Control of Rate of Intracellular Respiration 63 



any of these intermediates can react with water, respiration 

 will proceed even in the absence of ADP, and without the 

 concomitant synthesis of ATP, i.e. the phosphorylation is 

 "uncoupled" from the respiration. 



Fig. 2 shows that isolated heart sarcosomes, in the presence 

 of added Mg2+, are appreciably "uncoupled", since they 

 respire even in the absence of ADP. This "uncoupling" is not 

 observed in short-time experiments in the absence of added 

 Mg2+ (Chance and Baltscheffsky, 1958; Packer, 1957). The 

 possible nature of this uncoupling will be discussed later. 

 First, it is necessary to discuss the ways in which it is possible 

 to uncouple freshly prepared liver mitochondria, which have 

 very little respiration in the absence of ADP even in the 

 presence of Mg^^. 



Uncoupling can be brought about in many ways, e.g. 



(i) by the addition of 2 : 4-dinitrophenol (DNP), or other 



uncoupling agents; 

 (ii) by ageing of the mitochondria; 



(iii) by the addition of a preparation which can be isolated 

 from aged mitochondria. 



2 : 4-Dinitrophenol 



The stimulation of respiration of rat heart sarcosomes 

 caused by the addition of DNP is shown in Fig. 3. The initial 

 rate of respiration is about the same as that in the presence 

 of ADP. For the following reasons, it is believed that DNP 

 promotes the hydrolysis of X '^ I, rather than of A '^' I or 

 X^P: 



(i) The DNP-stimulated ATPase reaction of mitochondria 

 is not affected by the rate of oxidation or reduction of 

 members of the respiratory chain (Myers and Slater, 

 19576). Thus, A cannot be involved in the ATPase 

 reaction, 

 (ii) The addition of DNP abolishes the requirement of 

 inorganic phosphate for respiration (Loomis and 

 Lipmann, 1949). Thus, the hydrolytic reaction must 



