112 Britton Chance 



effects are obtained at 1-6 and 2-5 [iM DIB for reduced 

 pyridine nucleotide and cytochrome b, respectively, with 

 glutamate as substrate. These results indicate that the 

 uncoupling response of the pyridine nucleotide is very 

 similar to that of cytochrome b. The fact that pyridine 

 nucleotide becomes further oxidized in the presence of excess 

 uncoupling agents indicates that the inhibition of respiratory 

 activity with excess concentrations of the uncoupling agents 

 is due to interference with the reduction of pyridine nucleo- 

 tide. This is a point of considerable interest, since the isolated 

 dehydrogenases are not sensitive to uncoupling agents. This 

 result calls our attention to the possibility that the reduction 

 of pyridine nucleotide in mitochondria may be driven by 

 high-energy intermediates, not only with succinate as sub- 

 strate, but with other substrates as well. 



Respiratory control for the substrate level in mito- 

 chondria 



The affinity of mitochondria for most substrates is so low 

 that it is difficult to demonstrate control of electron transfer 



Additions of oiKg 



Heart Mw: ^ 

 +— 

 ADP 



430-410 mjj ^ 

 log Io/I= 0.0051: 



Cytochrome b 

 reduction | 



Fig. 16. Cyclic response of cyto- 

 chrome b of mitochondrial suspen- 

 sion to increases and decreases of 

 substrate concentration. Rat heart 

 mitochondria, pretreated with ADP ; 

 aK as substrate (Expt. no. 618). 



in response to small additions of substrate; amounts of sub- 

 strate sufficient to elicit maximum activity require a very 



