Control of Oxygen Utilization 115 



DPN-linked substrates, is due to the unreactivity of such a 

 bound form of pyridine nucleotide towards oxidation by 

 flavoprotein unless the following transfer reactions occur 

 (Chance and Williams, 19566). Here the two steps of the 

 phosphorylation reaction are written as suggested by the 

 data of Plant (1957): 



DPNH ^ I + X ^ DPNH + X ^ I (1) 



X--I + P^X--P (2) 



X -^ P + ADP ^ ATP (3) 



Application to metabolic control problems 



Two factors need to be considered in connexion with the 

 role of the mitochondria in control of metabolism: whether 

 control of respiratory metabolism is exercised by the sub- 

 strate level or through the phosphorylation process. 

 Control of respiration at the substrate level. Baker's yeast 

 suspended in phosphate buffer and aerated for 18 hours has 

 a very low respiratory activity. Under such circumstances, 

 the initiation of metabolism occurs directly upon the reduc- 

 tion of intramitochondrial pyridine nucleotide and cyto- 

 chrome, since an excess of ADP has accumulated in the 

 mitochondria. An example of the initiation of respiration in a 

 starved yeast cell suspension is given by Fig. 18. The initi- 

 ation of respiration caused by the addition of 34 mM ethanol 

 has been recorded polarographically by means of the platinum 

 electrode, and spectrophotometrically by the double-beam 

 spectrophotometer at wavelengths appropriate for pyridine 

 nucleotide and cytochrome h. In the right-hand portion of the 

 figure, the rapid reduction of cytoplasmic and mitochondrial 

 pyridine nucleotide through alcohol dehydrogenase activity 

 is indicated by the downward sweep of the trace, correspond- 

 ing to an increasing absorption at 340 vci\L with respect to 

 374 m(jL. The increased slope of the platinum microelectrode 

 trace indicates a higher respiratory activity. In the left-hand 

 portion of the figure, at wavelengths appropriate to cyto- 

 chrome h (428-405 mji,), a similar, although slightly less abrupt, 



