32 INTERMEDIARY METABOLISM AND GROWTH 



ADP-t-Pi 

 Oxidative phosphorylation X'^ 1 -== =^ATP + X-»-I 



DNP 



Glucose _ Glucose-6-phosphate 



Hexokinase +ADP 



X+DNP'^*I (relatively slow) 



HgO DNP + I 



Fig. 13. Postulated mechanism of dinitrophenol action. 



The breakdown of glvicose is inhibited by oxidative cellular processes. This 

 effect is known as the Pasteur effect. The inverse of this, that is, the inhibition of 

 respiration by glycolysis, has also been demonstrated in tissues which manifest an 

 unusually rapid glycolysis (Chance and Williams, 1956). Thus, the addition of 

 glucose to respiring ascites tumor cells is followed by an increase in oxygen con- 

 sumption and an oxidation of the steady state level of cytochrome b. This may be 

 attributed to the increase in the ADP pool due to the hexokinase reaction as in- 

 dicated in the scheme shown above. In about 40 sec, however, the above effects 

 are reversed; the cytochrome b is reduced once again, and the ADP level falls, 

 due to the competition for ADP between glycolytic enzymes and mitochondria. 

 As a result, the respiration rate is once again inhibited. 



J. Photosynthetic phosphorylation 



Plant chloroplasts contain multienzyme systems which catalyze three distinct light 

 dependent reactions: i) photolysis of water, 2) photosynthetic phosphorylation, and 

 3) CO2 fixation (Arnon, 1955; Arnon et al., 1956): 



light 



1) H2O > 2 (H) + (O) 



light 



2) ADP + Pi > ATP 



light 



3) CO2 + H2O * (CH2O) + O2 



There is an increasing order of complexity for the three photochemical reactions. Photolysis 

 can occur in preparations incapable of photosynthetic phosphorylation and CO2 fixation. 

 In turn, photosynthetic phosphorylation is found to proceed unimpaired in preparations 

 which can not fix CO2. CO2 fixation, however, occurs only in chloroplast preparations 

 capable of photolysis of water and of phosphorylation. 



In the presence of appropriate acceptor compounds, the light catalyzed photolysis of 

 water may be coupled with reductive processes in a reaction first studied by R. Hill. 

 Benzoquinone, ferricyanide, cytochrome c, flavins, menadione, and TPN^, or DPN^ can 

 be reduced by illuminated chloroplast grana under anaerobic conditions. Oxygen is 

 simultaneously evolved (Jagendorf, 1956; San Pietro and Lang, 1956) 



4) 2 (H) + (O) + benzoquinone — > hydroquinone + '/2O2 



chloroplasts 



chloroplasts 



5) 2 (H) + (O) + TPN* (DPN") > TPNH, (DPNH2) + '/2O2 



The (H) generated in reaction i) may be utilized by the chloroplasts for the reduction 

 of CO 2 into organic linkage (reaction 3), or the (H) may be reoxidized concomitant with 

 photosynthetic phosphorylation (reaction 2). The competition for (H) between the 



