OXIDASES 175 



OXIDASES 



It is again a matter of common observation that if, e.g., 

 an apple is cut open and the interior is left exposed to 

 air, in a short time it becomes brown. Everyone, too, must 

 have been struck by the difference in appearance between 

 mushrooms as bought in the shop and the same when 

 freshly gathered ; the dark brown appearance, especially of 

 the under surface, is an unpleasant change from the delicate 

 white and pink they exhibited whilst growing. These and 

 many other similar changes are due to oxidation brought 

 about by a class of enzymes known as oxidases ; that the 

 change is due to the presence of oxygen can be shown 

 by leaving freshly-cut slices of apple in vacuo or in an 

 inert atmosphere such as hydrogen, when no browning takes 

 place. 



Oxidases are very widely distributed enzymes, and for 

 this reason a great many vegetable extracts and juices 

 tend to darken on standing. A notable instance of such 

 a change is the case of the juice of the lac tree, which 

 furnishes the raw material of Japanese lacquer ; this juice 

 is a clear yellow when first drawn, exposed to air it rapidly 

 turns brown and finally black. It has been discovered 

 that this is due to an oxidising enzyme which has been 

 termed laccase. 



The browning of wine which takes place in course of time, 

 and which is known as ageing, is due to the oxidation and pre- 

 cipitation of the colouring matter ; this can be accelerated by 

 the addition of an oxidase. 



These enzymes have been studied in the same manner 

 as other cases already considered ; the following instances 

 from numerous researches will serve to illustrate the methods 

 employed. Laccase was investigated by Yoshida, who dis- 

 covered in Japanese lac an acid, urushic acid, which is 



