phosphorylated prior to absorption, 13, 18 and the phosphoryla- 

 tion may be clone in vitro with mucosal extracts. Interference with the 

 process by iodoacetate or by adrenalectomy in rats causes a prompt lack 

 of free riboflavin absorption and ensuing cessation of growth, which may 



