Control of Oxygen Utilization 



105 



tion, as indicated in Fig. 11. In carrying out these experi- 

 ments, the respiration rates were measured immediately 

 after adding calcium to avoid the diminution of respiratory 

 activity which characteristically occurs some seconds later. 



5mM EDTA 



20" 



— 1 

 30 



mM 



Fig. 11. Relationship between added magnesium and 

 calcium concentrations required for activation of respira- 

 tion. Experimental conditions similar to those of Fig. 

 10. (Expt. no. 473b) 



In the absence of added magnesium (a sucrose-phosphate 

 medium), a very small concentration of calcium gives 

 half-maximal stimulation; approximately 80 (jlm was ade- 

 quate in this experiment with succinate as substrate. 

 This calcium concentration is only slightly higher than 

 the ADP concentration required for half-maximal stimu- 

 lation of respiration. If the reaction is carried out in the usual 



