72 E. C. Slater and W. C. Hulsmann 



possible function, in vivo, of such naturally occurring un- 

 coupling agents. There are three possibihties which might be 

 considered : 



(1) The first possibility is applicable only to the uncoupling 

 agent isolated from aged mitochondria. It is possible that 

 when the lipid is bound to cytochrome as in the intact mito- 

 chondria, it is in some way involved in oxidative phosphoryla- 

 tion (possibly as I), but that when it is liberated from aged 

 mitochondria and added back to freshly prepared mito- 

 chondria, it acts as an uncoupling agent. 



(2) The second possibility, which is applicable to either 

 uncoupling agent, is based on Lardy 's (1952) suggestion that 

 an uncoupling agent might offer a more sophisticated type of 

 control mechanism than that provided by variation of the 

 ADP concentration (cf. Lardy, 1955; Pressman and Lardy, 

 1956). Under certain circumstances the rate of respiration 

 might be controlled by the rate at which one of the X - — ' I's 

 reacts with Pj and ADP [reactions (3a) and (Sb)]. If an 

 uncoupling agent specific for this X '^' I were present, this 

 rate-limiting step would be accelerated. This would mean that 

 the rate of hydrogen transfer, and therefore the rate of 

 synthesis of ATP would be increased in the other still coupled 

 steps of the respiratory chain. This increased rate of synthesis 

 of ATP in these two steps might more than compensate for 

 the loss of ATP in the uncoupled step. The feasibility of this 

 sort of control mechanism was demonstrated by Slater and 

 Lewis (1954) under rather special (and probably freak) 

 circumstances, using sarcosomes isolated from the thoracic 

 muscle of the blowfly. 



In the experiment illustrated in Fig. 4, the ATP synthe- 

 sized was immediately utilized for the synthesis of hexose 

 monophosphate (HMP) from glucose. Respiration was not, 

 therefore, hmited by the supply of ADP. It may be seen that 

 increasing concentrations of DNP caused an increase in the 

 rate of oxygen uptake, and particularly of the a-ketoglutarate 

 utilized. With low concentrations of DNP, the amount of 

 HMP formed was decreased, because DNP inhibited the 



