OXIDATIVE MECHANISMS IN ANIMAL TISSUES 



21 



molecular oxygen is too slow to be of physiological significance; 

 moreover, none of them except one recently isolated from yeast by 

 Haas, Horecker, and Hogness (14) reacts rapidly with cytochrome 

 c. How then are they Hnked to oxygen in the living cell? This ques- 

 tion we cannot at present answer. We are thus left with a gap be- 

 tween the flavoproteins on the one hand and the cytochromes on 

 the other. 



How large is this gap? Let us attempt to answer this question by 

 considering the various known systems in our chain in relation to 



Volts af 

 pH 70 



0.8. 



06. 

 04. 



02J 



+ 

 0.0 



Q2. 



0.4. 



OXYGEN 



Cytochrome Oxidase 



Cytochrome g 



Cytochrome c 



Succinate 

 Fumarate 



Cytochrome b 



Methylene 

 Blue 



Flavoproteins 



Pyridine Nucleotides 



Substrates 



HYDROGEN 



Acceptor 



le ("2) 



le ("2) 

 le 0<2) 



le (x2) 



2e+2H' 



2e+H* 



2e+2H^ 



Environ- 

 ment 



wp 



2, H^" 





>70% 



>30% 



Figure 2. — Oxidation-reduction systems concerned in biological oxidations 



The source of the potential values used is given in reference 2 except for the 



diphosphopyridine nucleotide system, which is taken from reference 5. 



their oxidation-reduction potentials. As shown in Figure 2, we are 

 able to plot fairly accurately according to their potentials all the 

 systems discussed above. Cytochrome oxidase is the chief exception, 

 but presumably we may place it between cytochrome a and oxygen. 

 We thus have interposed between the substrate and oxygen, reading 

 in the order of the potential of their systems, pyridine nucleotides, 

 flavoproteins, cytochrome h, cytochrome c, cytochrome a, and finally 

 cytochrome oxidase. Now if cytochrome h functions in this chain, 

 and it must be remembered that we are not certain that it does, 

 the possibility that another system lies between it and the flavo- 



