OXIDIZING-REDUCING ENZYMES 535 



reduced to a colorless compound called leucomethylene blue. Reduction of 

 the methjlene blue is due, not to removal of oxygen from the molecule, but 

 to a transfer of hydrogen from molecules of some other substance, which is 

 termed a hydrogen donator, to the molecules of methylene blue, which acts 

 as a hydrogen acceptor. Oxidation of the donator molecules thus occurs 

 simultaneously with the reduction of the acceptor molecules (methylene blue). 

 If air is now bubbled through this colorless liquid, the original blue color will 

 return within a short time due to oxidation {i.e. dehydrogenation) of the 

 leucomethylene blue back to methylene blue. These two reactions are indi- 

 cated by the following schematic equations : 



DH2 + Mb ^^'^"'""""^ D + MbH2 



Hydrogen Methylene Oxidized Leucomethylene 



donator blue donator blue (reduced) 



MbHs + >^ O2 -^ Mb + H2O 



As a specific example of this type of reaction we may take the oxidation 

 of succinic acid to fumaric acid which proceeds according to the following 

 equation : 



CH2COOH Dehydrogenase CHCOOH 



I + Mb — —^ > II + MbHs 



CH2COOH CHCOOH 



Succinic acid Fumaric acid 



Methylene blue, which is commonly used in testing for dehydrogenases, is 

 not a natural constituent of living cells; neither are some of the other reagents 

 which have been used in testing for such enzymes. However living organisms 

 apparently contain a number of naturally occurring compounds which act in a 

 manner analogous to methylene blue, i.e. as hydrogen acceptors. Among 

 these are the so-called "respiratory pigments" which are known to occur 

 in many plant tissues and the compounds cytochrome and glutathione which 

 have been found to be of widespread occurrence in both plant and animal 

 tissues (see later). 



Similarly, it has been found that living organisms contain a number of 

 compounds which can serve as hydrogen donators in reactions similar to that 

 just described. Among these are xanthine, fatty acids, aldehydes, sugars, 

 and amino acids. 



5. Catalase is one of the most ubiquitous of all enzymes and has been 

 found in practically all plant and animal tissues which have been tested for 

 its presence. This enzyme activates the reduction of hydrogen peroxide into 

 water and molecular oxygen according to the equation: 



■.--. ^ Catalase _ 



2 H2O2 > 1 H2O + O2 



