490 NIACIN 



contributes slightly to absorption intensity at 260 m/x, and the slightly 

 reduced absorption intensity of reduced DPN and TPN at 260 mn is 

 probably due to a disturbance in the conjugated double bonds of the 

 pyridine ring. The absorption at 340 m/x of reduced DPN and TPN is a 

 function of the addition of hydrogen to the pyridine ring. Since both DPN 

 and TPN react to form the same type of reduced compounds, an identical 

 spectrum at this wavelength would be expected. 



There has been little uniformity in the reported values for the extinction 

 coefficients of DPNH2 and TPNH2 at 340 mju. The published values, re- 

 cently reviewed by Drabkin,62 have varied from 4.78 X 10« to 6.28 X 10« 

 sq. cm. X mole~^ More recent studies using DPN- or TPN-dependent 

 reactions which go to completion with pure substrates have shown identical 

 extinction coefficients for DPNH2 and TPNH2 at 340 m/x of 6.22 X 10« 

 sq. cm. X mole~^^^ 



TPN is destroyed by ultraviolet light, as is DPN. Electrophoresis de- 

 terminations show two different dissociation constants, pKi = 1.8 and 

 pK2 = 6.1."* TPN is optically active, Hsggm^ = —24.6 and [ol^iGmn = 

 -29.4.«'' 



F. MECHANISM OF ACTION 



1. The Coenzymes 



DPN and TPN function in oxidation-reduction systems by virtue of 

 their ability to accept hydrogen atoms (dehydrogenation) from certain 

 substrates and transfer these hydrogen atoms to other hydrogen acceptors 

 such as the flavin enzymes. In other words, DPN and TPN function by 

 reversibly alternating between the oxidized (I) and the reduced state (II), 

 as depicted below. 



H (II) 

 C 



/ \ 

 H HC C— CONH2 



\l II 



2H+ + 2e C CH 



H N O- H+ 



I I I I 



Ribose — O — P — O — R Ribose — O — P — O — R 



II II 



O O 



«2 D. L. Drabkin, /. Biol. Chem. 157, 563 (1945). 

 63 B. L. Horecker and A. Romberg, /. Biol. Chem. 175, 385 (1948). 

 " H. R. Rosenberg, Chemistry and Physiologj^ of the Vitamins, p. 236. Interscience 

 Publishers, New York, 1942. 



