OXIDASES 95 



In view of the importance ascribed to this class of enzyme 

 in respiration processes, it is desirable here to reconsider 

 them.* A freshly prepared I per cent alcoholic solution of 

 guaiacum resin, made by dissolving pieces of the resin from 

 which the outer layer has been scraped off, forms a delicate 

 reagent for a certain type of oxidizing system which turns 

 the solution blue on being brought into contact When 

 properly prepared, the guaiacum solution is not turned blue 

 on exposure to the air, nor by the addition of hydrogen 

 peroxide. If, however, a mixture of guaiacum and hydrogen 

 peroxide be brought into contact with the juice of certain 

 plant tissues, either the expressed sap or a freshly-cut surface, 

 such as a slice of horse radish, a blue colour is immediately 

 assumed. From this it may be concluded that the tissue 

 contains a material capable of setting in motion some oxidizing 

 mechanism which causes the oxidation of guaiacum to the 

 blue compound. Horse radish and like substances exhibiting 

 a similar action are said to contain a peroxidase, that is, an 

 enzyme which can activate hydrogen peroxide so as to make 

 it turn guaiacum blue. Peroxidase is, in fact, assumed to 

 act in much the same way as does ferrous sulphate upon a 

 mixture of hydrogen peroxide and tartaric acid : if a little 

 hydrogen peroxide be added to a solution of tartaric acid, no 

 change is observed ; on the addition of a few drops of ferrous 

 sulphate, however, a yellow coloration is immediately pro- 

 duced which changes to violet on the addition of caustic soda. 

 The ferrous sulphate activates the hydrogen peroxide to 

 oxidize the tartaric acid to dihydroxymaleic acid. Two 

 kinds of oxidizing enzymes are now generally recognized 

 in the plant world, namely, the direct acting oxidases 

 which turn blue an alcoholic solution of guaiacum with- 

 out the addition of hydrogen peroxide, and the indirect 

 acting peroxidases which are unable to produce a change 

 in colour until hydrogen peroxide has been added. To 

 account for these facts, it was assumed that whereas the 

 peroxidase required the addition of a peroxide in order that 

 it might have something from which to liberate active oxygen, 

 the oxidase contained not only the peroxidase but also a 



* See Vol. I., p. 392. 



