484 



NIACIN 



cient he observed, 6.3 X 10=* (340 him), indicates a product of exceptional 

 purity. ^^^ 



The more recent methods which have been developed to isolate DPN 

 and TPN have been described in Section II (see p. 458). 



D. OCCURRENCE OF DPN AND TPN 



Pyridine nucleotides have been found in every living cell examined thus 

 far. Table VII lists a number of substances and the concentrations of 

 DPN and TPN found. Many of these figures must be regarded as approxi- 

 mations only because of analytical limitations. Many cells contain sub- 

 stances which rapidly destroy these coenzymes once their cellular structure 

 is disrupted. Furthermore, it is known that TPN and DPN are intercon- 

 vertible (see p. 493). Nevertheless it is clear that these coenzymes have a 

 rather universal distribution, although their concentration varies widely in 

 different substances. DPN and TPN tend to occur together, although the 

 amount of TPN seems always to be less than DPN. In animal tissues, a 

 fairly constant proportion (35 to 45 %) of the DPN is in the reduced state.^" 



Yeast, red blood cells, liver, and skeletal and cardiac muscle are good 

 sources of DPN. Fresh yeast may contain as much as 0.5 g. per kilogram^^ 

 and rabbit cardiac muscle 0.4 g. per kilogram.^^ Yeast contains very little 

 TPN. Animal tissues, especially red blood cells, liver, and muscle are good 

 sources of TPN, containing as much as 40 to 80 7 per gram.^^ 



E. STRUCTURE AND PROPERTIES 



1. DiPHOSPHOPYRIDINE NUCLEOTIDE (DPN) 



a. Structure 



H 

 C 



\ 



N=C— NHs 



CH 



CH 



C— CONHo HC C — N 





CH 



\ 



CH 



N— C— N 



H— C- 



H— C— OH 



i 

 H— C— OH 



H— C 



o- 



O 



H— C- 



H— C— OH 

 H— C— OH 

 H— C 



O 



CHo— O— P— O— P— O CHo 



II I 



O OH 



