90 Discussion 



two reagents. By contrast, when magnesium is added the respiration 

 becomes considerably faster than is found with ADP or DNP, and 

 respiratory control is entirely removed. Loss of respiratory control 

 indicates breakdown of an intermediate in the energy-transfer chain, 

 but the fact that a higher respiration rate is observed suggests that the 

 intermediate concerned cannot be identical with the one whose break- 

 down is catalysed by DNP, and that it is formed by a reaction which 

 comes earlier in the energy- transfer chain. Alternative explanations of 

 the magnesium effect involve its action at more than one point in the 

 reaction sequence and for that reason seem to me to be less likely. It 

 may be noted that this evidence is distinct from the kinetic evidence 

 adduced by Chance and Williams (1956, loc. ciL), which is concerned 

 with the relative rates of change of the oxidation-reduction state of the 

 respiratory carriers when respiration is increased by ADP, DNP and by 

 oxygen. 



