ENZYMES. FERMENTATION 41 j 



XII. Oxidases. 



A. Catalase. 



A catalase is present in most animal and vegetable tissues. 

 Solutions may be prepared by extracting the tissues with water ; the 

 extracts are usually not very active and a piece of tissue is used 

 directly. 



Since catalase acts upon hydrogen peroxide with the formation of 

 oxygen only hydrogen peroxide can be used as substrate. 



E.g. a piece of liver is placed in a test tube and covered with a dilute 

 solution of hydrogen peroxide. An evolution of oxygen occurs. 



B. Peroxidase. 



Peroxidases are very abundant in plant tissues. Active solutions 

 are best prepared from horse-radish, potato, or fungi, by grinding up 

 the material, treating with water and filtering from insoluble matter. 



A substrate is usually present in the plant tissue together with 

 the enzyme. On bruising the tissue it becomes brown like the cut 

 surfaces of apples and pears, or it may become blue or red as in some 

 species of fungi. The substrate is adihydric or trihydric phenol such as 

 hydroquinone or pyrogallol. In the tissue an organic peroxide, or oxy- 

 genase, is also frequently present On bruising the tissue, oxygen is- 

 taken up from the air and the peroxide is formed. The peroxidase acts 

 upon the peroxide giving " active " or nascent oxygen, which oxidises 

 the substrate. These oxidases are sometimes called direct oxidases. 



Sometimes the colour is only given after hydrogen peroxide or 

 other peroxides (especially organic peroxides), such as are present in 

 oil of turpentine which has been exposed to the air, are added. Such 

 oxidases are called indirect oxidases. 



For purposes of demonstration a variety of phenolic substances are 

 used as substrate. 



(1) Guaiacum. A freshly prepared I per cent, solution in alcohol _ 

 (tincture of guaiacum). It changes to blue on oxidation. 



(2) Guaiaconic acid, the constituent of guaiacum. A -5 to i per 

 cent, solution in alcohol. 



