338 



CELLS, TISSUES, AND ORGANISMS 



(.•onceiiicd witli maintaining tlit- normal status of the mucous mem- 

 l)iane, the skin, and othc^r cpithehal structures. The biochemistry of 

 these processes is not known. \ itamin A is essential for the normal 

 growth of young animals. 



Nicotinic Acid. Nicotinic acid is readily converted by animals to 

 di- and tri-phospliopyridine nucleotides (DPNH and TPNH). These 

 two coenzymes have a seemingly inexhaustible series of roles in bio- 

 logical oxidations and reductions. Recently Arnon (1959) has empha- 

 sized the function of DPNH in photosynthesis as the primary transfer 

 agent for oxidizable hydrogen. The discovery that nicotinic acid was 

 the curative agent for blacktongue and pellagra was soon followed by 

 the remarkable finding that tryptophan could function as the biological 

 precursor of nicotinic acid for various animal species, including man. 

 This biochemical peculiarity could not have been anticipated on struc- 

 tiu'al groimds. It places nicotinic acid in a separate class from the other 

 \itamins, for, nutritionally speaking, there is no such thing as nicotinic- 

 acid deficiency; there is only tryptophan deficiency, and although tryp- 

 tophan can supply the requirement for nicotinic acid, the reverse is not 

 true. Moreover, the conversion of tryptophan to nicotinic acid is medi- 

 ated by three other B-complex vitamins, thiamine, riboflavin, and vita- 

 min Be, as Figures 2 and 3 show ( Dalgliesh, 1956 ) . 



Thus it seems that nicotinic-acid deficiency in animals may be 

 caused by a combined lack of tryptophan and nicotinic acid in the diet. 

 Even if a sufficient supply of tryptophan is present, the deficiency 

 could be caused by a lack of the vitamins that have a coenzymatic 

 function in the conversion of tryptophan to nicotinic acid. 



aC0-CH2-CH-C00H 

 NH 

 NH-CHO 

 Serine 



Co 



-CH; 



CH-COOH 

 NHg 



Tryptophan 



Formyl Kynurenine 



NH 

 Indole 



T 



Anthranilic acid 



Kynurenine > .3 OH Kynurenine 



Bo 



Shikimic acid 



B. 



COOH 



^a. 



OH 

 3- OH Anthranilic acid 



Figure 2. Formation of 3-hydroxy anthranilic acid from tryptophan and regeneration of 

 tryptophan from indole by tryptophan synthetase. 



