330 THE BIOCHEMISTRY OF B VITAMINS 



may provide one means whereby the animal kingdom cannot exceed the 

 balance that exists, just as the plant kingdom is held in check by the 

 carbon dioxide content of the air, the supply of which is influenced by 

 the animal population. It may be surmised that the dichotomy may have 

 developed by a stepwise genetic process, leaving in the primitive forms 

 of both kingdoms {T hallo phytes and protozoa) members having the 

 ability to synthesize intermediate numbers of B vitamins; and on this 

 basis the fact that the protozoa and fungi vary in their abilities would 

 be expected. Arriving thus at a common primitive progenitor with fully 

 developed synthetic abilities does not seem as improbable as arriving at 

 one with none, according to present concepts 168 of evolution. 



Despite the un verifiable nature of these suggestions, one is tempted to 

 wonder why nicotinic acid stands out as an exception to the fact that 

 animals cannot synthesize B vitamins. (There is some evidence that 

 Tetrohymena gelii and Drosphila melanogaster may be exceptions to 

 this). If one were forced into the ludicrous discussion as to which one of 

 the B vitamins is the most vital, he would probably select niacin. Ulti- 

 mately, life is characterized by activities; these require energy, and 

 energy is generally derived in animals from high-energy phosphate bonds, 

 which are most frequently created by a reduction involving Cozymase 

 and thus nicotinic acid. An extremely primitive system might conceivably 

 subsist with this B vitamin, whereas the other B vitamins could not 

 serve this prime purpose of energy production. (In plants, the utilization 

 of radiant energy presumably involves a reduction of fixed carbon dioxide, 

 and here again it may well be that nicotinic acid is the first vitamin 

 involved in the linear process.) It may be considered providential, at 

 least, that more flexibility is permitted animals in niacin synthesis, allow- 

 ing the conversion from tryptophan. It may also be because of the par- 

 ticularly crucial place assigned to niacin in metabolism, that animals, 

 which are still limited by the tryptophan nutrition and the efficiency of 

 the process, tend to store nicotinic acid in as nearly finished a product 

 as possible, nicotinamide, 169 whereas the less limited plants store it 

 primarily as nicotinic acid. Indeed, our existing scanty knowledge sug- 

 gests that the routes of synthesis of niacin may differ in the animals and 

 higher plants, since asparagin, glutamic acid, proline and ornithine 

 seem to be intimately involved in plant synthesis, 170 but not in animals, 

 while tryptophan generally seems to be the major precursor in animals, 

 but not in plants. This point in the discussion at least should receive 

 elucidation in the near future. 



