Steady-state determinations were made with a reaction mixture of the following 

 final composition: Sucrose, 107 mM ; MgCL, 15.5 mM ; KCl, 25 mM ; Na.,HP04- 

 KHoPOj, pH 7-4, 12-5 mM (or tris-HCl, pH 7-4, i8-8 mM); ADP, 18 mM ; 

 sodium ^-hydroxybutyrate, 4-5 mM ; sodium succinate 3-0 mM; mitochondrial 

 protein, approx. 6 mg./ml. Total volume 1 4 ml. Mixture aerated for 30 sec. 

 Temp. 17-20. 



Hatefi [22] has also described results of experiments on beef-heart mito- 

 chondria which support such a role for ubiquinone. He found also that 

 when phosphate in the medium was replaced by tris the ubiquinone went 

 into the completely oxidized state. This was interpreted as being due to the 

 release of the inhibitory etfect of phosphate on the oxidation of ubiquinol. 

 In the present work this phosphate effect could not be demonstrated in 

 rat-liver mitochondria ; the steady-state levels of ubiquinone were almost 

 the same in the absence of phosphate, in the presence of phosphate, and 

 in the presence of ADP without added phosphate (Table VI). 



Recently a number of workers have put forward hypotheses implicating 

 phosphorylated derivatives of quinones in oxidative phosphorylation [23, 

 24, 25]. Ubiquinol monophosphate, a possible intermediate in these 

 postulated reaction mechanisms has been synthesized by Dr. K. J. M. 

 Andrews of Roche Products Ltd., Welwyn. In a preliminary experiment 

 this substance was added to rat-liver mitochondria in the presence of ADP 

 but no stimulating effect on the rate of oxygen uptake was observed. Also 



