Control of Oxygen Utilization 93 



electrode and the reference electrode in conjunction with a 

 spectrophotometric apparatus is given in Fig. 1. The glass- 

 enclosed platinum tip is inserted well beneath the surface of 

 the mitochondrial suspension and is vibrated up and down 

 at an amplitude of about 1 mm. by the reed of a "Brown 

 Converter" at 60 cycles per sec. Rapid vibration of the 

 platinum tip is thereby obtained without undue disturbance 

 of the solution. The electrode is polarized at —0-6 v and 

 reduces oxygen to either peroxide or water, depending upon 

 the experimental conditions at the tip of the electrode. Thus, 

 a current proportional to the oxygen concentration is obtained. 

 This current passes through a salt bridge and calomel reference 

 electrode and can be measured by any convenient electronic 

 system. As we have made the circuit for the vibrating 

 electrode, the current proportional to oxygen concentration 

 is obtained as an alternating voltage which can be more 

 readily amplified and recorded than a steady current. 



Examples of respiratory control in mitochondria 



In Fig. 2 ( A-C) we see that some mitochondrial preparations 

 which exhibit a low respiratory rate in the presence of sub- 

 strate alone show a stimulation of respiration upon addition 

 of ADP but a negligible slackening of respiration upon con- 

 sumption of an amount of oxygen equivalent to the amount 

 of added ADP. In Fig. 2A, the vibrating platinum electrode 

 is inserted into a cuvette containing a suspension of heart 

 sarcosomes in the usual saline medium for the assay of 

 oxidative phosphorylation in liver mitochondria. In the 

 presence of 2 -5 [jlm a-ketoglutarate (a-Kg), the respiration rate 

 is 0*3 [JLM Oa/sec. Shortly after the addition of 290 [lm ADP, 

 the respiration increases approximately threefold. However, 

 instead of slackening after approximately 40 [xm oxygen has 

 been consumed, as would be expected from the P/0 value for 

 this type of preparation, the respiration continues practically 

 unabated until the remaining oxygen (240 [jlm) is exhausted. 

 In Fig. 2B we represent the response of another preparation of 



