238 MARTIN KLINGENBERG 



Conclusions 



The experimental results of an influence of ATP on the respiratory 

 chain are summarized as follows: 



1. ATP can affect the reduction of DPN in the presence of flavin- 

 specific substrates, such as succinate, etc. Thus the energy requirement of 

 the DPN reduction in the presence of these substrates is directly 

 demonstrated. 



2. ATP can affect also the reduction of a flavoprotein, the reduction of 

 which had been shown to depend on operative oxidative phosphorylation. 



3. ATP can influence the overall electron transport by inducing 

 respiratory control. 



4. ATP addition causes a "crossover point" of redox changes at the 

 respiratory chain between cytochrome b and c. cf. (15). 



These results are interpreted as demonstrating the reversibility of 

 oxidative phosphorylative reactions. At least two phosphorylation steps 

 of the respiratory chain are shown to be reversible. 



DPN<^-^Fp<^-(b)^-±-c a O2 



In the DPN-flavin region a complete reversal of the oxidative phosphory- 

 lation, including also a reversal of electron transfer, can be effected. In the 

 cytochrome region the reversal of the reactions between ATP and the 

 respiratory chain is seen. The interaction of ATP at the cytochrome 

 level also controls the overall electron transport of succinate or glycerol- 

 phosphate oxidation. 



There are tw^o aspects which should be briefly mentioned on the basis 

 of these results. Firstly, the elucidation of the mechanism of oxidative 

 phosphorylation depends greatly on the knowledge about the inter- 

 mediates of the phosphate transfer reactions. The reversibility of oxidative 

 phosphorylation presents in principle the possibility to estimate the energy 

 content of the intermediates. Second is the physiological meaning of 

 energy-dependent hydrogen transfer from flavin to pyridine nucleotide in 

 mitochondria, as originally proposed by Krebs [18]. In this case, hydrogen 

 from succinate or fatty acid oxidation w^ould not be transferred to oxygen, 

 generating ATP in oxidative phosphorylation, but, with expenditure of 

 energy, to the DPN or TPN systems of the mitochondria. It appears 

 possible to imitate such a system in experiments with liver mitochondria 

 where hydrogen in the presence of ATP can be transferred from succinate 

 to oc-ketoglutarate with the formation of glutamate [19]. 



