Control of Oxygen Utilization 



117 



more delicately balanced levels of metabolic activity. It is, 

 in fact, possible not only to demonstrate control of respiratory 

 metabolism, but also to show that this is due to limitation of 

 intracellular ADP concentration. Yeast cells are appropriate 

 for this type of study. Fig. 19 represents, in its initial phases, 

 a repetition of the experiment of Fig. 18 in which aerated, 



Starved yeast, 



aerated 



Spectrophotometric Trace 

 ^ 430-405nn;j ^ 



Fig. 19. Identification of ADP as a rate-controlling intermediate in the 

 respiratory metabolism of baker's yeast cells. Combined spectrophoto- 

 metric and polarographic recordings of cytochrome b reduction and res- 

 piratory activity. The large numbers refer to the metabolic state of the 

 yeast (2 starved, 3 active and phosphorylating, 4 inactive due to lack of 

 ADP or phosphate, 5 anaerobic). The small numbers below the respiratory 

 trace indicate oxygen uptake rates in (xM/sec. (Expt. no. 493b-14) (From 

 Chance and Williams, 19566. Reproduced by permission of the Editors. 

 Advanc, Enzymol.) 



starved yeast is treated with ethanol. In this case, however, 

 a lower concentration of ethanol is used. As in the previous 

 experiment, a rapid reduction of cytochrome h is indicated by 

 the downward deflexion of the spectrophotometric trace 

 represented by the recording of absorbancy changes at 430 m^ 

 with respect to 405 m(x. Respiration starts rapidly and 

 achieves a steady-state rate of 2 -1 (jlm 02/sec., as indicated on 

 the trace. The mitochondria are now in an active state, 

 respiring rapidly with an excess of both substrate and intra- 

 cellular ADP. They are therefore in state 3. About a minute 



