6 Sir Hans Krebs 



Table II 



Effects of 2 : 4-dinitrophenol on oxygen 



CONSUMPTION of SHEEP HEART MUSCLE SUSPENSIONS. 

 10 PER CENT TISSUE ; 20° ; 60 MIN. 



O2 {[imole) used by 

 4 ml. suspension 



that the enzyme systems catalysing the interaction between 

 substrate and oxygen are not used to full capacity in the 

 absence of dinitrophenol, in other words that the activity 

 of the pyruvate-oxidizing enzyme system is not rate-limiting. 

 It is the generally accepted interpretation of the effects of 

 dinitrophenol that this substance uncouples phosphorylation 

 (as Loomis and Lipmann discovered in 1948) and that under 

 physiological conditions the oxidation of reduced pyridine 

 nucleotide is obligatorily coupled with the synthesis of 

 adenosine triphosphate (ATP) from inorganic phosphate and 

 adenosine diphosphate (ADP). This concept offers, in fact, a 

 satisfactory explanation for many observations. Thus, what 

 limits the rate of oxygen consumption and pyruvate removal 

 in the absence of dinitrophenol is not the amount of enzyme, 

 but the level of either ADP or inorganic phosphate. We 

 can exclude the latter in the present experiments because it 

 was added in the medium in relatively high concentrations, 

 and increasing the concentration of phosphate did not 

 stimulate respiration. Under the particular conditions of 

 these experiments the limiting factor must therefore be the 

 amount of ADP available for the coupled oxidation of the 

 electron carriers. The conclusion that dinitrophenol leads to 

 a raised ADP level has, of course, been reached before on the 



