510 



NIACIN 



absorption at 340 m^ when small amounts of pyruvate and lactic dehydro- 

 genase were added. 



DPN may also be determined as described l)y Cori et al}^^ utilizing a 

 reaction originally proposed by Warburg and Christian^'*^ in which D-glyc- 

 eraldehyde-3-phosphate is oxidized to D-glyceric acid phosphate by the 

 appropriate crystalline dehydrogenase plus DPN as a hydrogen acceptor. 

 Arsenate is required to make this reaction go to completion (arsenolysis) . 

 The DPNH2 formed is measured spectrophotometrically at 340 m^. This 

 system is more complicated than the one described above since fructose- 



.20 



.80 



.40 .60 



ml. DPN added 

 Fig. 8. Spectrophotometric assay for DPN; response to known amounts of DPN. 

 (From Kornberg."^) 



1 , 6-diphosphate is used to produce D-glyceraldehyde-3-phosphate accord- 

 ing to the following reaction: 



Fructose-1, 6-diphosphate + aldolase 



D-Glyceraldehyde-3-phosphate — 



isomerase 



:^ Dihydroxyacetone phosphate 



+ dehj'drogenase 

 + DPN 

 D-Glyceric acid 3-phosphate + DPNH2 



This system, however, has the advantage not only of providing for the 

 quantitation of DPN, but of being a sensitive method for the determination 



180 G. T. Cori, M. W. Slein, and C. F. Cori, /. Biol. Chem. 173, 605 (1948). 



