498 NIACIN 



gen. This process is thought to constitute one of the important regulatory 

 mechanisms in carbohydrate metabolism. 



The source of the inorganic phosphate which accumulates has long been 

 uncertain. It was widely held that most of it resulted from the hydrolytic 

 cleavage of ATP. In certain systems, however, it could be clearly shown 

 that ATP was not the source of the inorganic phosphate.^" • "^^ "^ Korn- 

 }3gj.gii4, 115 recently established at least one source of this inorganic phos- 

 phate when he discovered an enzyme in yeast and in liver which catalyzed 

 the reversible reaction; 



Nicotinamide mononucleotide + ATP ;^ DPN + inorganic pyrophosphate 



This reaction plus the ubiquitous inorganic pyrophosphatase could yield 

 orthophosphate. Schrecker and Romberg"^ have demonstrated a similar 

 phenomenon with flavin mononucleotide. 



It should also be noted that this mechanism provides for a biosynthesis 

 of DPN from nicotinamide mononucleotide (p. 506). The reverse of this 

 reaction, i.e., the phosphorolysis of a dinucleotide by inorganic pyrophos- 

 phate, resembles the action of inorganic phosphate on the reversible split- 

 ting of polysaccharides referred to above (p. 497). 



2. DPN AND TPN IN Glycolysis 



Several pathways are known by which glucose can be utilized. "'' The 

 classical series of reactions which seem to prevail in most organisms in- 

 volves some eleven steps, with pyruvic acid as the end product."^ Glucose 

 is phosphorylated and converted through a series of steps to fructose- 1 ,6- 

 diphosphate. The latter compound is then split into two trioses, dihydroxy- 

 acetone phosphate and 3-phosphoglyceraldehyde. The latter compound 

 forms an addition product with phosphate, 1 ,3-diphosphoglyceraldehyde, 

 which is dehydrogenated by DPN and converted to 3-phosphoglyceric acid 

 as depicted on p. 496. The latter compound is converted in three steps to 

 pyruvic acid. The pyridine nucleotides are required for only one of these 

 eleven steps as indicated above. 



3. In Pyruvate Metabolism 



Pyruvic acid can be utilized in a number of ways.^*' When the supply 

 of oxygen is limited, pyruvic acid can be converted to lactic acid. 



^^^ C. F. Cori, A Symposium on Respiratory Enzymes. Madison, 1942. 



"3 R. J. Cross, J. V. Taggart, G. A. Covo, and D. E. Green, /. Biol. Chem. 177, 655 



(1949). 

 1" A. Kornberg, /. Biol. Chem. 182, 779 (1950). 



116 A. Kornberg and W. E. Pricer, Jr., J. Biol. Chem. 191, 535 (1951). 

 116 A. W. Schrecker and A. Kornberg, /. Biol. Chem. 182, 795 (1950). 

 1" E. S. G. Barron, Advances in Enzymol. 3, 149 (1943). 

 118 O. Meyerhof, Biol. Sijmposia 5, 141 (1941). 

 lis E. Stotz, Advances in Enzymol. 5, 129 (1945). 



