26 A SYMPOSIUM ON RESPIRATORY ENZYMES 



reduced forms react directly with oxygen at a rapid rate. Thus 

 they differ in this respect from the flavoproteins responsible for the 

 oxidation of the reduced pyridine nucleotides. The reduced form 

 of the fZ-amino acid oxidase flavoprotein also differs from all other 

 flavoproteins in that it does not react with methylene blue or with 

 dyestuffs of even higher oxidation-reduction potential (6). This varia- 

 tion in the behavior of different flavoproteins containing the same 

 prosthetic group resembles the variation in the behavior of the 

 various iron porphyrin protein compounds. Obviously the protein 

 partner exerts a marked influence on the behavior of the prosthetic 

 group. 



Now such flavin systems as those just mentioned are probably 

 of minor importance in furnishing the energy required by the cell; 

 it can be shown that they are not affected by cyanide, which blocks 

 the bulk of the oxygen consumption of the cell. In fact, these 

 systems might be looked upon as incinerators for disposing quickly 

 of unwanted products. Franke and Hasse (13) have termed them 

 "rudimentary." It may be that they represent the earliest types of 

 mechanism to emerge for the furtherance of biological oxidations 

 and thus might be classed as primitive. They differ from 

 cytochrome-linked systems in that hydrogen peroxide appears as a 

 by-product of their reaction with oxygen. Thus there arises the 

 question of the relationship of catalase and peroxidase to such 

 systems. It is interesting to note that, of the tissues examined, the 

 liver, among the richest in catalase, has the highest flavin and the 

 lowest cytochrome c content. 



Finally, one other interpretation of the relatively low cytochrome 

 c content of tissues must be considered. This is the possible exist- 

 ence of pathways as yet unidentified which parallel the cytochrome 

 system or supplement it. The existence of such unknown pathways 

 has already been postulated as a result of certain experimental data 

 obtained from a study of the action of inhibitors of respiration that 

 are believed to poison cytochrome oxidase. Two of the most com- 

 monly employed inhibitors of this type are cyanide and azide. Let 

 us consider, therefore, two examples of experiments involving the 

 use of these respiratory poisons and examine the validity of the 

 conclusions that may be drawn from such experiments. 



The respiration of the unfertilized arbacia egg is insensitive to 

 cyanide. Upon fertilization the egg consumes oxygen at a markedly 

 increased rate, and the additional oxygen consumed is found to be 

 cyanide-sensitive. These facts have been interpreted to indicate 



