METABOLISM OF THE B VITAMINS 361 



vitamin. 161 Rabbits similarly excrete nicotinuric acid, but humans ap- 

 parently do not. 102 In vitro studies with hog intestinal hippurase have 

 shown that it cleaves nicotinuric and hippuric acids at roughly equal 

 rates, and it has been suggested therefore that nicotinuric acid is prob- 

 ably formed by hippurase in vivo. 16 * Although this point lacks further 

 verification, it seems likely that the liver and kidneys may well perform 

 this function along with their more extensive hippuric acid-synthesizing 

 functions. 



Dinicotinylornithine has been isolated from dried chicken droppings, 130 

 but nothing is known as yet concerning its metabolism or significance. 

 The structure is of particular interest, however, because of the relationship 

 of ornithine to nicotinic acid synthesis in the higher plants (p. 84) . The 

 recent isolation of quinolinic acid from rat urine is mentioned on page 

 355, but little more is yet known concerning the significance of this 

 substance in the urine. 107a - c 



A considerable portion of ingested nicotinic acid remains still to be 

 accounted for, and several probable, but as yet undetected, products have 

 been suggested in the preceding discussion. In the studies previously 

 mentioned employing nicotinic acid in which the acid group was labelled 

 with C 14 , it was shown that 15 per cent of the C 14 of the acid group 

 disappears from mice as exhaled carbon dioxide, so that considerable 

 decarboxylation, possibly preceded by ring rupture, occurs. These studies 

 further showed that the gross metabolism of niacin and its amide is 

 identical in the mouse. 144 It thus appears likely that in addition to the 

 3-carboxylic acid derivatives of pyridine now known, other pyridine 

 derivatives may also occur and eventually be detected. In view of this 

 likelihood, a reinvestigation of the numerous pyridine derivatives already 

 discovered in nature with regard to their urinary occurrence would seem 

 to be in order. 124 - 125 - 164 



Metabolic Products of the Other B Vitamins. As previously stated, 

 little is actually known of the metabolic products of the other B vitamins, 

 although one might hazard extensive guesses as to the probable hydro- 

 lytic, oxidative, and conjugative changes that they may logically undergo 

 by analogy with the metabolism of other structurally similar compounds. 

 Rather than venture so far, therefore, it seems better to outline what 

 little is known in this regard, leaving the speculation for a time when 

 it might be better strengthened by the discovery of new and suggestive 

 data in this largely unstudied realm of knowledge. 



On normal levels of dietary intake, only a small portion of the B vita- 

 mins consumed can be accounted for on the basis of their excretion in 

 urine. This may be due to their conversion to as yet unidentified but 

 closely related decomposition products, 165 - 166 or to their conversion to 



