. Biological structure and function; proceedings. Biochemistry; Cytology. 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
. Biological structure and function; proceedings. Biochemistry; Cytology. 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 succinat
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