Control of Oxygen Utilization 109 



dicoumarol and ADP has been mentioned previously (Chance, 

 1956), and is repeated here (Fig. 14). Here we compare the 

 absorption bands of the cytochromes which disappear upon 

 addition of ADP with those which disappear upon addition of 

 dicoumarol. If, for example, there were a selective effect of 

 dicoumarol, then one would expect differences in the re- 

 sponses to these agents. Treatment of the mitochondria with 



ADP addition 

 to state 4 



Dicoumarol 

 addition to 

 state 4 



- +0.00H 550 570 



Q. 



o 



Fig. 14. Spectroscopic changes caused by addition of ADP, 

 upper trace, compared with those caused by dicoumarol, 

 lower trace. Both traces show an oxidation of cytochromes c 

 and b. The data are plotted so that a decrease of absorption 

 corresponds to an upward deflexion on the traces. Rat liver 

 mitochondria, 400 [j.m ADP, 14 [jlm dicournarol, ^-hydroxy- 

 butyrate as substrate. (Expt no 468d-6J (From Chance, 

 1956. Reproduced by permission of the Editors, Proc. Ill 

 int. Congr. Biochem.) 



ADP causes absorption bands due to cytochromes c + c^ 

 (551 m[x) and b (562 mpi) to disappear, as indicated by the 

 double-humped peak in the difference spectrum. Addition of 

 dicoumarol affects the absorption bands to the same relative 

 extent. 



An interesting feature of Fig. 14 is that the absorbancy 

 changes caused by dicoumarol are slightly larger than those 

 caused by ADP, which appears to be inconsistent with the 

 nearly identical respiratory stimulation caused by these two 

 substances. In order to determine whether this is due to 



