NUCLEOTIDES AND MITOCHONDRIAL FUNCTION 235 



oxidizing effect of ATP, which otherwise is very small. Azide at this 

 concentration does not yet inhibit noticeably the respiration (cf. also [i6]). 

 The reductive effect of ATP on cytochrome h and the oxidative effect of 

 ATP on cytochromes c and a are reversed by addition of phosphate, ADP 

 or dinitrophenol, as was found with the ATP-dependent reduction of 

 DPX and flavoprotein. 



Fig. 7. The simultaneous inhibition of respiration and oxidation of cyto- 

 chrome a in the presence of succinate on addition of ATP. Amperometric record- 

 ing of oxygen consumption and simultaneous spectrophotometric recording at the 

 a-band of cytochrome a. 



The interaction of ATP with the respiratory chain causes a "crossover 

 point" of the redox changes of the respiratory components between cyto- 

 chrome h and cytochrome c. The "crossover point" can be interpreted to 

 indicate the reaction step at which the electron transfer is inhibited by 

 ATP. Thus, it is to be expected that in the overall electron transport also, 

 i.e. in the oxygen uptake, an inhibitory effect of ATP can be observed. As 

 shown in the upper recordings of Figs. 7 and 8, after addition of ATP, the 

 respiration of skeletal-muscle mitochondria, both with glvcerolphosphate 

 and succinate, is inhibited to about 50^' o- After further addition of phos- 

 phate or dinitrophenol, respiration is again accelerated synchronously with 



