Control of Rate of Intracellular Respiration 77 



mitochondria. Chance found that the addition of succinate 

 to aerobic mitochondria caused an increase of the extinction 

 at 340 m[jL, measured with respect to the extinction at 374 my., 

 and concluded that the spectral change corresponded to the 

 reduction of DPN. If, however, DPN '^ I has the same 



Table IV 



Effect of addition of substrates to liver mitochondria 



ON LEVEL of FORMS OF DIPHOSPHOPYRIDINE NUCLEOTIDE 



Substrate was allowed to react with liver mitochondria suspended in • 18 m 

 sucrose for 90 seconds at 0°. Final volume, 0-5 ml. No change in DPN or 

 DPNH contents occurred when suspension was incubated in absence of sub- 

 strate. 



Diphosphopyridine nucleotide 

 {[imoleslg. protein) 



spectral properties as other addition compounds of DPN, it 

 would also be expected to absorb in the same region as DPNH. 

 Chance's spectrophotometric method might not, therefore, 

 distinguish between DPNH and DPN -^ I. 



Chance's (1956) report that succinate was able to reduce 

 DPN was rather unexpected, because DPN is not a member 

 of the succinate chain. Chance (1958) suggested that "this 

 reduction was brought about by a reversal of electron trans- 

 port through cytochrome b and flavoprotein that leads to a 

 reduction of pyridine nucleotide. This reaction is mediated 

 by endogenous high-energy intermediates present in the 



