Rate-limiting Factors in Cell Respiration 7 



basis of many other experiments (Lardy and Wellman, 1952; 

 Chance, 1956; Aldridge, 1957). 



There are many conditions in vitro when dinitrophenol does 

 not stimulate respiration. Tyler (1949) described experiments 

 on rat brain slices showing that the oxidation of glucose, 

 lactate and pyruvate was stimulated by dinitrophenol whilst 

 the oxidation of succinate, citrate or fumarate was not. He 

 also found that the dinitrophenol effect was absent from 

 homogenates though it occurred in sUces. The dependence of 

 the dinitrophenol effect on the substrate is also illustrated by 

 data given in Table II. In heart muscle the results obtained 

 differ much from those with brain slices. There is a marked 

 dinitrophenol effect with pyruvate, oxoglutarate and succin- 

 ate, and a smaller one with citrate and acetate. There is no 

 effect in the suspension to which no substrate has been added 

 but which is rich in carbohydrate and lactate, or after addition 

 of lactate or fumarate. 



When dinitrophenol does not accelerate respiration, factors 

 other than ADP and phosphate must be rate-limiting. This 

 is not surprising because two substances (apart from phos- 

 phate) are necessary as substrates of oxidative phosphoryla- 

 tion : reduced DPN and ADP. Both these substances behave . 

 like catalysts in that they are present in small amounts 

 and require continuous regeneration to remain available. If 

 dinitrophenol acts by speeding up the regeneration of ADP 

 from ATP, it is obvious that this can stimulate respiration 

 only if the speed of this regeneration is not already faster than 

 the speed at which reduced pyridine nucleotide can be 

 regenerated. As soon as the regeneration of reduced DPN is 

 slow its level, rather than that of ADP, may become the limit- 

 ing factor. Hence, dinitrophenol can be expected to stimulate 

 only when readily oxidizable substrates are available. In the 

 case of other substrates, hydrogen transfer from substrate to 

 carrier, i.e. the activity of the dehydrogenases concerned, 

 must remain rate-limiting even in the presence of dinitro- 

 phenol. 



A second factor which determines the rate of reaction 



