362 THE BIOCHEMISTRY OF B VITAMINS 



very dissimilar products. In this connection it should be pointed out that 

 they may be completely metabolized to carbon dioxide, water, and am- 

 monia. Tracer studies have indicated that the nutritionally active isomer 

 of inositol is at least partially converted to glucose in the rat, 167 and the 

 probable existence of similar conversions of some other vitamins may 

 make it impossible ever to obtain completely balanced data on intake and 

 excretion. This seems even more likely since the urinary and fecal excre- 

 tion so frequently exceeds the intake (pp. 300 and 368) , and it is uncertain 

 what portion of the B vitamins present in both urine and feces originated 

 in the diet. 



A variety of types of metabolic conversions break down the B vitamins 

 to their known excretory products. Urine apparently contains no cocar- 

 boxylase, so that the thiamine present in urine must largely result from 

 the action of phosphatases on the pyrophosphate. 168 Thiamine is split 

 in the human body in apparently much the same way as by the thiaminase 

 of aquatic animals (p. 292), and the resulting pyrimidine moiety, called 

 pyramin (2-methyl-4-amino-5-hydroxymethylpyrimicline) is found in the 

 urine in considerable quantities. 169 When an individual is saturated with 

 thiamine, practically all excess thiamine given can be recovered either as 

 thiamine or pyramin, 170 so that it would appear that there are not nor- 

 mally other major end products of thiamine metabolism. However, al- 

 though the amount of thiamine excreted is highly characteristic for the 

 individual (p. 255) , pyramin excretion seems to be a relatively constant 

 process. 171 Since on the same diets one individual may consistently ex- 

 crete three times as much thiamine as another, while excreting an essen- 

 tially identical level of pyramin, it is apparent that still another pathway 

 of thiamine metabolism must exist. 



Riboflavin is principally excreted as such, but varying amounts up to 

 half of the total may be excreted as the phosphate. 172 Aquaflavin, or 

 uroflavin, a degradation product of riboflavin, is also found in most urine 

 samples. 173 In cow, goat, and sheep milk and urine there is a considerable 

 discrepancy between riboflavin determinations done fluorometrically and 

 microbiologically, and it appears that an as yet structurally unidentified 

 degradation product of riboflavin is present which exhibits marked fluores- 

 cence but no microbiological activity. 174 As this metabolic product is not 

 present in the urine of humans or rats, it is possible that the product is 

 formed in the rumen of the animals mentioned. Certain microorganisms 

 are well known for their ability to oxidize riboflavin to lumichrome, 175 

 and such a reaction may be responsible for the presence of this fluores- 

 cent pigment in the urine and milk of ruminants. 



Pantothenic acid is partially excreted as such, but a large part of that 

 ingested has an unknown fate. 176, 177 It is well established that there is 



