OXIDIZING-REDUCING ENZYMES 533 



peroxidase is to bring the plant extract or tissue to be tested into contact with 

 an alcohoh'c solution of gum guaiacum in the presence of hydrogen peroxide. 

 If a peroxidase is present the solution of gum guaiacum will turn blue in 

 color. This is due to the formation of a blue oxidation product of the guaiacol 

 (a phenolic compound) which is a constituent of the gum guaiacum. Perox- 

 idases are supposed to operate b}^ splitting the peroxide into water and active 

 ox3'gen, the latter combining with the compound which is oxidized. Perox- 

 idases are of extremely widespread occurrence throughout the plant kingdom, 

 having been found in practically every plant which has been tested for their 

 presence, 



3. Oxidases. — It is common observation that when a potato tuber is cut, 

 the exposed tissues usually turn a brownish color. Similar reddish, brownish, 

 or blackish colorations can be discerned in many other plant tissues after cut- 

 ting or crushing. The frequently observed darkening of plant juices as they 

 are pressed out of a tissue is another example of this phenomenon. Such 

 colorations of plant tissues upon the ready access of atmospheric oxygen are 

 due to the oxidation of certain cell constituents which is brought about by 

 enzyme systems called oxidases. 



Chodat and Bach (1903) postulated that an "oxidase" actually represents 

 a system composed of two enzymes — an oxygenase and a peroxidase. This 

 view has been concurred in by most subsequent workers although there is no 

 general agreement regarding the exact manner in which the two components 

 of this enzyme system operate. 



In general the oxygenase is supposed to produce hydrogen peroxide or 

 organic peroxides by the oxidation of certain compounds, using atmospheric 

 oxygen in this process ; or, according to another view, to become oxidized 

 itself in the presence of atmospheric oxygen, forming a peroxide. According 

 to Onslow and Robinson (1926) catechol and similar compounds are oxidized 

 in the presence of oxygenase, hydrogen peroxide being one of the products 

 of the reaction. Such compounds are known to be of widespread distribution 

 in the plant kingdom. 



The peroxidase component of the system is supposed to accomplish the 

 oxidation of phenolic compounds '^ by catalyzing the transfer of oxygen from 

 the peroxide to the compound which is oxidized, as previously described. 



Derivatives of phenol (carbolic acid ) : CH 



HC 



HC 



/\ 



COH 



CH 



CH 



