OXIDATIVE MECHANISMS IN ANIMAL TISSUES 27 



that in fertilized and unfertilized eggs the pathways for the oxida- 

 tion of substrates are different. It should be noted, however, that at 

 best the evidence merely indicates the possible existence in un- 

 fertilized eggs of a system alternate to the cytochrome system. 

 Cyanide and azide do not effect the reduction of methylene blue by 

 substrates acting through the pyridine nucleotide and flavoprotein 

 systems. Hence these systems may be functioning in the unfertilized 

 egg. Supporting evidence is furnished by the findings of Krahl 

 et al. (18) that the unfertihzed egg contains flavin-adenine di- 

 nucleotide and is also rich in diphosphopyridine nucleotide (17). 

 Now does the cyanide insensitivity of the unfertilized egg indicate 

 that an iron porphyrin system is not functioning in its respiratory 

 mechanisms? Such an interpretation is indeed possible. One may, 

 for example, postulate that the respiration is of a primitive type and 

 passes directly through a flavoprotein to oxygen. Such a contention 

 is supported by the fact that the presence of cytochrome a, b, or 

 c cannot be demonstrated in the unfertilized arbacia egg. It is 

 possible, however, to demonstrate in the egg the presence of hemin 

 substances (4). Moreover, Krahl and his co-workers (19) have re- 

 cently shown that the eggs contain a substance resembling cyto- 

 chrome oxidase, in amounts equal in activity to the cytochrome 

 oxidase of mammalian tissues. This substance was found to be 

 cyanide-sensitive if it was functioning in an oxidation requiring the 

 addition of cytochrome c. Are we to conclude, then, that this egg 

 "cytochrome oxidase" plays no role in the respiration of the un- 

 fertilized egg because such respiration is cyanide-insensitive? Is it 

 not possible that a reaction can occur directly between "cytochrome 

 oxidase" and flavoprotein in the arbacia egg? In view of the wide 

 variety of properties exhibited by flavoproteins such a reaction 

 might well occur. But it appears that such a postulate is contradicted 

 by the evidence cited above that this egg "cytochrome oxidase" can 

 be inhibited by cyanide. 



Before we decide what is the correct interpretation of these data, 

 let us review our knowledge about the mechanism of cyanide poison- 

 ing. Cyanide apparently inhibits respiration by reacting with cyto- 

 chrome oxidase, since it prevents the air oxidation of the three re- 

 duced cytochromes. At low concentrations it apparently does not 

 combine with the cytochromes. Presumably it combines with the 

 ferric form of cytochrome oxidase, since it is methemoglobin and not 

 hemoglobin that reacts with cyanide. We may conclude, then, that 

 somehow cyanide prevents the reduction of ferric cytochrome oxi- 



