62 E. C. Slater and W. C. Hulsmann 



X--I + Pi ^X--P + I (3a) 



X -- P + ADP ^ X + ATP (Sb) 



(Pi is inorganic phosphate. X and I could be interchanged in 

 the last two reactions, i.e. the intermediate might be I ^^ P, 

 rather than X '-^ P). 



AHg and B are adjacent members of a phosphorylative 

 step in the respiratory chain, e.g. AHg could be reduced 

 diphosphopyridine nucleotide (DPNH) and B could be flavo- 

 protein (fp.) I is a postulated intermediate which enables the 

 energy made available in the oxido-reduction between AH2 

 and B to be retained in the energy-rich compound A '^ I. 

 This energy is then transferred to a second hypothetical com- 

 pound X in reaction (2), giving X '^-' I, and restoring A, the 

 member of the respiratory chain. A is then reduced to AHg in 

 the preceding step of the respiratory chain, and BHg is oxidized 

 to B in the following step. In reactions (3a) and (36), the 

 energy of X '^' I is used to make ATP, and free X and I are 

 liberated. It is believed that there are three different I's 

 (Ij, I2 and I3) corresponding to the three phosphorylative 

 steps in the chain (Hiilsmann and Slater, 1957). The question 

 of whether the same or different X's are involved in the dif- 

 ferent steps is left open. 



According to this scheme, the continuity of respiration 

 requires that X and I are reformed from X '---' I. This is 

 brought about by reactions (3a) and (3b) which require in- 

 organic phosphate and ADP, respectively. Thus respiration 

 proceeds, with the synthesis of ATP, until either the inorganic 

 phosphate or the ADP is exhausted. This provides a control 

 mechanism whereby the rate of synthesis of ATP is governed 

 by the needs of the cell for ATP. 



Stimulation of respiration by uncoupling agents 



The extent to which the respiratory rate is under the 

 control of the ADP concentration depends upon the stability, 

 in the absence of ADP, of the various energy-rich intermediates 

 in the above scheme, namely, A '^ I, X ^^ I and X '^ P. If 



