METABOLISM OF THE B VITAMINS 353 



nucleosidase which splits only nicotinamide from DPN, whereas rab- 

 bit kidney contains a pyrophosphatase which forms adenylic acid and 

 nicotinamide mononucleotide from DPN; n and it may be that the 

 coenzyme synthesis similarly has more than one route. The synthesis 

 of the coenzyme does not directly involve pyridoxine or its derivatives. 72 

 The DPN of chick embryos increases with the niacin content, suggesting 

 that the ability to synthesize coenzymes is present in the animal from 

 the earliest stages of its development. 73 Apparently only leucocytes and 

 other nucleated cells are able to perform the synthesis. 41, 74 The tissues 

 of pantothenic acid-deficient rats and ducks exhibit a deficiency of coen- 

 zyme A in a manner analogous to many of the other B vitamins. Nitrogen, 

 arsenite, and glucose are reported to interfere with coenzyme A synthesis 

 in vitro. 75 With regard to biotin, there is some evidence to indicate that 

 adenylic acid is involved in the synthesis of its coenzyme, 228 which is re- 

 ported to be more active biologically than biotin itself. 76 Steps involved 

 in the formation of active forms of the other B vitamins are unknown 

 and must await further elucidation of the structure of the functional 

 forms. 



Choline Synthesis. Any discussion of choline synthesis must be con- 

 sidered in the light of the fact that in many ways choline is not a typical 

 B vitamin, and appears to occur and function in relatively large amounts 

 in the living organism. In an earlier chapter it has been shown that in 

 Neurospora and other organisms, choline is synthesized via the proc- 



CH 2 OH CH 2 OH CH 2 OH CH 2 OH CH 2 OH 



CH— NH 2 — > CH 2 — > CH 2 — > CH 2 — > CH 2 



I I I I I 



COOH NH 2 NH— CH 3 N— CH 3 N" 



CH 3 



CH 3 CH3 CH3 



and that the necessary methyl groups may be derived from a source such 

 as methionine or betaine. Apparently the same series of reactions occurs 

 in all the higher animals if sufficient serine and methionine are available, 79 

 and in practice it is necessary to limit the methionine content of the diet 

 in order to produce choline deficiencies. Isotopic studies in the rat have 

 done much to show that the pathway indicated above does in fact exist 

 in animals. so 



Nicotinic Acid Synthesis. It has previously been mentioned that ani- 

 mals have the ability to convert tryptophan to nicotinic acid (p. 83), 

 and that in Neurospora the pathway presumably involves kynurenine, 

 3-hydroxy-kynurenine, and 3-hydroxy-anthranilic acid (p. 280) , but that 



