Rate-limiting Factors in Cell Respiration 5 



A priori any of the separate stages could be expected to be 

 rate-limiting but under physiological conditions it appears to 

 be generally a reaction involving substrates, and not one of 

 the reactions involving other stages, which is rate -limiting. 

 This applies to animal tissues but not necessarily to micro- 

 organisms. Only under unphysiological conditions, e.g. in the 

 presence of cyanide or other specific inhibitors, does one of 

 the intercarrier reactions become rate-limiting. 



As already mentioned, the fact that the oxygen consumption 

 can be increased by certain special substrates which are known 

 to reduce either DPN or flavoprotein shows that the transport 

 of hydrogen from DPN or flavoprotein is not used to full 

 capacity in the absence of these special substrates. It follows 

 that the limiting step is the interaction between substrate and 

 DPN, i.e. the first stage of the electron transport system. 

 When an added substrate increases the rate of respiration it is 

 due to the fact that this substrate reacts more readily with 

 DPN (or flavoprotein, as the case may be) than the endogen- 

 ous substrate. This, then, is the reason why pyruvate or 

 a-ketoglutarate, or succinate, stimulates the rate of respira- 

 tion. 



The question next arises of what determines the rate of 

 reaction between substrate and pyridine nucleotide (or sub- 

 strate and flavoprotein). There are two aspects of this 

 question, which I propose to consider. The first concerns the 

 variations of the rate in the presence of one and the same 

 substrate but under varying conditions. Factors which are of 

 importance are obviously the concentrations of the immediate 

 reactants, those of the substrate and of the enzymes and 

 coenzymes attacking it. If, for example, pyruvate were the 

 limiting factor, constant rates of pyruvate removal might be 

 expected when an excess of pyruvate is present and this 

 constant rate would depend on the activity of the saturated 

 enzyme system. A simple experiment, however, shows that 

 other factors can be rate-limiting; the addition of 2 : 4-di- 

 nitrophenol (DNP) may cause a doubling of the rate of oxygen 

 consumption and of pyruvate removal (Table II). This proves 



