70 BIOLOGICAL CHEMISTRY 



OXIDISING ENZYMES 



are divided into three classes. Peroxidases, which accelerate 

 oxidation in the presence of hydrogen peroxide. Oxidases, 

 which accelerate oxidation in the presence of molecular 

 oxygen. Catalase, which accelerates the decomposition of 

 hydrogen peroxide. 



PEROXIDASES 



are widely distributed throughout the animal and vegetable 

 kingdoms. They are classified according to the nature of the 

 substance that they oxidise into polyphenol, tyrosin, alcohol, 

 uric acid, etc., peroxidases. These names indicate the 

 substances that are used in vitro, as indicators for the presence 

 of these peroxidases. 



OXIDASES 



are not so widely distributed as the peroxidases. They are less 

 stable, and it is claimed that they are really a mixture of a 

 stable peroxidase with an unstable substance called oxygenase. 

 The oxygenase is capable of taking up molecular oxygen with 

 the formation of a peroxide-like substance. Iron, manganese, 

 etc., have been found associated with oxidases, and they may 

 be the peroxide forming substance, but in some cases the 

 peroxide forming substance is probably of organic nature ; 

 for instance something allied to catechol.* 



CATALASES 



are not true oxidising enzymes, as they decompose 

 hydrogen peroxide with the liberation of molecular oxygen. 

 This process does not accelerate oxidation, and it is believed 

 that the catalase is a protective mechanism to prevent the 

 poisonous action of hydrogen peroxide on protoplasm. It 

 may be that the decomposition of hydrogen peroxide is 

 merely a surface effect of the colloids in the tissues and fluids 

 as even the inorganic colloids accelerate the decomposition 

 of hydrogen peroxide. 



REDUCING ENZYMES 



have been described. These cannot be enzymes in the 



sense of catalytic agents as described in this chapter, because 



they would then necessarily be oxidising enzymes as well. 



Oxidation leads to the liberation of energy, but reduction 



causes an accumulation of energy. Therefore reduction can 



take place only when energy is supplied from some other 



source. The so-called reducing enzymes may be oxidising 



enzymes that accelerate oxidation, but take the oxygen from 



* M. W. Onslow, Biochem. Journ., 1919, vol. 13, p. i. 



