OXIDATIVE MECHANISMS IN ANIMAL TISSUES 23 



of such a reaction occurring with the flavoproteins. Another example 

 is the catalysis by the cytochrome system of the oxidation of certain 

 organic substances, such as p-phenylenediamine. It should be noted, 

 however, that such extraneous substances or their oxidized products 

 may by their lack of specificity react in a way that is harmful to the 

 cell mechanism. Methylene blue, for example, though not reduced 

 by the d-amino acid oxidase system, gradually inactivates it in the 

 presence of light (6). Mr. Kerr, working in my laboratory, has 

 recently been investigating the mode of action by which butter 

 yellow produces liver tumors. He has found that the oxidation 

 catalyzed by a heart muscle preparation of extremely small quanti- 

 ties of p-phenylenediamine (an apparent breakdown product of 

 butter yellow) completely inactivates the succinic oxidase activity 

 of such a preparation. Thus the marked specificity of interaction of 

 these compounds may also serve to prevent unwanted and harmful 

 reactions from occurring within the cell. 



Next arises the question whether the energy released at each step 

 in this chain is utilized by the living organism and if so, how. At 

 present direct evidence for the utilization of energy furnished by 

 individual oxidative processes such as these is limited to the demon- 

 stration of coupled phosphorylation reactions. Such demonstrations 

 have thus far been largely confined to that portion of the chain in- 

 volving the pyridine nucleotides. Whether the large bulk of energy 

 release that occurs through the cytochromes is useful for phosphor- 

 ylations or for energy-utilizing mechanisms other than phosphoryla- 

 tion has not yet been definitely ascertained. It should be noted, 

 however, that Korr (16) has pointed out that in the fertilized arbacia 

 egg respiratory and functional activity are both inhibited by 

 cyanide. Restoration of the respiratory rate by the addition of a 

 substance such as methylene blue to replace the inactivated cyto- 

 chrome system does not, however, restore functional activity. 



A point that should perhaps be mentioned in connection with 

 this pathway is the use of the term "hydrogen transport" to describe 

 biological oxidations. E' values of the cytochrome c system exhibit 

 a zero slope in the neutral pH region. Preliminary experiments of 

 the author indicate the same to be true for the cytochrome a and 

 h systems. This indicates that only electron transfer occurs with 

 these systems and that hydrogen is not concerned in the reaction. 

 If the cytochrome oxidase system behaves similarly, only a fraction 

 of the total energy released in biological oxidations involves hydro- 

 gen transport. Thus only the electrons of the substrate can be con- 



