236 MARTIN KLINGENBERG 



the increase of reduction of cytochrome a. It appears that in these mito- 

 chondria, which are partly uncoupled, the respiratory control is increased 

 by ATP. The "crossover point" between cytochrome h and cytochrome c 

 shows further that the respiration is inhibited by the action of ATP at this 

 point. We conclude that by the "crossover point" of ATP action the 

 reversal of oxidative phosphorylation also at this step of the respiratory 

 chain is demonstrated. It should be noted that in studies of ADP action on 

 the respiratory chain in liver mitochondria, by Chance and Williams, a 

 "crossover point" between cytochrome h and c in the opposite sense had 



400 r 



//, atom O 



liter 



300 - 



200 L 



605- 

 630 m/i 



i4E = 

 0-0025 



Cyt. a 

 reduction 



Fig. 8. The inhibition of respiration and oxidation of cytochrome a in the 

 presence of glycerol- 1 -phosphate on addition of ATP (cf. legend of Fig. 2 and 7). 



been observed [17], which led to assume a phosphorylation step at this 

 point. 



An inhibition of respiration with glycerolphosphate or succinate can 

 also be obtained by albumin addition (Fig. 9). This inhibition is not 

 abolished by phosphate but only by further addition of ADP. Thus 

 albumin can also increase the respiratory control, in agreement with its 

 assigned role of binding the endogenous uncoupling agents of mitochondria. 



The kinetics of the redox changes initiated by ATP or albumin can also 

 be explained on the basis of the proposed mechanism. The oxidation of 

 cytochrome c or cytochrome a on addition of ATP is very rapid, followed 



