Tryptophan, Nicotinic Acid, and the Pyridine Nucleotides

The pyridine nucleotides are the functional form of nicotinic acid, but still comparatively little work has been done on their relation to tryptophan-nicotinic acid metabolism. Two possibilities must be considered the pyridine nucleotides may be formed from tryptophan without intermediacy of nicotinic acid and only give nicotinic acid on breakdown, or nicotinic acid first formed from tryptophan may be incorporated into pyridine nucleotides. The latter now seems the more likely possibility, though the former has not been excluded. 

Injected tryptophan causes a rise in erythrocyte DPN in the rat (562). The problem was taken up by Elvehjem and his school (c/. review, 224), who at first found tryptophan to be more active than nicotinamide in stimulating synthesis of rat-liver DPN and TPN (924, 925). Nicotinamide had, however, a sparing effect in young, but not in adult, rats (925). In pyridoxine deficiency conversion of tryptophan to pyridine nucleotides 

Bacterial synthesis of pyridine nucleotides may proceed somewhat differently. Hughes (421) has produced evidence for the following pathway of cozymase synthesis  

However, isotopically labeled nicotinic acid readily gives rise to labeled DPN and TPN attached to the outside of certain bacterial cells (95), suggesting that exchange reactions may also occur. 

The biosynthesis, function, and degradation of the pyridine nucleotides have been reviewed by Singer and Kearney (812a). 

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