450 NIACIN 



4. In Pentose Biosynthesis 500 



5. In Lipid Metabolism 501 



6. In Nitrogen Metabolism 502 



7. In Photosynthesis 503 



J. Biological Synthesis of DPN and TPN 504 



1. Synthesis of Nicotinic Acid 505 



2. Amidation of Nicotinic Acid 505 



3. Synthesis of Ribose 505 



4. Synthesis of Nicotinamide Mononucleotide 505 



5. Biosynthesis of DPN 506 



6. Biosynthesis of TPN 506 



K. Destruction of DPN and TPN 507 



L. Inhibitors of DPN and TPN 508 



M. Assay Procedures of DPN and TPN 509 



1. General Principles 509 



2. DPN 509 



3. TPN 511 



N. Tissue Distribution and Fate 513 



1. Excretion 513 



2. Tissue Distribution and Turnover Rate 514 



3. Urinary Metabolites 514 



O. N'-Methylation of Nicotinamide 516 



V. Specificity of Action 517 



A. Modifications of the Nicotinic Acid Molecule 517 



B. Tryptophan and Derivatives 521 



VI. Biogenesis 521 



A. Demonstration of Nicotinic Acid Biosynthesis 521 



B. Mechanism of Biosynthesis 522 



1. The Ornithine Scheme 522 



2. Biosynthesis from Tryptophan 523 



C. Site of Biosynthesis 529 



1. Synthesis in Tissues 529 



2. Synthesis by Intestinal Microorganisms 530 



3. Utilization of L- and D-Tryptophan as Precursors for Nicotinic 

 Acid 531 



4. Tryptophan-Nicotinic Acid Relationships in Other Organisms . 532 

 VII. Estimation 534 



A. Biological Assay 534 



B. Chemical Methods 534 



1. Nicotinic Acid 534 



2. Differential Assay of Nicotinic Acid, Nicotinamide, and Related 

 Compounds 535 



3. Ni-Methylnicotinamide 536 



4. 6-Pyridone of N'-Methylnicotinamide 537 



5. Trigonelline (N'-Methyl Nicotinic Acid Betaine) 537 



6. DPN and TPN (Coenzymes I and II) 537 



7. Other Methods 538 



C. Microbiological Methods 538 



VIII. Standardization of Activity 540 



IX. Occurrence 541 



A. Occurrence in Food 542 



