METABOLISM OF THE B VITAMINS 345 



hope to understand this, as well as the innumerable clinical reports deal- 

 ing with the marked beneficial effects of large doses of some one vitamin 

 in an individual where the pathology bears no apparent relation to the 

 medication. 



Distribution of the B Vitamins 



Much specific information has been included in an earlier chapter on 

 the occurrence and distribution of the B vitamins that might be recon- 

 sidered at this point. It would seem more appropriate, however, to con- 

 sider in more general terms and from a somewhat dynamic standpoint 

 the various general relationships that exist in the distribution process, 

 leaving the interpretation of the voluminous data found in the literature 

 on B vitamin occurrence for those whose needs justify the labor that must 

 necessarily be involved in so arduous a task. The allocation by the cir- 

 culation of the various B vitamins to the tissues is a complex process, and 

 we can at best do little more than guess at the principles involved. 



State and Levels in the Circulation. Once the B vitamins are intro- 

 duced into the circulation, they are rapidly distributed between the 

 cellular elements and the plasma, and almost as rapidly between the 

 blood and the tissues, so that an equilibrium is usually maintained. It 

 is generally true that the plasma does not in itself modify the B vitamins, 

 although in some cases "binding" may occur. The blood is generally in- 

 capable of any vitamin liberation, being unable to break, for instance, 

 the avidin-biotin complex. 24 Parenteral administration of the complex, 

 however, results in its destruction by tissue oxidative processes. 



The thiamine in blood plasma is largely in the free state, but not 

 entirely so, whereas that in the cellular elements of the blood is mostly, 

 if not entirely in the form of cocarboxylase. 25 Apparently all nucleated 

 cells are capable of performing the phosphorylation, and adult red blood 

 cells are believed to have acquired their cocarboxylase content prior to 

 loss of the nucleus while in the bone marrow. 26 The total blood thiamine 

 in normal humans generally ranges between 8 and 9 fig per cent, although 

 some workers have reported values as high as 14.5 fig per cent. 27 Of this, 

 apparently 70-90 per cent is esterified. 28 " 30 Pig blood apparently contains 

 about 20 fig per cent; 31 oxblood, 5.7 fig per cent; 25 pigeon blood 20.2 

 fig per cent; 25 rat blood 7 ^g per cent; 32 - 33 and rabbit blood about 28.3 

 fig per cent of total thiamine. 34 Various reported figures relating to the 

 distribution of free and esterified thiamine between plasma and cells, 

 and in pregnancy, and placental and amniotic fluid, 36 and infant blood 37 

 may be found in the literature, but are not at present sufficiently sub- 

 stantiated to merit general acceptance. 



