RESPIRATORY METABOLISM 373 



(2) Cytochrome is a group of pigments or enzymes, which in the 

 living cell are oxidized under aerobic, and reduced under anaerobic 

 conditions, but which cannot be oxidized directly by molecular O,. These 

 serve as hydrogen acceptors for anaerobic dehydrogenase systems. 



(3) Oxidase. — The term oxidase includes all enzymes which are 

 capable of performing oxidations in the presence of molecular oxygen. 

 To this group belongs the respiratory enzyme of Warburg, which is 

 perhaps identical with the oxidase of cytochrome, which in turn is 

 also referred to as indophenol oxidase because of one method of detect- 

 ing its activity. This enzyme brings about the oxidation of reduced 

 cytochrome by molecular oxygen, and water is supposed to be oxidized 

 to hydrogen peroxide during the process. Aerobic dehydrogenases are 

 sometimes classified as oxidases. 



(4) Catalase is an enzyme present in aerobic organisms and usually 

 absent in anaerobes. This enzyme converts hydrogen peroxide to water 

 and molecular oxygen, and its place in the respiratory chain is given 

 below. 



(5) Peroxidases are enzymes which in the presence of an oxidizable 

 substrate convert hydrogen peroxide to water and activated oxygen, 

 thereby causing oxidation of the substrate. The exact role of peroxidases 

 in cellular respiration is not understood. The peroxidases are iron com- 

 pounds, and other iron compounds, such as cytochrome and methemo- 

 globin, exhibit some peroxidase-like activity. 



(6) Yelloiv respiratory pigment, or enzyme, is a flavo-protein capable 

 of reversible oxidation and reduction whicii may be reduced in a re- 

 action involving oxidation of substrate (through the intermediary action 

 of a co-enzyme) and which can then be reoxidized in the presence of 

 molecular Oo (with formation of HoOo) or other hydrogen acceptors. 



(7) Glutathione is an amino-acid complex capable of reversible oxi- 

 dation and reduction, and which may act as a hydrogen acceptor through 

 the reduction of an -S-S- group to two -SH groups (cysteine to 2 

 cysteine) which are auto-oxidizable in the presence of molecular Og. 



The details of how these substances function in the living cell are 

 subject to considerable controversy, but for our present purpose we may 

 regard the general outline for the first four items listed above as follows: 



(1) substrate -|- 2 oxidized cytochrome ^ 



dehydrogenase 



oxidized substrate -j- 2 reduced cytochrome 



